The Structure and Permeability of Integument REFERENCESlemastm/Publications/PDF/Snake... · 2016. 2. 16. · sociations with hyaluronic acid, mucopo- lysaccharides and small amounts

Oxford University Press is collaborating with JSTOR to digitize, preserve and extend access to American Zoologist.

http://www.jstor.org

The Structure and Permeability of Integument Author(s): Harvey B. Lillywhite and Paul F. A. Maderson Source: American Zoologist, Vol. 28, No. 3 (1988), pp. 945-962Published by: Oxford University PressStable URL: http://www.jstor.org/stable/3883392Accessed: 14-01-2016 20:44 UTC

REFERENCESLinked references are available on JSTOR for this article:

http://www.jstor.org/stable/3883392?seq=1&cid=pdf-reference#references_tab_contents

You may need to log in to JSTOR to access the linked references.

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at http://www.jstor.org/page/ info/about/policies/terms.jsp

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected].

This content downloaded from 131.252.127.37 on Thu, 14 Jan 2016 20:44:06 UTCAll use subject to JSTOR Terms and Conditions

http://www.jstor.org

http://www.jstor.org/publisher/oup

http://www.jstor.org/stable/3883392

http://www.jstor.org/stable/3883392?seq=1&cid=pdf-reference#references_tab_contents

http://www.jstor.org/page/info/about/policies/terms.jsp



Amer. Zool., 28:945-962 (1988)

The Structure and Permeability of Integument1

Harvey B. Lillywhite

Department of Zoology, University of Florida, Gainesville, Florida 32611

AND

Paul F. A. Maderson

Department of Biology, Brooklyn College, Brooklyn, New York 11210

Synopsis. The skin is a heterogeneous, multimembrane system with multiple diffusion

pathways and potentially numerous rate-limiting barriers for specific molecules that are

exchanged with the environment. A broad survey of animals indicates that integumentary coverings are morphologically, biochemically and embryologically diverse but with com? mon themes of adaptation. The evolutionary proliferation of intercellular junctions, fibrous or mineralized protective barriers, and lipoid waterproofing barriers emphasize the gen? erality of diffusion limitations. However, the regulation of specific exchange processes such as the flux rates of respiratory gases entails a complex interaction of multiple factors affecting both diffusion and perfusion limitations. Consideration of specific pathways and rate limitations for diffusion of various substances suggests that the permeation of skin by specific molecules can be partially independent of other exchange processes. Our understanding of regulated permeability is, however, lacking in mechanistic and integrated analysis in most cases. Comprehensive understanding of the integument as a regulatory pathway of communication with the environment will require comparative studies of specific transport pathways, their rate-limiting resistances, and the interactions as well as individual regulation of transported molecules.

Introduction

The integument provides physical protection for internal organs and regulates the exchange of materials between the

organism and its environment. While var? ious other roles are evident in many species, the integument is primarily a barrier and

transporting surface. For different ani?

mals, a wide range of substances potentially permeate the skin, and numerous factors affect their rates of permeation. To under? stand the limitations of diffusion and the cutaneous regulation of diffusional

exchange, we must consider both morpho? logical and physiological features of integ? ument.

This article considers the structural

organization of skin as it relates to the movement of various materials between the environment and the internal milieu. Stud? ies of integumentary morphology are long standing, and, in recent years, research on

1 From the Symposium on Cutaneous Exchange of Gases and Ions presented at the Annual Meeting of the American Society of Zoologists, 27-30 December 1986, at Nashville, Tennessee.

94

permeability has been advancing rapidly with increasing sophistication. Rarely, however, are these two realms of investi?

gation satisfactorily united in context, and

rarely do the approaches of investigators consider competing demands on the link between function and structure. Thus, it is hoped that the focus of this review will stimulate interest and future study in these

neglected areas. Recent and excellent reviews of integu?

mentary morphology in the various animal taxa are numerous, and this information will not be repeated here (see, for example, Bereiter-Hahn et al, 1984, 1986). Rather, common structural themes will be empha- sized in relation to transport phenomena as elucidated from physicochemical and

physiological studies. Discussion will be limited to water, ions and respiratory gases as these have received the most attention from physiologists.

The Generalized Morphology of Integument

Before considering the permeability of

specific components of integument, it is

[b



946

A

H. B. Lillywhite and P. F. A. Maderson

B

Fig. 1. Generalized epidermal organization in (A) a turbellarian, (B) a terrestrial insect, (C) an amphibian, and (D) a squamate reptilejust prior to skin shedding. The examples illustrate features of simple and complex integuments in both invertebrate and vertebrate animals (not drawn to identical scales). The symbols are as follows: a, alpha keratin; b, basal laminae or basement membrane; B, beta keratin; c, cement; cl, clear layer; en, endocuticle; ep, epicuticle; ex, exocuticle; gly, glycocalyx; IG, inner epidermal generation; m, mucus; ms, mesos layer; OG, outer epidermal generation; sc, stratum corneum; se, syncytial epithelium; sg, stratum germi- nativum; tw, terminal web; w, wax.

important to review some general organi- zational features. These considerations will be restricted to triploblastic animals, because in diploblastic taxa lacking a mesoderm the epithelia are not free to specialize at a tissue level that is comparable to the outer epithelia of more advanced forms

(Fig. 1). The skin of invertebrates is embryolog-

ically as well as structurally diverse, reflect-

ing both phylogenetic and ecological fac? tors. Invertebrates typically possess a monostratified epidermis that is derived from ectoderm and overlies a basal lamina that is either amorphous or fibrous (base? ment membrane). The epidermis of tur- bellarians is representative of the simplest level of organization; it is a single layer of

epithelial cells which overlies muscles and lacks an external cuticle. The epidermis of certain Platyhelminthes, Nematoda and

pseudocoelomate phyla is syncytial rather than cellular. This form is restricted, how-

ever, and has no counterpart among higher invertebrates or vertebrates. In more advanced invertebrates, the epidermis may be thickened and protected by secreted

mucus, cuticle or mineralized structures. The skin of adult vertebrates is a mul?

ticellular, stratified epithelium of ectoder- mal origin overlying fibrous and vascular mesoderm. The epidermis never forms a

confining exoskeleton strictly comparable to that of arthropods, molluscs or echi- noderms. All types of cells that are present throughout the epidermis are represented by precursors in the basal layer, which pro- liferates cells that are eventually lost from the animal's surface. In fishes, mitotic

activity is detectable throughout the epi? dermis although it is most prevalent in the

deeper layers. In tetrapod vertebrates, mitotic activity is limited to the basal layer. Keratin is formed by epidermal cells in all

groups including fishes, but it is a prevalent structural feature only in epidermis of



Skin Structure and Permeability 947

amniotes. The form of integument may include patterned folds or scales in addi? tion to appendages that result from localized epidermal and/or dermal cell prolif? eration and differentiation (Maderson, 1972). Cellular renewal ofthe epidermis is a vertebrate characteristic that maintains

integumentary health and function. In

contrast, the maintenance of invertebrate skin involves replacement of cuticle and

underlying apical cell membranes without cellular renewal.

The resistance ofa given layer of cellular cutaneous tissue to chemical invasion is

directly related to its water content (Krogh, 1919) as well as ionic and viscous proper? ties. Therefore, limitations of diffusion are related to the compositional features of a tissue in addition to its thickness and area

(e.g., Shick etal, 1979). Additionally, three

morphological trends in the adaptive evo? lution of integuments have had profound effects on the permeability of animal sur? faces.

First, the integuments of probably all

higher animals have been "tightened" by intercellular junctions which, among other

things, form occluding barriers by restrict-

ing the diffusion of fluids and solutes between cells. In vertebrate tissues "tight junctions" (=zonulae occludentes) form belt- like regions of intimate contact between

plasma membranes of adjacent cells. These are generally impermeable to lanthanum tracer and contribute to the maintenance of osmotic and electrical gradients across

epithelial layers. Tight junctions are gen? erally absent from invertebrate epidermis with the exception of tunicates, in which

they are leaky. Septate junctions appear to substitute for tight junctions in other invertebrates where they are also compar- atively leaky, being permeable to tracers such as lanthanum. The epidermis of fresh? water and terrestrial organisms typically has more septa than that of marine inverte?

brates, which experiences reduced concen? tration gradients across the epithelial layer.

A second trend in evolution is the

strengthening of the epidermis and its derivative structures by means of fibrous

polymer layers of protein or, in various

invertebrates, a chitin-protein complex.

Collagen and other similar structural

proteins are common components of inver? tebrate cuticle and frequently form associations with hyaluronic acid, mucopo- lysaccharides and small amounts of liquid. The structure and composition of these structural elements are related to the mechanical and durability requirements of the integument. Some proteins undergo sclerotization or "tanning" which stiffens the cuticle and causes the protein to become water-insoluble by cross-linking the adjacent chains, making a more resistant and less extensible structure. Further harden-

ing is achieved by mineralization. The third important modification of

cutaneous form is the presence of lipids either as a superficial covering, as in insects, or sequestered in diffuse or localized layers within the epidermis, as in various verte? brates. These lipids comprise a variably effective barrier to water diffusion, and they affect the movement of other molecules across the skin as well.

