ANATOMY OF THE PERITONEAL MEMBRANE

George E. Digenis

The increasing use of peritoneal dialysis in the treatment of end-stage renal disease has stimulated interest in the structure of the peritoneal membrane and the mechanism(s) by which water and various solutes cross this membrane during peritoneal dialysis .

Despite extensive anatomical and physiological studies of the microvasculature and a better understanding of the movement of various solutes across the capillaries many controversies surround peritoneal transport and many questions remain unanswered (1). Furthermore we know little about the contribution to solute transport of the peritoneal lymphatics which are found in the diaphragmatic peritoneum and also in the avascular areas of mesentery. The lymphatics, especially those of the diaphragmatic peritoneum appear to be important in the absorption of solutes, and are the principal route of drainage of fluid from the peritoneal cavity (2-10) .In contrast, strong indirect evidence ( 11 ) suggests that the major route of solute removal during peritoneal dialysis is through the peritoneal capillaries .

Today it is widely accepted that the peritoneal membrane consists of three layers: a) the capillary endothelium -with its basement membrane and pericytes; b) the peritoneal interstitium and c) the mesothelium with its basement membrane (Fig. 1). This paper will discuss these three components of the peritoneal membrane and their contribution to the fluid and solute exchange between blood and fluid in the peritoneal cavity.

CAPILLARIES

Starling (1894) was the first to recognize that hydro static pressure and colloid osmotic pressure were the forces regulating fluid balance across the capillary membrane (12). Subsequently Pappenheimer (1953) proposed that the passage of water and water soluble molecules across the capillaries could be explained if the capillary wall was regarded as containing aqueous

channels or pores. According to this theory , the total cross-sectional area of these pores, which may be cylindrical, comprises less than 0.2% of the histological surface of the capillaries. It was calculated that the radius of these pores was 30-45 A and their population density 1 -2 x 109/cm2 of capillary wall. This concept 1 -2 x 10 = 49 = 4/cm = 42 = 4 = 1 assumes that the pore size is sufficient to allow even large molecules such as plasma proteins to penetrate the capillary wall. Pappenheimer also speculated that the main process of solute exchange between blood and the interstitial tissue is diffusion while ultrafiltration contributes only to the transport of large molecules (13).

Palade (14) added pinocytosis -a term originally used by Lewis (15), as an additional transport mechanism across the endothelial wall. This mechanism is said to transport chemicals from one surface of an endothelial cell to the other by intracellular vesicles with a diameter

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of about 600 A. A vesicle begins as an invagination of the cellular membrane, develops a neck and then separates; when free, the vesicle moves randomly in the cytoplasm and, when it meets the cell boundary again, it empties its contents by a reversal of the procedure (Fig. 2). Using dextrans of various molecular weights, Grotte ( 16) and Mayerson et al ( 17) concluded that the two systems, i.e. diffusion through pores and pinocytosis, coexist across the capillary wall. Renkin (18) also accepted this idea of the simultaneous existence of a small (pores) and a large (vesicles) transport system. The first system (i.e. Pappenheimer's pores) having openings about 30 A in radius, permits the rapid exchange of small solutes and limits the penetration of substances with a molecular weight higher than 20,000 daltons. This system is responsible for the maintenance of fluid balance across the capillary wall by the mechanism proposed by Starling. The transport of larger molecules takes place either passively (i.e. diffusion) through a few very large pores, or actively but more slowly through the vesicles. More recent work by Pappenheimer (36) supports this view of two transport "phases" and asserts that the capillary membrane should no longer be considered as a permeable membrane punctured by minute holes like a cellophane membrane.

The mechanism(s) of transcapillary filtration has been extensively studied (19-23) using single capilla ries of the mesentery .The introduction of the electron microscopy did much to elucidate the ultrastructure of capillaries (24-35). However despite some correlation between the physiological and morphological studies,

no one has made an unequivocal anatomical identification of Pappenheimer's pores.