Permeation of Water, Gases and Ions: General Principles

The integument is a complex, hetero-

geneous structure comprising a cascade of resistances to movements of molecules

through multiple membranes in series.

Considering the multilayered complexity of this structure, three fundamental ques? tions arise: (1) What are the pathways of molecular movement across the skin? (2) What steps in the permeation process are

rate-limiting? (3) How do various permeant molecules differ with respect to their facil-

ity to pass a given structural barrier?

Although the following sections review information that bears on these topics, the

questions remain largely unanswered. Much current knowledge regarding the

permeability properties of individual bio?

logical membranes is incomplete or con-

troversial, so, clearly, understanding the mechanistic diversity of molecular trans?

port across integument will require an

ongoing commitment to rigorous and

imaginative research. To the extent that a multicellular integ?

ument consists of a number of cell mem? branes in series, the rate limiting step(s)



948 H. B. Lillywhite and P. F. A. Maderson

for transport of molecules through the skin can be the passage across the membrane/

cytosol or membrane/environment inter?

face, movement in the interior ofthe mem?

brane, or diffusion in the cutaneous matri- ces between the membranes, neglecting unstirred layers at the skin surface (see Feder and Pinder, 1988). If transport across the membranes is fast compared with dif? fusion in the cytosol, movement can then be limited by diffusion polarization related to diffusion-limited liquid zones in series with the membranes (Neumcke, 1971). Intercellular routes of transport are con? sidered in the next section.

Natural membranes occurring within the skin are mosaic structures containing lipid bilayers in addition to pores or molecular channels. While thermal mobility of molec? ular species in a diffusing system is inversely related to the molecular mass, the relative

diffusivity of molecules through skin

depends on considerations of solubility and

partition coefficients as well. Thus, the extent to which molecular size or lipid sol?

ubility regulates the penetration of mole? cules into cutaneous cells depends on the fractional membrane area occupied by channels and the characteristics of the channels (see Finkelstein and Cass, 1968; Finkelstein, 1984). Additionally, mem? brane proteins may facilitate the flux of water or dissolved molecules either by spe? cific transport or perhaps by forming a low- resistance pathway between the protein and

lipid. Various membrane proteins that serve as channels for specific ions may also

permit the concomitant permeation of

water, nonelectrolytes and dissolved gases. In generalized terms, molecules cross the

protein-free bilayer by a solubility-diffusion mechanism, dissolving into the hydrophobic regions and then diffusing through the bilayer subject to the boundary con? ditions that are established. Lipophilic molecules are very permeant, whereas

hydrophilic molecules are relatively impermeant. The movement of lipophilic mol? ecules across the bilayer depends on the

degree of packing and thermal mobility of the hydrocarbon chains and on the charge of polar head groups of phospholipids.

The free diffusion ofa molecular species across an interface or barrier can be

described by Fick's diffusional equation. Both general definitions and interpreta? tions of permeability coefficients are based

upon, or related to, the Fick relationship. With respect to water movement, two dif? ferent permeability measurements are fre?

quently used. The diffusional water per? meability coefficient (Pd) expresses the

isotopic flux of water through a unit of area

per unit of concentration difference of iso?

topic water. On the other hand, if concen? tration differences of impermeant solutes cause an osmotic flow of water, the osmotic flow through a unit of area per unit of concentration difference of solute describes the osmotic permeability coefficient (Pf). The term filtration coefficient or hydraulic con-

ductivity applies if the driving force for water flow is a differential of hydrostatic pres? sure. In contrast to diffusion, bulk osmotic or hydrodynamic flow involves the vecto- rial movement of an assembly of molecules

being driven by an imposed potential. Con-

sequently, the various permeability coeffi? cients may differ quantitatively because of the physical nature ofthe water movement

pathway (see Schafer and Andreoli, 1972; Finkelstein, 1984).

If water movement occurs by a solubility- diffusion mechanism (i.e., diffusion through a lipid bilayer in which water is poorly sol?

uble), then it can be shown that bulk and

isotopic water flow are equal such that Pf/ Pd = 1 (Cass, 1968). On the other hand, if water transport is through channels, Pf/Pd

generally exceeds 1, and the ratio increases with increasing channel radius (neglecting problems of unstirred layers). The greater value of Pf results from the fact that osmotic water transport occurs by laminar or qua- silaminar flow (Mauro, 1957).

Aqueous pores probably comprise the

major route for water transport in certain

epithelia including integument of amphib? ians and possibly some aquatic inverte? brates (Hevesy et al, 1935; Koefoed-John- son and Ussing, 1953). However, in many cell membranes the number of ion con-

ducting channels are apparently too few to

provide a significant pathway for water movement (Finkelstein, 1984). With few

exceptions, the Pf attributable to mem? brane channels is calculated to be about 1 % that of the value for the entire mem-




brane, and therefore Pf/Pd approximate

unity. Thus, the bulk of transcellular water movement in cutaneous tissues appears to occur by a solubility-diffusion mechanism

involving the lipid bilayer pathway. Even

so, water flux may occur at the lipid/protein interface as well as via channels formed

by proteins, so that water permeability may be governed largely by the presence of

membrane-spanning proteins and their interactions with the lipid bilayer. Diffu- sional flux of water through lipid bilayers is reduced by orders of magnitude in the absence of protein (Carruthers and Mel-

chior, 1983). The permeation mechanism for water in

lipid bilayers is conceivably driven by the interaction of lipid polar head groups with water rather than the solubility of water in the lipid hydrocarbon (Carruthers and

Melchior, 1983). In any event, water dif- fuses (exchanges) between the various

hydration shells of the polar head group into the hydrocarbon core (Hauser and

Phillips, 1979), driven by the transmem- brane water concentration energy gradi? ent. Divalent cations compete with water for interaction with the negatively charged phospholipid groups and potentially dis-

place the water molecules (Hauser et al,

1976).

Permeability coefficients for water movement through cell membranes vary by orders of magnitude depending on the membrane composition and physical state

(Finkelstein, 1978; Carruthers and Mel?

chior, 1983). Fluid membranes are more

highly permeable than are those in a liquid- crystalline state. In fluid state membranes,

elevating temperatures, decreasing the chain length of hydrocarbon tails of phospholipids, increasing hydrocarbon satura?

tion, and reducing amounts of cholesterol

(within certain limits) in lipid bilayers all act to increase water permeability. Consid?

ering the effects and molecular dimensions of these changes, modifications of gaseous and ionic permeability might also be

expected, albeit in complex fashion. Whereas the lipid bilayer membrane may

represent the principal pathway of water

movement, electrolytes are virtually impermeant. Ions and hydrophilic non-

electrolytes require a hydrophilic environ-

ment for movement through lipid bilayers. Thus, permeation of cells by ions requires integral membrane proteins in the form of channels or carriers. The penetration of channels by ions is dependent on such fac? tors as the steric hindrance of molecules at the channel entrance, the viscous drag and

charge interaction between molecules and the walls of the channel, and the configu? ration of the channel length. Clearly, the

importance of these factors will depend on the relationship between the size of the

transported molecule and the radius of the

conducting channel. Transport of ions and water may be single-file in narrow channels (Finkelstein, 1984). In this situ-

ation, the rate of movement of an ion such as Na+ may be equal to that of a water molecule moving the length of the chan? nel. The equality of the two rates means that the movement of water can be a major barrier to ion transport. The exit step is an additional barrier to the ion so that the rate of movement of ions across the entire membrane (channel) may be less than that for water. Moreover, these considerations raise the possibility that water permeability of a channel could be salt-dependent, so that in essence the ion can conceivably block a channel to flow of water (Finkelstein, 1984).

The permeation of membranes by ions is usually measured as isotopic flux or eiec? trical conductance. Extensive literature on this subject will not be reviewed here. Stud? ies of integument (e.g., frog skin) indicate that ion movement may be active or passive and that ion transport therefore may influ? ence the permeation of water and dissolved

gases. On the other hand, solvent drag of

electrolytes may accompany flow of water within aqueous membrane channels.

The diffusion of gases through mem? branes is described by the application of Fick's laws for diffusion and Henry's laws for partial pressure relations and gas sol-

ubility (discussed in Feder and Burggren, 1985). Because of its high solubility, car? bon dioxide is far more permeable in most tissues than is oxygen, which accounts for the consistent observation that cutaneous fractional C02 loss exceeds cutaneous oxy? gen uptake in bimodal, skin breathing ver? tebrates. Presumably, both gases permeate




tissues wherever water movement occurs in the bulk phase, i.e., through channels or intercellular spaces. Additionally, respira? tory gases dissolve in thin lipid membranes at membrane-water interfaces in propor? tion to the partial pressures in the aqueous phase. It is assumed that for oxygen the

boundary processes are rapid, so that the rate limiting step is the diffusion of gas within the membrane. Diffusion of oxygen in the aqueous phase may, in certain cir-

cumstances, be facilitated by virtue of microturbulence induced by the pumping actions of respiring mitochondria or the

presence of haem or haem-like carriers (e.g., Lee and Smith, 1965). These factors may account for diffusion coefficients that are

higher than that through free water

(MacDougall and McCabe, 1967). The movement of C02 across an epithe?

lium involves diffusion ofthe dissolved C02

gas across lipid bilayers and also reactions

among the several chemical forms of C02 in the aqueous layers. At least two forms of C02, HC03~ and C032_, cannot easily diffuse across most cell membranes. Numerous studies indicate that different

steps in the movement of C02 may be rate

limiting under different conditions. In the absence of carbonic anhydrase, the diffu? sion of C02 through aqueous compart? ments may be rate-limiting because the

uncatalyzed hydration-dehydration of C02 is too slow for [HC03~] to facilitate C02 diffusion through the aqueous layer. How?

ever, presence of carbonic anhydrase stim? ulates C02 flux 10-100-fold in proportion to the [HCCV] over the pH range 7-8

(Gutknecht et al, 1977). Carbonic anhy? drase is widely distributed in respiratory epithelia including the integument of a

variety of taxa (e.g., Simkiss and Wilbur, 1977; Hackman, 1984). Thus, in certain circumstances the diffusion of C02 through membranes becomes the rate-limiting step. Permeabilities of bilayer membranes to

C02 also vary at least two orders of mag? nitude depending on their lipid fluidity and

composition.

Cellular Junctions and the Paracellular Diffusion Pathway

The basic organization of any epidermis results in two pathways for transepidermal

molecular movement. The first pathway is located between the cells (lateral, intercellular spaces), where solutes and fluids can flow down their respective chemical and, in some cases, eiectrical gradients. The sec?

ond, parallel pathway is through the cells, where movement may be by diffusion

through the lipid bilayer or by bulk flow

through channels in the cell membrane and is potentially a multi-step process. Cellular

physiologists often categorize simple epi? thelia according to the relative permeabil? ity of these two pathways. In so-called tight epithelia, junctional contacts between cells have high resistance, so that most passive diffusion occurs through the cell mem? branes. In contrast, leaky epithelia are those in which most diffusion occurs between

cells, that is, it is paracellular. While these distinctions are potentially applicable to

complex, multi-layered integuments, information is presently insufficient to

attempt such categorization. The junctional contacts between cells are

structurally modified so that cells can

adhere, interact and dissipate tensional stresses throughout a tissue. Additionally, specialized junctions confer an occluding function that allows concentration gradi? ents to be maintained across the epidermal layers. The term ''tight junction" has been

loosely applied to a broad range of intimate contacts between plasma membranes,

although the term was originally intro? duced to designate the zonula occludens

(Farquhar and Palade, 1963). The term will here be used to designate any belt-like

region of membrane apposition which occludes the intercellular space. Typical tight junctions join cells at their apical edges by a continuous belt, thus forming a planar array.

Freeze fracture studies of tight junctions show the presence of a network of fibrils of diverse complexity within the mem?

branes, thought to be the sealing elements ofthe junction. In some tissues the number of intramembrane fibrils and junctional permeability are directly related (Claude and Goodenough, 1973), although other features appear to be related to the per? meability properties ofthe junction. Often the fibrils are not continuous but are inter-

rupted at more or less regular intervals,




which might constitute leak pathways through the junction. Differences of per? meability might also depend on the chem? ical composition of the molecules within the junctions rather than, or in addition

to, their morphological features. Studies on a variety of "leaky" epithelia of endo? thelial origin indicate that junctional permeabilities are larger to cations than to

anions; i.e., the tight junction constitutes a

negative environment for ion diffusion. The relative cation- or anion-selectivity may also change, depending on the pH of the

junction environment. Such ionic selectiv-

ity is not well characterized for tight epi? thelia, mainly because of the small area

occupied by the junctional pathway. One of the principal questions relating

to permeability and transport in skin is: What fraction of the transepithelial diffu? sion of substances goes through the junc? tional or paracellular pathway? So far, the most reliable information on this question comes from studies of epithelial slices of

frog epidermis which do not constitute a normal multilayered integument. Clearly, however, frog skin is very "tight," having resistances of 10-50,000 ohm-cm2 com?

pared with resistances ranging from less than 10 to a few hundred ohm ? cm2 in leak- ier epithelia of endothelial origin. Various data indicate that more than 90% of the

transepithelial conductance is localized in the paracellular pathways of leaky epithe? lia. By contrast, less than 10% ofthe total conductance resides in the paracellular route of tighter epithelia such as frog skin

(Erlij and Martinez-Palomo, 1978), but such determinations are imprecise. It must be remembered that, with respect to different ions, epithelia are selectively permeable and that both passive and active permeation may result in net fluxes that are variably partitioned between cellular and paracel? lular routes.

Tight junctions restrict the diffusion of both fluids and solutes between cells. In addition to electrolytes, various nonelec-

trolytes (and undoubtedly gases) are known to permeate tight junctions, presumably through hydrated pores. Consensus is presently lacking, however, regarding the eval- uation of relative hydraulic conductivities of the transcellular and paracellular path?

ways. Based on geometrical considerations, several investigators have rejected the

notion that tight junctions are important routes for water permeation, and some believe that the hydraulic conductivity of the junctional pathway can account for only 10% ofthe whole epithelium (Wright et al,

1972). However, the junctional pathway may account for most of the water move? ment in "leaky" epithelia (Levitt, 1981). Further research is required to partition the transport of water with respect to cel? lular and extracellular routes, especially in relation to varying permeability require? ments of animal integuments.

In frog skin the extracellular spaces are sealed by tight junctions in at least two lay? ers of cells: the superficial, cornified cell

layer and the outermost cell layer of the stratum granulosum. Recent evidence indi? cates that the cornified cells are ineffective diffusion barriers and, therefore, the bar? rier function of cellular junctions is local? ized to the outside border ofthe outermost cells in the stratum granulosum (Erlij and

Ussing, 1978). Undoubtedly, however, the

permeability properties of the cellular

junctions are, in part, functions of the osmotic state of the cells, which depends on the permeability of the outermost skin and its mucous covering (see below).

Changes of cell volume induce changes in active transport (Erlij and Ussing, 1978) in addition to altering the structure of intercellular spaces. Changes in the mor?

phology of intercellular spaces can affect diffusion of water and solutes even inde-

pendently of cellular swelling or shrinkage. For example, osmotic flow of water within

epithelia can cause dilation or collapse of lateral extracellular spaces (depending on the flow direction). Calculations show that diffusion through these spaces can become

rate-limiting as they collapse and that such

changes in the dimensions of lateral spaces can produce asymmetry of water flow

(Wright etal, 1972). Other factors can modify the properties

of the paracellular pathway. Low pH, for

example, weakens intercellular junctions thus leading to increased paracellular per? meation by ions (Ferreira and Hill, 1982; Marshall, 1985).

Most investigations of cellular junctions




have been restricted to vertebrate tissues and to invertebrate tissues of endothelial

origin. Tight junctions are absent from the vast majority of invertebrate tissues, being replaced by septate junctions. These are structures that, in transverse sectional view,

comprise a series of septa spanning 15-18 nm intercellular spaces. Septate junctions occur in all invertebrate integuments stud? ied to date and always form a belt around the apical edges of cells lining the outer

epithelia. Circumstantial evidence suggests that they restrict paracellular diffusion in a manner that is functionally analogous to that of vertebrate tight junctions (Green, 1984).

Circumapical tight junctions are present in all classes of vertebrates, although they appear to occur sparsely in skin of mam? mals (Matoltsy, 1984). It is not clear whether the emphasis placed on tight junc? tions in amphibian skin merely reflects the extensive use of that system as a model for

study of membrane permeability.

Structural and Secreted Barriers

The epidermis of numerous inverte? brates with an exposed plasma membrane surface is primarily an absorptive, trans-

porting organ. Primitively, as well as in specialized forms such as parasitic tapeworms, the integument is important for nutrient

absorption in addition to ion regulation or

gaseous exchange. During the evolution?

ary radiation of eumetazoan animals, a

variety of factors (including increasing size,

terrestriality and mobility) necessitated structural reinforcement of the skin for

protection and support. In general terms, this evolution proceeded from a flagellated or ciliated epidermis to a "hypodermis" covered with a secreted cuticle. Although primitive cuticles may have regulated the net flux of nutrients as a primary function, their appearance and radiation was often correlated with skeletal, protective or loco-

motory needs of the organism (Rieger, 1984).