The endothelial layer differs among the capillaries of various tissues or organs and even in the same tissue may display heterogenous capillaries. Majno (37) described three types of capillary endothelium: a) the "continuous" type, with a continuous endothelial layer and a continuous basement membrane, which is found in the muscles, heart, nervous system etc., b) the "fenestrated" type, with some openings, in the endothelium and with a continuous basement membrane, found in the glomerulus and endocrine glands, and c) the "discontinuous" type with large gaps in the endothelium and discontinuous or absent basement mem-brane, found in the sinusoids of the liver, spleen and bone marrow. It is apparent that there are large differences in the permeability of these three types of capillaries.

The peritoneal capillaries belong to the "continuous" type (37). They have continuous endothelium -without interendothelial gaps and a continuous basement membrane; the cells around them (pericytes) are discontinuous. A most important characteristic of this type of endothelium is the varying number of cytoplasmic vesicles (27, 30).

The permeability of capillary endothelium has been studied chiefly with electron-dense substances (tracers) of various molecular weights; these are given intravenously or intra arterially and are identified by electron microscopy in the capillary wall and the surrounding tissues. The most commonly used tracers are the following: (a) Ferritin with a molecular weight of about 450,000

.o daltons (30, 38-40), and a d1ameter of 110 A. (b) Iron dextran which has particles of varying size and

with a diameter less than 70 A. ( c ) Horseradish peroxidase with a molecular weight of approximately 40,000 daltons and a diameter of

.o approx1mately 40 A (43-47) and

(d) Microperoxidase with a molecular weight of 1500 1900 daltons and a diameter of 20 A (32-35).

Investigators, who have administered one substance to the luminal surface and a different one to the abluminal surface of the endothelium, have been able to study the site(s) of precipitation of the created chemical compounds (48). Such studies in capillaries of the "continuous" type show that the tracer passes through the layer of the endothelial cells with the exception of the brain capillaries, which seem to be impermeable at least to horseradish peroxidase (25) .However the route

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followed by the various tracers has not been completely clarified. Thus some authors support the concept of transcellular transport and consider that the intercellular spaces are sealed by tight junctions, which surround the cells completely (29, 30, 49, 50), while others assume that these junctions have "windows", which pennit the substances to pass through them. According to the latter investigators, the intercellular spaces represent the route for solute exchange between plasma and interstitial tissues (35,51,52).

Simionescu et al (53,54) found that about 25% of the intercelluar junctions of the endothelium of the postcapillary venules are open, have gaps of 30-60 A and appear to be rapidly penneated by the tracer

(microperoxidase). Nakamura and Wayland (55) who studied the micro circulation of cat mesentery with in viva fluorescence

microscopy, found a progressively increased permeability from the arterial to the venous part of capillaries.

Vesicular transport, another route of transport across the endothelium (45), seems to be responsible for the "slow" passage of large molecules, and represents the large pore system proposed by physiologists. The movement of vesicles across the endothelium has been attributed to Brownian motion (56).

Simionescu et al (32, 34) accepting the concept proposed first by Bums and Palade (30) believe that vesicles are fused with each other forming channels through the cytoplasm of capillary endothelial cells. According to this hypothesis, each channel consists of a chain of linked vesicles and extends from the luminal to the interstitial surface of the endothelial cell (Fig. 2).

The most recent contribution to the morphology of vesicles, that ofBundgaard et al (57), proposes that the

vesicles are pennanent structures, which constitute an

elaborate system of invaginations of the surface of endothelial cell (Fig. 3) .However, this theory does not exclude the existence of transendothelial channels (58) .

INTERSTITIUM

The peritoneal interstitial tissue can be viewed as the space between the endothelial cells of the blood capillaries and the mesothelial cells (Fig. 1) .It is largely composed of muccopolysaccharides, which fonn a barrier for diffusion of large molecular weight solutes (59).