The differentiation of a true cuticle,

completely absent from vertebrates, involves the secretion of fibrous and/or

granular materials onto the outer surface of the epidermis. However, supportive

fibrous and granular materials may also be

associated with the base of the epidermal cells or with the apical cytoplasm. In more advanced eumetazoans, the fibrous matrix of the cuticle may become specialized by division into distinct layers in addition to the incorporation of chitin, collagen and minerals. Various acids, mucopolysaccha- rides and lipids may also form associations with the fibrous matrix.

Chitin is a high molecular mass polysac- charide that forms a fibrous component of

many cuticles and confers tensile strength combined with flexibility. The chitin mol? ecule is long and straight and is stabilized

by a covalently bonded backbone and intramolecular hydrogen bonds between

adjacent residues along the chain. Micro- fibrils of chitin associate with many differ? ent proteins having a wide range of molec? ular mass. Interface bonding between chitin and the immediately neighboring protein is strong. The chitin fibers of insect cuticle lie on a matrix of cross-linked protein, forming a system which has a large strength to mass ratio and high tensile and com?

pressive strengths that are stronger than either of the two components alone. Sev? eral crystalline forms of chitin are known,

commonly forming lattices that are

arranged in the manner of a plywood. The ratio of protein to chitin varies widely, but

together these two components normally account for more than 90% ofthe organic content of cuticle. In sclerotized cuticles mineralization involves calcium or mag- nesium carbonates and phosphates whose

quantity is inversely related to the protein content. Mineral deposition occurs both within and between fibrils and may be crys? talline in form.

Invertebrate collagens and vertebrate keratins also have two-phase structures with helicoidal arrangements present in some of the fiber layers. The strength of these sys? tems is increased by having fibers of high tensile strength and elasticity oriented in a matrix of weaker proteins or polysaccharides. Keratin complexes are heterogeneous collections of proteins containing many sulphydryl (SH) and disulphide (SS) linkages of cystine. These form the main constituents of the dead horny cells in




superficial layers of epidermis and its deriv- atives (hair and feathers) in amniote ver? tebrates. They may be associated with fibrous or globular proteins, variations that allow wide differences in structure and mechanical properties of keratinized cells. Mineralization of horny tissue involves cal? cium that is usually bonded to phospholipid within the keratin complex.

Both fibrous cuticle and subepidermal tissue must be traversed by diffusion, so these structures may represent a significant resistance to mass transfer, as compared with the epidermal resistance. Keratins of vertebrates and cuticles of invertebrates

vary from thin, transparent membranes to

thick, tough and rigid armour. In very gen? eral terms, permeabilities of fibrous struc? tures are related to their thickness, and where hard and soft tissues occur on the same body surface, the important sites of

exchange are at the soft tissue. Thick, cal? cified or sclerotized structures such as the shells of molluscs, exoskeleton of crusta? ceans and calcareous ossicles of echino- derms are virtually impermeable to gases, ions and water. At the other extreme, thin and highly permeable cuticles may cover

exchange structures such as gills or occur on the general body surface immediately after molting (e.g., Mangum et al, 1985). Thickness is not an absolute predictor of

permeability, however, because the dimen? sion is not independent of the tissue com?

position (see Lillywhite and Maderson, 1982).

How do the structural features affect the resistance of fibrous polymers to molecular diffusion? From a strictly mechanistic view-

point, any alteration in a polymer structure that affects the free volume (equivalent to "holes" or "pores") should alter perme? ability to invading molecules. Moreover,

permeating molecules will have perme? ability coefficients related to their size,

shape and polarity. Due to steric hind-

rance, the diffusion coefficient is expected to decrease considerably with increasing molecular mass ofthe diffusing substance. Studies of avian eggshells demonstrate that their permeability to 02, C02 and water

vapor varies in direct proportion to the differences in diffusion coefficients, sug?

gesting a common diffusion pathway for

gaseous molecules (Paganelli et al., 1978; Ackerman and Rahn, 1980).

Generally, studies on permeability of

polymers, including collagen, show that

alterations of side chains, crystallinity, polar

groups, plasticizers and fillers all affect per?

meability (Lieberman et al, 1972). With

respect to side chains, permeability is

affected below a critical molecular mass of

fiber cross-linkages. Additionally, cross-

linking beyond certain critical levels

increases the resistance to diffusion. Sta-

bility of the cross-linkage structure is

dependent on a large number of inter- molecular forces: mobility and free vol? umes of biological polymers are signifi? cantly affected by covalent, ionic and

hydrogen bonding as well as van der Waals attractive forces between nonpolar amino acid side chains.

Proteins that are sclerotized are hard- ened and stabilized by the presence of aro- matic cross-links, which decrease perme? ability. Hardening is also attributable to mineralization in some species. Many crus? tacean cuticles are both calcified and scler? otized. Proteins from nonsclerotized cuti? cles generally have a higher content of amino acids with bulky side chains and so inhibit close packing ofthe molecules. The woodlouse cuticle, for example, is rela?

tively permeable compared with that of insects. X-ray diffraction data suggest that the proteins may be globular with a high proline content, which limits the degree of helix formation and packing that is possi? ble (reviewed in Hackman, 1984). In con?

trast, proteins from sclerotized cuticles have

generally smaller and nonpolar amino acids that are able to pack more closely together, thereby permitting formation of many van der Waals bonds.

Hydration significantly affects the per? meability of fibrous protein polymers because of the influence of water on the molecular structure. Proteins bind water

very strongly at low relative humidities, and the water content increases in proportion to humidity exposure. The mobility of side chains and the distance between them increases with increasing water content as water enters between the protein mole-




cules and reduces the degree of crystallinity. Introduction of water molecules into the polymer structure increases the avail? able free volume as well as the mobility of

cross-linkage groups. Dehydration reduces

permeability not only by reversing these effects but also by promoting noncovalent

bondings between protein chains (Vincent and Hillerton, 1979). Consequently, the

permeability of dry protein films is

extremely low (Lieberman et al, 1972).

Obviously the pH of fibrous structures is significant with respect to ion diffusion, the stability of molecular associations, and the bonding of water. Arthropod cuticles tend to be maintained at a pH close to the isoelectric points of their proteins, such that

cross-linkages between protein chains are

strongest. Marked deviations of pH from these isoelectric points potentially alter the water content of cuticles because bondings between chains are replaced by bondings between protein and water (Hackman, 1984). Ion exchange can be markedly affected in some cuticles which have a matrix including acid polysaccharides (Gomme, 1984).

Fluid secretions may contribute to the cuticle of various animals, especially in taxa

lacking a fibrous or mineralized covering of significant thickness. These secretions serve a variety of functions including mechanical protection, pH regulation, ion

exchange, antibiotic actions, friction

reduction, pheromonal roles and preven- tion of dehydration. Most secretions are

generally mucous or proteinaceous in vary? ing proportions. Mucous secretions are

predominantly water with varying amounts of proteoglycans and glycoproteins (gly- coconjugates), various ions and often lesser

quantities of sugars, amino acids and lipids (Campbell et al, 1967; Wilson, 1968; Dap- son, 1970). Secretions may be either acidic or alkaline, the latter possibly counteract-

ing acidification of the skin due to C02 excretion (Friedman et al, 1967).

In fundamental terms, an epicutaneous film of mucus may be regarded as an unstirred layer whose resistance to diffu? sion is proportional to the thickness, vis-

cosity and solute concentration. Mucus

covering the skin of carp, Cyprinus carpio,

has an oxygen diffusivity that is about 70% that of the surrounding water (Ultsch and

Gros, 1979). However, very thin films are

typically involved where the underlying tis? sue is absorptive or respiratory, and a vari?

ety of evidence indicates that such films do not limit diffusion significantly. The secre? tions themselves are sources of water, sol? ute and gaseous efflux, although water and

electrolytes might be recycled at the skin surface (Machin, 1977; Whitear, 1977).

Mucous secretions are important in pre- venting dehydration of exposed epidermal surfaces in dehydrating environments sug? gesting a fundamental dichotomy of cuticular organization in terrestrial animals. With few exceptions, either the epidermis is provided with a lipid and/or fibrous dif? fusion barrier to prevent excessive water loss from the skin, or the epidermis is cov? ered with a wet film so that evaporation occurs from secreted fluids rather than from the epidermal cells. In terrestrial gas? tropods and amphibians, mucus secretion is a fundamental mechanism for the trans? fer of water to evaporating surfaces, which are sculptured to spread and retain the fluid

(Machin, 1964a, b; Lillywhite, 1971). With? out mucus extrusion, the diffusional per? meability of the epidermis is unable to sus- tain normal levels of evaporative water loss, and the skin dries (see also Lillywhite, 1975). With respect to cutaneous gas exchange, it is not necessary for oxygen or

C02 to dissolve in water before entering or leaving the outer epidermal membrane if the latter is exposed to air. Rather, the maintenance of "wet" respiratory struc? tures is essential to preserve the functional

morphology of the diffusion membrane,

including patency of its perfusion vessels. This point is often not clearly represented in textbooks.

Lipids and Water Diffusion: Biochemical Barriers

Surveys indicate that numerous terres? trial arthropods and vertebrates have lost the ability to exchange gas and solutes

through the integument by developing a

protective and relatively waterproof epi? dermis. The maintenance of internal water volume and concentration is an important




problem faced by all animals, but one that is magnified in terrestrial species, which

may have limited access to water and live in dehydrating conditions. Mechanisms for

reducing the diffusion of water through skin are varied and not entirely under-

stood; however, the most effective reduc? tion of water permeability is achieved in those integuments where lipids are depos? ited as a more or less continuous sheet. It

appears that the "tight" nature of such a diffusion barrier eminently affects gases and solutes as well as water.

Lipoid materials occur at various places throughout all integuments. However, the use of lipids to form layered, watertight barriers is best known in certain arthro?

pods and vertebrates where the resistance mechanisms have been well studied (see

Hadley, 1980, 1981; Lillywhite and Mad-

erson, 1982 for reviews). Evidence suggests that diverse chemical

compounds in all of the cuticular layers may contribute to the impermeability of

integument in terrestrial arthropods. But the principal recognized water barrier in most insects and arachnids consists of lipid layers or impregnated waxes associated with the epicuticle. In particular, the

importance of surface lipids in restricting cuticular water loss is firmly established. Water efflux from the cuticle increases sig? nificantly when these lipids are removed

by solvent extraction or mechanical abrasion. Many studies have shown correlations between the quantity of extracted cuticular

lipids, which may vary seasonally as well as

interspecifically, and their effectiveness as a diffusion barrier. This relationship has

exceptions however, and compositional features of lipids are also important.

Chemical analyses indicate that cuticular

lipids of most insects and arachnids consist of a heterogeneous mixture of compounds with long chain hydrocarbons predomi- nating (Hadley, 1981). Free fatty acids, wax

esters, cholesterol and other categories of

lipid are usually present in smaller amounts, whereas alcohols and phospholipids are often not present. Lipid barriers exhibit-

ing the greatest resistance to water flux are characterized by predominance of largely nonpolar, long-chain, saturated and usu-

ally branched alkanes that appear not to

be organized into specifically oriented lay? ers (see also Hadley, 1984). The long chain molecules pack closely and increase the

intensity of van der Waals interactions between hydrocarbon molecules, thus cre?

ating a nonfluid membrane that resists

penetration by water.

Lipids are present in the epidermis of all classes of vertebrates, although their influ? ence on permeability is not significant in most amphibians and is not well studied in birds. Some species of South American

phyllomedusine frogs secrete lipids onto the skin surface and then use their limbs to spread the secretion over the body (Blay- locketal, 1976). The composition of these

lipids has been analyzed for one species, Phyllomedusa sauvagei, and was found to contain largely long-chain wax esters

(McClanahan et al., 1978). The superficial film of lipid retards water loss while frogs are inactive in trees where they are exposed to the (usually dry) atmosphere.

Numerous reptiles live exposed to dry conditions and generally such species pos? sess quite impermeable integuments. Vari? ation in the integumentary water perme? ability of squamate reptiles is correlated with habitat and determined more by lipid composition of the epidermis than by the thickness or structure of its keratin (Lil? lywhite and Maderson, 1982). The lipid barrier resides principally in the mesos

layer, which is overlain by layers of beta keratin whose role is primarily structural

(Fig. 1). The mesos lipids comprise a com?

plex mixture of polar and neutral lipids which appear to be similar in composition to those comprising the diffusion barrier of mammalian epidermis (see below). The mesos lipids occur as intercellular sheets and are derived from lamellar granules that extrude their contents into the intercel? lular space (Landmann, 1986). Part ofthe contents remain within mesos cells while the expelled lipids envelop the cells from all sides and fill the intercellular spaces to form a continuous sheet.

A similar process occurs in mammalian

epidermis, wherein lipids are deposited intercellularly from lamellar granules sit- uated at the boundary of the stratum gran-




ulosum and stratum corneum. The expelled contents of the granules form broad lipid sheets that lie parallel with the envelopes and filaments of keratinized cells. The

major group of lipids that contribute to these extracellular sheets are highly satu-

rated, unbranched, long-chain ceramides

(Wertz, 1986). The high degree of satu? ration resists oxidative damage, and the

paucity of branching makes these mole? cules capable of forming tightly packed, nearly crystalline arrays of hydrophobic covering.

Does a lipoid water barrier necessarily impede diffusion of other important mol? ecules such as respiratory gases and ions? Information is conflicting on this point, and a satisfactory understanding of the mechanistic constraints governing independent adjustments of permeability to water, ions and gases must await further research that

specifically addresses these interactions.

Damage to the cuticle of insects appears to affect water permeability more so than per? meability to gases, and the waterproofing waxes of insect eggs are reported to be very permeable to oxygen (reviewed in Buck,

1962). Also, in aquatic reptiles permeabil? ity of skin to water and gases does not nec?

essarily parallel permeability to electro-

lytes. In addition, there is an apparent asymmetry in the diffusion of water and

electrolytes through the skin, which can be altered by extraction of lipid (Stokes and

Dunson, 1982; Dunson and Stokes, 1983). These findings are controversial, but at least a circumstantial case can be made sug? gesting that permeabilities of skin to dif? ferent substances may have some indepen? dent components of adjustment.

On the other hand, numerous surveys indicate that highly water impermeable integuments are also poor exchangers of

gases and ions. Moreover, strong correla? tions between water loss and C02 loss dur?

ing ventilatory activity of insects (e.g., Quinlan and Hadley, 1982) are indicative ofthe dual resistance of cuticle to both C02 and water. Recently, pulmonary and cuta? neous gas exchange and cutaneous evap? orative water loss were measured simulta?

neously in the xeric-adapted frog Phyllomedusa sauvagei (Stinner and Shoe-

maker, unpublished data). Evaporative water loss varied widely between individual

frogs, presumably due to variation in the thickness or distribution ofthe waterproof? ing lipids that covered the outermost epi? dermis. The interesting finding in the pres? ent context was that cutaneous oxygen uptake and C02 loss varied directly with the magnitude of cutaneous water loss (Fig. 2). These data indicate that reducing cuta? neous water loss by use of a lipid barrier entails an attendant reduction in cutaneous

gas exchange.

Vascular Morphology and Perfusion

Perfusion ofthe skin with blood is essen? tial to integumentary function in all higher animals characterized by a well-organized circulation. Blood circulation not only ser- vices the metabolic needs of active cuta? neous tissue, but also provides the trans?

port medium for substances exchanged at the skin surface. Morphological aspects of

integumentary perfusion are fundamental to the regulation of internal fluid compo? sition, as the capillary architecture deter? mines both the functional area and the dis? tance through which diffusion and

exchange occur. The morphology of cutaneous vascula-

ture has been thoroughly studied in com-

paratively few species. Although great variation exists, several generalizations concerning vascular patterns are evident. Vascularization is essentially a property of the dermis, and capillaries rarely penetrate the epidermal structures. Presumably, extensive vascularization of epidermis compromises the barrier and protective functions of the skin. In mammals, capil? lary units extend as a "candelabra" of loops projecting vertically to contact the basal membrane and form papillary intrusions ofthe epidermis. These drain into venules

arranged horizontally in units that tend to outnumber the arterioles. Endothelium

adjacent to the epidermis is fenestrated and sometimes associated with microfilaments or myofilaments.

Other vertebrates display variations of this general scheme. Often, there is an elaborate network of anastomosing vessels




60

3* 50

40 < >

uj 30 O z <

o X UJ

<

20

10

<?*??

? ?

"% *o <p ^.

CP

^?cr" ?cTo0

5 10 15 20

CUTANEOUS WATER LOSS (mg-g-i-h"1)

25

Fig. 2. Relationship between cutaneous gas exchange and cutaneous evaporative water loss in the frog Phyllomedusa sauvagei. Measurements were made as frogs rested with variably groomed layers of lipids that were secreted from integumentary glands onto the skin surface. Rates of oxygen uptake (open circles) and

C02 loss (dark circles) are indicated as percentages of the total oxygen or C02 exchange (pulmonary plus cutaneous flux). Data are from J. Stinner and V. Shoemaker, unpublished (with permission).

contained within loose connective tissue of the superficial dermis which has a low nutritional requirement. In bony fishes, there is a secondary microvascular system immediately below the epidermis but over-

lying the scales and corresponding to their

topography (Vogel, 1985). The cutaneous vascular systems of squamate reptiles underlie the keratinized elements of the outer scale surfaces but are extensively developed within the inner and outer scale

regions as well as the free edges of scales

(Drane and Webb, 1980). The various pat? terns that are seen very often suggest that the cutaneous vascular system is function?

ally related to exchange processes between the animal and its environment. Capillary networks are often less developed in regions of skin that are less exposed to environ? mental medium or are subjected to exces? sive abrasion.