Wayland and Silverberg (60) believe that the interstitium represents a network of aqueous channels through collageneous gels (Fig. 4). The presence of fluid in the peritoneal cavity during peritoneal dialysis may produce a short circuit in the nonnal interstitial flow of fluid which fonns the lymph (61). If this is true, the real "peritoneal membrane" is only the mesothelial layer of the peritoneum rather than the system of three layers endothelium, interstitium, mesothelium. Whatever the real anatomical structure of the peritoneal membrane, the qualities of the mesothelial cells are important to our understanding of the

kinetics of peritoneal dialysis (62).

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MESOTHELIAL CELLS

Some authors assert that there is a remarkable similarity between

endothelium and mesothelium and that these two types of cells share many morphological features in both light and electron

microscopy (37,46,48,59,63, 64). Their permeability can be

modified in a similar way by various physiological or pharmacological factors (65, 66). Furthermore, the study of

mesothelium in normal rodents, normal men and uremic patients showed that in these three groups the ultrastructure is closely

similar (67).

Odor (64), the first investigator who reported on the fine structure of mesothelium under electron microscopy, suggested

that the presence of microvilli (Figs. I, 5) in the peritoneal surface of the mesothelial cells increases their surface area.

The most characteristic feature of the mesothelial cells is the

large number of the cytoplasmic vesicles (64, 68, 69) (Figs. 6, 7). These vesicles are e,ither free in the cytoplasm or they form

clusters (69); most of them are round (41), while some are elongated (70) (Fig. 8).

The intercellular boundaries between mesothelial cells are

tortuous (Fig. 9) and are joined by all three types of connection, i.e. desmosomes, tight junctions (Fig. 6) and zonulae adherens (41,

68), but tight

junctions are the most common (69). The mesothelial surface appears to be continuous (Fig. 5).

The permeability of peritoneal mesothelium has been studied by various tracers, administered intraperitone ally (41,46-48,64,71). These studies have given contradictory results. Some workers support the view that the transmesothelial transport takes

place via the vesicles (41, 64), others suggest the presence of "windows" through the intracellular junctions (46, 47)

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and others interpreted their findings to support both vesicular and intermesothelial transport ( 48, 71) .

Our studies (70) of the movement of iron dextran from the blood to the peritoneal cavity during peritoneal dialysis support the concept of intracellular transport through vesicles, at least for substances with a high molecular weight (Fig. 10).

Although many believe that solutes pass through the peritoneal membrane by simple diffusion (72, 73) through intercellular spaces, there is evidence that the mesothelium plays an active role in the transport of substances accross the peritoneal membrane; this evidence can be summarized as follows: (a) When tested for the enzyme -adenosine triphos

phatase, the junctional region between adjacent cells gave a strong reaction implying that it may contribute to the active transport of materials along intercellular spaces (74).

(b) The presence of some pharmacological agents changed the flux of Rb86 through mesentery without affecting the migration of p32 ( 65, 66) .

( c ) The addition of phenazine methosulfate in in vitra preparations of endothelial and mesothelial membranes changed their permeability suggesting the presence of an oxidative metabolism and A TP formation (75).

(d) Transport of solutes across isolated mesentery, although compatible with the kinetics of passive diffusion (73), is sensitive to temperature, chemical and pharmacological agents and metabolic changes (65,75-78). In fact, changes in tempera

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ture induce statistically significant changes in the permeability coefficient of mesentery with respect to large

insoluble lipid molecules (77) .

(e) ATPase activity not sensitive to Uabaine has been found to be bound to the vesicular membranes of isolated mesentery (79),

and finally (f) The rate of labelling vesicles by ferritin was much

slower at low temperatures (80).

In conclusion during the last few years we have gained a significant insight on the ultrastructure of normal peritoneum.

Further research is required to elucidate the mechanism(s) of transport across the peritoneum and the changes that may occur

after long-term peritoneal dialysis.

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