Among the more extreme specializations of cutaneous vasculature are those that have developed in relation to respi? ratory and nutritional functions in inver? tebrates. For example, the tentacles of

pogonophores bear pinnules consisting of

single epidermal cells that are elongated

distally. The cuticle covering the pinnule is nonfibrous and exceedingly thin, while

the inner cytoplasm is largely occluded by two blood capillaries which unite near the

tip of the cell. Outgrowths of these capil? laries associate with cisternae ofthe smooth

endoplasmic reticulum in some species. The

capillaries are essentially extracellular,

however, inasmuch as they are formed by

invaginations of the basal plasma mem?

brane and basal lamina which connect with

blood spaces between the epidermal cells

and subjacent muscle cells (George, 1977). Some of the cutaneous blood vessels of

earthworms are similarly lined with extra? cellular basement membrane (Hama,

1969). These vascular specializations doubtless have a respiratory role.

The pogonophore example involves both

morphological reduetion ofthe cuticle and

modification ofthe blood transport system as means of reducing the diffusion path between the environment and internal fluid

spaces. Epithelial thinning is also apparent in various vertebrates in which cutaneous

gas exchange is demonstrated to be impor-




tant. As noted by others (e.g., Feder and

Burggren, 1985), the diffusion barrier of

specialized epithelia in fish gills may be

fractions of a jtim, whereas diffusion bar?

riers ofthe general integument are greater, reflecting the protective and supportive functions that evidently must be accom-

modated by the cutaneous morphology. In

many amphibians the epidermal diffusion barrier separating blood from the outside medium varies from about 12 ixm in lung? less salamanders to more than 60 um in

some bufonids, whicji is adequate to sup? port diffusional exchange (Czopek, 1965). Cutaneous capillaries may be deeper in the skin of xeric frogs (Drewes et al, 1977), whereas in frogs (Telmatobius) or salaman? ders (Cryptobranchus) that rely on cuta? neous gas exchange the skin vasculature is

very superficial and may penetrate the epi? dermis (Rabl, 1931; Guimond and Hutch-

ison, 1973; Hutchison et al, 1976).

Epidermal thickness in the skin of fishes varies considerably, even among species that rely on cutaneous gas exchange (Mittal and Munshi, 1971). Both eels and pond loaches can supply nearly the whole of their

oxygen demand cutaneously, although the

epidermis is 263 um thick in Anguilla anguilla (Jakubowski, 1960) and 339 iim thick in Misgurnus fossilis (Jakubowski, 1958). On the other hand, the epidermis ofthe air-breathing fish Mastacembelus pan- calus may be as thin as 7 /im at places where blood capillaries penetrate the epidermis (Mittal and Munshi, 1971). This latter

species occupies stagnant pools that are

subject to drying, and the cutaneous mor?

phology is presumably an adaptation that facilitates cutaneous oxygen extraction when the fish is exposed to air.

Both density of capillaries and the dif? fusion distance (epidermal thickness) that overlies them show variability related to demands for exchange processes. Amphib? ians and fishes that inhabit moist or aquatic environments and are variously dependent on cutaneous transfer of respiratory gases possess dense networks of skin capillaries (reviewed in Randall, 1970; Feder and

Burggren, 1985). However, there is no consistent relationship between the extent of cutaneous vascularization, thickness of

the epidermis, and the amount of oxygen

uptake occurring across the skin. This is

because the exchange process must take into account the total functional surface

area of skin with capillaries, the diffusion

gradient, blood flow and oxygen loading characteristics, metabolic requirement for

oxygen, and competing requirements that

also shape the skin morphology. Moreover,

integumentary structure and vasculariza?

tion may vary regionally and through time.

Both theoretieal and empirical studies have shown that molecular flux of a sub? stance having a low permeability coeffi-

cient is independent of blood flow and thus

governed largely or exclusively by the con? centration gradient and permeability con? stant. However, as permeability increases, movement or uptake becomes more

dependent on blood flow. Therefore, con?

sidering the movement of a molecular

species across the skin, two factors in com? bination limit the transport between blood and the external environment: (a) diffusion

limitation, which is the resistance of the

epithelium to molecular transport, and (b)

perfusion limitation, which is the lack of molecules available for transport across or

away from the epithelium as a result of insufficient blood flow. A limited but grow? ing number of studies have provided indices of diffusion limitation and perfusion limi?

tation, largely with respect to the mass transfer of water and respiratory gases in

amphibian integument and noncutaneous

epithelia. Studies of skin from Bufo bufo and Rana

pipiens indicate that both osmotic and diffusive water exchange are perfusion- dependent when skin permeability is high (Christensen, 1974, 1975; Mahany and

Parsons, 1978). Thus, when skin perme? ability is increased with arginine vasotocin

(ADH), perfusion rate markedly affects water flux through the cutaneous tissue.

Moreover, while arginine vasotocin increases osmotic water exchange substan-

tially, diffusive water transfer is always more

dependent on blood flow than on ADH- induced permeability changes. Similarly, water exchange is shown to be dependent on blood flow in other epithelia such as gut and muscle (e.g., Dobson et al, 1971).




Studies of cutaneous gas exchange in a

variety of amphibians have demonstrated diffusion limitations in addition to perfusion-dependent enhancement of the skin

diffusing capacity. Inasmuch as these find?

ings are reviewed elsewhere, extensive comments will not be made here (see Feder and Burggren, 1985; Malvin, 1988; Piiper, 1988). As in studies of water permeability, the regulation of cutaneous gas exchange involves a complex interaction of multiple factors involving both diffusion and perfu? sion limitations. Perfusion dependent changes of the skin diffusing capacity are often related to conditions in the ambient

medium, including the ventilatory distur- bance of external unstirred layers (Feder and Pinder, 1988). However, a compre? hensive picture of how perfusion is regulated in relation to diffusion limitations must remain a goal for future research,

especially in contexts of conflicting per? meability requirements for different permeating substances.

Clearly, blood flow can alter the mass transfer of substances in several ways. (1) Increasing the rate of flow in patent, perfused capillaries potentially increases transfer but may have little or no effect in diffusion limited systems (Piiper, 1988). (2) If the increment of perfusion involves

increasing the number of perfused capil? laries (recruitment), then the functional

exchange surface is increased. Capillary recruitment has been shown to enhance

gas transfer across integument of amphib? ians and humans (Feder and Burggren, 1985). (3) Considering the morphology of cutaneous vasculature, capillary recruit? ment might also entail reductions in dif?

fusing distance as well as increases in dif?

fusing area. The relative contributions of these two aspects remain to be quantified, however. (4) It is conceivable that increases in perfusion can modify mass transfer rates

indirectly, and this might apply in several

exemplary situations. In the case of ion

transport, one explanation for dependence of sodium flux on perfusion flow rate is that sodium is actively transported and some component of this movement is sen? sitive to perfusate oxygen or metabolite

availability (Claiborne and Evans, 1981).

As a related example, blood flow changes

may indirectly affect cutaneous oxygen

uptake through an effect on metabolism of skin tissue. In some species of fish, cuta? neous oxygen consumption equals or exceeds the cutaneous oxygen uptake from

the external medium (Kirsch and Non-

notte, 1977). Consequently, the diffusion

gradient for oxygen across the epidermis

may be greatly influenced by the level of

mitochondrial respiration in the skin. (5) In terrestrial species, blood flow conceiv-

ably affects the hydration status of fibrous

proteins and thus the free space available for molecular diffusion within the fibrous matrix (see p. 953). (6) Finally, changes of

perfusion patterns are potentially impor? tant with respect to the timing and geom? etry of skin ventilation. Studies have shown

that larvae of air-breathing teleosts have cutaneous blood flow that runs countercurrent to the water stream (Liem, 1981). Thus, regulation of skin perfusion might be important with respect to the timing and frequency of movements of the pec- toral fins that generate posteriorly directed

respiratory water flow. Conceivably, reg? ulation of cutaneous ventilation and per? fusion matching, if only at a more localized

scale, might be part of a complex pattern of responses in other species of animals.

Conclusion

We have discussed the structural attri? butes of integument as they relate to

exchange processes involving gases, ions and water. Unlike simple membranes on which the understanding of many mechanistic principles is based, animal integu? ments are heterogeneous, multimembrane

systems capable of regulating exchange pathways and barriers for numerous sub? stances in relation to widely different and sometimes changing environmental demands. It is possible to quantify the

transport or diffusing capacity of a given integument for a particular substance, using a "black box" approach. However, as noted

by Jorgen Gomme (1984), "A comprehensive understanding of the integument as a barrier to and pathway of communication with the environment can be achieved only when a comparative study ofthe individual




transfer processes is integrated into an

analysis of their individual regulation and mutual interdependency." Evolutionary considerations and broad surveys of the occurrence and function of junctional complexes and fibrous, mineralized or

lipoid barriers emphasize the generality of diffusion limitations. On the other hand,

insight into the characteristics of individ? ual permeation processes suggest some

scope for independent adjustment of per? meability for specific substances. The chal?

lenge for contemporary and future research is for comparative investigations in functional cutaneous morphology to elu- cidate meaningful understanding of dif? fusional pathways, the rate limitations for

specific molecules, and the variation that is possible both physiologically and through evolutionary time.

ACKNOWLEDGMENTS

We express our thanks to Martin Feder and David Evans for helpful comments

during the preparation of the manuscript. H.B.L. was supported by the National Institutes of Health research grant number

HL33821 while this work was prepared. P.F.A.M. was supported partly from PSC- CUNY funds.

References

Ackerman, R. A. and H. Rahn. 1980. In vivo 02 and water vapor permeability of the hen's eggshell during early development. Respir. Physiol. 45: 1-8.

Bereiter-Hahn, J., A. G. Matoltsy, and K. S. Richards. (eds.) 1984. Biology ofthe integument. 1. Inverte? brates. Springer-Verlag, New York.

Bereiter-Hahn, J., A. G. Matoltsy, and K. S. Richards. (eds.) 1986. Biology ofthe integument. 2. Vertebrates. Springer-Verlag, New York.

Blaylock, L. A., R. Ruibal, and K. Platt-Aloia. 1976. Skin structure and wiping behavior of phyllomedusine frogs. Copeia 1976:283-295.

Buck, J. 1962. Some physical aspects of insect res? piration. Ann. Rev. Entomol. 7:27-56.

Campbell, J. P., R. M. Aiyawar, E. R. Berry, and E. G. Huf. 1967. Electrolytes in frog skin secre? tions. Comp. Biochem. Physiol. 23:213-223.

Carruthers, A. and D. L. Melchior. 1983. Studies of the relationship between water permeability and bilayer physical state. Biochemistry 22:5797- 5807.

Cass, A. 1968. Water and ion permeability of thin lipid membranes. Ph.D. Diss., Rockefeller Uni? versity, New York.

Christensen, C. V. 1974. Effect of arterial perfusion on net water flux and active sodium transport across the isolated skin of Bufo bufo bufo (L.). J. Comp. Physiol. 93:93-104.

Christensen, C. V. 1975. Effects of dehydration, vasotocin and hypertonicity on net water flux

through the isolated, perfused pelvic skin oiBufo bufo bufo (L.). Comp. Biochem. Physiol. 51A:7- 10.

Claiborne, J. B. and D. H. Evans. 1981. The effect of perfusion and irrigation flow rate variations on NaCl efflux from the isolated, perfused head of the marine teleost, Myoxocephalus octodecimspi- nosus. Marine Biol. Letters 2:123-130.

Claude, P. and D. A. Goodenough. 1973. Fracture faces of zonulae occludentes from "tight" and

"leaky" epithelia. J. Cell Biol. 58:390-400. Czopek, J. 1965. Quantitative studies on the mor?

phology of respiratory surfaces in amphibians. Acta Anat. 62:296-323.

Dapson, R. W. 1970. Histochemistry of mucus in the skin of the frog, Rana pipiens. Anat. Rec. 166: 615-626.

Dobson, A., A. F. Sellers, and S. O. Thorlacius. 1971. Limitation of diffusion by blood flow through bovine ruminal epithelium. Amer. J. Physiol. 220: 1337-1343.

Drane, C. R. and G. J. W. Webb. 1980. Functional morphology of the dermal vascular system of the Australian lizard Tiliqua scincoides. Herpetologica 36:60-66.

Drewes, R. C, S. S. Hillman, R. W. Putnam, and O. M. Sokol. 1977. Water, nitrogen and ion balance in the African treefrog Chiromantis petersi Bou- lenger (Anura: Rhacophoridae) with comments on the structure of the integument. J. Comp. Physiol. 116:257-268.

Dunson, W. A. and G. A. Stokes. 1983. Asymmet- rical diffusion: A reversal of sodium and water movement through the skin of sea snakes. Phys? iol. Zool. 56:106-111.

Erlij,D.andA.Martinez-Palomo. 1978. Roleoftight junctions in epithelial function. In G. Giebisch, D. C. Tosteson, and H. H. Ussing (eds.), Mem? brane transport in biology. III. Transport across multimembrane systems, pp. 27-53. Springer-Verlag, New York.

Erlij, D. and H. H. Ussing. 1978. Transport across amphibian skin. In G. Giebisch, D. C. Tosteson, and H. H. Ussing (eds.), Membrane transport in biology. III. Transport across multi-membrane systems, pp. 175-208. Springer-Verlag, New York.

Farquhar, M. G. and G. E. Palade. 1963. Junctional complexes in various epithelia. J. Cell Biol. 17: 375-412.

Feder, M. E. and W. W. Burggren. 1985. Cutaneous gas exchange in vertebrates: Design, patterns, control and implications. Biol. Rev. 60:1-45.

Feder, M. E. and A. W. Pinder. 1988. Ventilation and its effect on "infinite pool" exchangers. Amer. Zool. 28:973-983.

Ferreira, K. T. G. and B. S. Hill. 1982. The effect of low external pH on properties of the paracellular pathway and junctional structure in isolated frog skin. J. Physiol. 332:59-67.




Finkelstein, A. 1978. Lipid bilayer membranes: Their

permeability properties as related to those of cell membranes. In T. E. Andreoli, J. F. Hoffman, and D. D. Fanestil (eds.), Physiology of membrane disorders, pp. 205-216. Plenum Medical Book Co., New York.

Finkelstein, A. 1984. Water movement through membrane channels. In F. Bronner and W. D. Stein (eds.), Current topics in membranes and trans? port, Vol. 21, pp. 295-308. Academic Press, New York.

Finkelstein, A. and A. Cass. 1968. Permeability and electrical properties of thin lipid membranes. J. Gen. Physiol. 52:145s-l72s.

Friedman, R. T., N. S. Laprade, R. M. Aiyawar, and E. G. Huf. 1967. Chemical basis for the H+ gra? dient across frog skin. Amer.J. Physiol. 212:962- 972.

George, J. D. 1977. The pogonophore epidermis, its structure, functions and affinities. Symp. Zool. Soc. Lond. 39:195-222.

Gomme,J. 1984. Permeability and epidermal trans? port. In J. Bereiter-Hahn, A. G. Matoltsy, and K. S. Richards (eds.), Biology of the integument. 1. Invertebrates, pp. 323-367. Springer-Verlag, New York.

Green, C R. 1984. Intercellular junctions. In J. Be? reiter-Hahn, A. G. Matoltsy, and K. S. Richards (eds.), Biology ofthe integument. 1. Invertebrates, pp. 5-16. Springer-Verlag, New York.

Guimond, R. W. and V. H. Hutchison. 1973. Aquatic respiration: An unusual strategy in the Hellben- der Cryptobranchus alleganiensis alleganiensis (Dau- din). Science 182:1263-1265.

Gutknecht, J., M. A. Bisson, and F. C Tosteson. 1977. Diffusion of carbon dioxide through lipid bilayer membranes: Effects of carbonic anhydrase, bicar? bonate, and unstirred layers. J. Gen. Physiol. 69: 779-794.

Hackman, R. H. 1984. Cuticle biochemistry. In J. Bereiter-Hahn, A. G. Matoltsy, and K. S. Rich? ards (eds.), Biology ofthe integument. 1. Invertebrates, pp. 583-610. Springer-Verlag, New York.

Hadley, N. F. 1980. Surface waxes and integumen? tary permeability. Amer. Sci. 68:546-553.

Hadley, N. F. 1981. Cuticular lipids of terrestrial plants and arthropods: A comparison of their structure, composition, and waterproofing func? tion. Biol. Rev. 56:23-47.

Hadley, N. F. 1984. Cuticle: Ecological significance. In J. Bereiter-Hahn, A. G. Matoltsy, and K. S. Richards (eds.), Biology ofthe integument. 1. Inver? tebrates, pp. 685-693. Springer-Verlag, New York.

Hama, K. 1969. The fine structure of some blood vessels of the earthworm, Eisenia foetida. J. Bio? phys. Biochem. Cytol. 7:717-724.

Hauser, H., A. Darke, and M. C. Phillips. 1976. Ion binding to phospholipids: Interaction of calcium with phosphatidylserine. Eur. J. Biochem. 62:335- 344.

Hauser, H. and M. C. Phillips. 1979. Interactions of the polar groups of phospholipid bilayer mem? branes. Prog. Surf. Membr. Sci. 13:297-413.

Hevesy, G., E. Hofer, and A. Krogh. 1935. The per? meability of the skin of frogs to water as deter-

mined by D20 and HaO. Scand. Arch. Physiol. 72:199-214.

Hutchison, V. H., H. B. Haines, and G. Engbretson. 1976. Aquatic life at high altitude: Respiratory adaptation in the Lake Titicaca frog, Telmatobius culeus. Respir. Physiol. 27:115-129.

Jakubowski, M. 1958. The structure and vascularization ofthe skin of Pond-loach (Misgurnusfossilis L.). Acta Biol. Cracov. (Zool.) 1:113-127.

Jakubowski, M. 1960. The structure and vascularization of the skin of the eel (Anguilla anguilla L.) and the viviparous blenny (Zoarces viviparus L.). Acta Biol. Cracov. (Zool.) 3:1-22.

Kirsch, R. and G. Nonnotte. 1977. Cutaneous res? piration in three freshwater teleosts. Respir. Physiol. 29:339-354.

Koefoed-Johnsen, V. and H. H. Ussing. 1953. The contribution of diffusion and flow to the passage of D20 through living membranes: Effect of neu- rohypophysial hormone on isolated anuran skin. Acta Physiol. Scand. 28:60-76.

Krogh, A. 1919. The rate of diffusion of gases through animal tissues, with some remarks on the coefficient of invasion. J. Physiol. 52:391-408.

Landmann, L. 1986. Epidermis and dermis. In J. Bereiter-Hahn, A. G. Matoltsy, and K. S. Rich- ards (eds.), Biology ofthe integument. 2. Vertebrates, pp. 150-187. Springer-Verlag, New York.

Lee, D. L. and M. H. Smith, 1965. Hemoglobins of parasitic animals. Exp. Parasit. 16:392-424.

Levitt, D. G. 1981. Routes of membrane water trans? port: Comparative physiology. In H. H. Ussing, N. Bindslev, N. A. Lassen, and O. Sten-Knudsen (eds.), Water transport across epithelia, Alfred Ben- zon Symposium 15, pp. 248-257. Munksgaard, Copenhagen.

Lieberman, E. R., S. G. Gilbert, and V. Srinivasa. 1972. The use of gas permeability as a molecular probe for the study of cross-linked collagen struc? tures. Trans. N.Y. Acad. Sci. Series II, 34:694- 708.

Liem, K. F. 1981. Larvae of air-breathing fishes as countercurrent flow devices in hypoxic environ? ments. Science 211:1177-1179.

Lillywhite, H. B. 1971. Thermal modulation of cuta? neous mucus discharge as a determinant of evap? orative water loss in the frog, Rana catesbeiana. Z. Vergl. Physiol. 73:84-104.

Lillywhite, H. B. 1975. Physiological correlates of basking in amphibians. Comp. Biochem. Physiol. 52A:323-330.

Lillywhite, H. B. and P. F. A. Maderson. 1982. Skin structure and permeability. In C. Gans and F. H. Pough (eds.), Biology ofthe Reptilia, Vol. 12 Phys? iology C, Physiological ecology, pp. 397-442. Aca? demic Press, New York.

MacDougall, J. D. B. and M. McCabe. 1967. Diffu? sion coefficient of oxygen through tissues. Nature 215:1173-1174.

Machin, J. 1964a. The evaporation of water from Helix aspersa. I. The nature of the evaporating surface. J. Exp. Biol. 41:759-769.

Machin, J. 19646. The evaporation of water from Helix aspersa. II. Measurement of air flow and the




diffusion of water vapor. J. Exp. Biol. 41:771- 781.

Machin, J. 1977. Role of integument in molluscs. In B. L. Gupta, R. B. Moreton, J. J. Oschman, and B. J. Wall (eds.), Transport of ions and water in animals, pp. 735-762. Academic Press, New York.

Maderson, P. F. A. 1972. When? Why? and How?: Some speculations on the evolution of the ver? tebrate integument. Amer. Zool. 12:159-171.

Mahany, T. M. and R. H. Parsons. 1978. Circulatory effects on osmotic water exchange in Rana pip? iens. Amer. J. Physiol. 234:R172-R177.

Malvin, G. M. 1988. Microvascular regulation of cutaneous gas exchange in amphibians. Amer. Zool. 28:999-1007.

Mangum, C. P., B. R. McMahon, P. L. deFur, and M. G. Wheatly. 1985. Gas exchange, acid-base bal? ance, and the oxygen supply to the tissues during a molt ofthe blue crab Callinectes sapidus. J. Crus? tacean Biol. 5:188-206.

Marshall, W. S. 1985. Paracellular ion transport in trout opercular epithelium models osmoregula? tory effects of acid precipitation. Can. J. Zool. 63: 1816-1822.

Matoltsy, A. G. 1984. Structure and function ofthe mammalian epidermis. In J. Bereiter-Hahn, A. G. Matoltsy, and K. S. Richards (eds.), Biology of the Integument. 2. Vertebrates, pp. 255-271. Springer-Verlag, New York.

Mauro, A. 1957. Nature of solvent transfer in osmo- sis. Science 126:252-253.

McClanahan, L. L., J. N. Stinner, and V. H. Shoe- maker. 1978. Skin lipids, water loss, and energy metabolism in a South American tree frog (Phyl- lomedusa sauvagei). Physiol. Zool. 51:179-187.

Mittal, A. K. andj. S. Datta Munshi. 1971. A com? parative study of the structure of the skin of cer? tain air-breathing fresh-water teleosts. J. Zool. 163:515-532.

Neumcke, B. 1971. Diffusion polarization at lipid bilayer membranes in the presence of a homo? geneous chemical reaction in the solutions. T.I.T. J. Life Sci. 1:85-90.

Paganelli, C. V., R. A. Ackerman, and H. Rahn. 1978. The avian egg: In-vivo conductances to oxygen, carbon dioxide, and water vapor in late devel? opment. In J. Piiper (ed.), Respiratory function in birds, adult and embryonic, pp. 212-218. Springer- Verlag, Berlin.

Piiper, J. 1988. Models for cutaneous gas exchange and transport. Amer. Zool. 28:963-972.

Quinlan, M. C. and N. F. Hadley. 1982 A new system for concurrent measurement of respiration and water loss in arthropods. J. Exp. Zool. 222:255- 263.

Rabl, H. 1931. Integument der Anaminier. In L.

Bolk, E. Goppert, E. Kallius, and W. Lubosch (eds.), Handbuch der vergleichenden Anatomie der Wirbelthiere, Vol. 1, pp. 271-374. Urban and Schwarzenberg, Berlin.

Randall, D. J. 1970. The circulatory system. In W. S. Hoar and D. J. Randall (eds.), Fish physiology, Vol. 4, pp. 133-172. Academic Press, New York.

Rieger, R. M. 1984. Evolution ofthe cuticle in the lower Eumetazoa. In J. Bereiter-Hahn, A. G. Matoltsy, and K. S. Richards (eds.), Biology ofthe integument. 1. Invertebrates, pp. 389-399. Springer- Verlag, New York.

Schafer,J.A. andT.E. Andreoli. 1972. Water trans? port in biological and artificial membranes. Arch. Intern. Med. 129:279-292.

Shick, J. M., W. I. Brown, E. G. Dolliver, and S. R. Kayar. 1979. Oxygen uptake in sea anemones: Effects of expansion, contraction, and exposure to air and the limitations of diffusion. Physiol. Zool. 52:50-62.

Simkiss, K. and K. M. Wilbur. 1977. The molluscan epidermis and its secretions. Symp. Zool. Soc. London 39:35-76.

Stokes, G. D. and W. A. Dunson. 1982. Permeability and channel structure of reptilian skin. Amer. J. Physiol. 242:F681-F689.

Ultsch, G. R. and G. Gros. 1979. Mucus as a diffusion barrier to oxygen: A possible role in oxygen uptake at low pH in carp (Cyprinus carpio) gills. Comp. Biochem. Physiol. 62A:685-689.

Vincent,J. F. V. andj. E. Hillerton. 1979. The tanning of insect cuticle?a critical review and a revised mechanism. J. Insect Physiol. 25:653-658.

Vogel, W. O. P. 1985. Systemic vascular anasto- moses, primary and secondary vessels in fish, and the phylogeny of lymphatics. In K. Johansen and W. W. Burggren (eds.), Cardiovascular shunts, Alfred Benzon Symposium 21, pp. 143-151. Munksgaard, Copenhagen.

Wertz, P. W. 1986. Lipids of keratinizing tissues. In J. Bereiter-Hahn, A. G. Matoltsy, and K. S. Rich? ards (eds.), Biology ofthe integument. 2. Vertebrates, pp. 815-823. Springer-Verlag, New York.

Whitear, M. 1977. A functional comparison between the epidermis of fish and of amphibians. Symp. Zool. Soc. London 39:291-313.

Wilson, R. A. 1968. An investigation into the mucus produced by Lymnaea truncatula, the snail host of Fasciola hepatica. Comp. Biochem. Physiol. 24: 629-633.

Wright, E. M., A. P. Smulders, and J. M. Tormey. 1972. The role ofthe lateral intercellular spaces and solute polarization effects in the passive flow of water across the rabbit gallbladder. J. Mem? brane Biol. 7:198-219.



Documents

The Structure and Permeability of Integument REFERENCESlemastm/Publications/PDF/Snake... · 2016. 2. 16. · sociations with hyaluronic acid, mucopo- lysaccharides and small amounts