comparative skeletal structure.pdf

7/28/2019 comparative skeletal structure.pdf

1/12

Comparative SkeletalStructureClinton Rubin, State University of New York, Stony Brook, New York, USA

Mani Alikhani,State University of New York, Stony Brook, New York, USA

Janet Rubin, Emory University, Atlanta, Georgia, USA

The sophisticated organization of the skeleton achieves a structure that can withstand the

extremes of functional load-bearing. The growth, development and repair of the

skeletal structure is realized through the tightly regulated remodelling of bone tissue,

orchestrated by cells that specifically form or resorb the matrix.

Bone

Bone is a highly specialized form of connective tissue that

provides an internal support system and facilitateslocomotion. The first vertebrates appeared on Earth inthe Ordovician period, some 500 million years ago, andsince that time bone has evolved to become a complexliving tissue in which the extracellular matrix is miner-alized, conferring marked rigidity and strength to theskeletonwhilestillmaintaining some degree of elasticity. Inaddition to understanding the palaeontological record, thestudy of bone is important because diseases of the skeleton,such as osteoporosis, are major health concerns that affecttens of millions of people. Clearly, an improved under-standing of the basic science of bone will provide atremendous opportunity to develop novel approaches to

treat and reverse these crippling diseases.Bone regulates its mass and architecture to meet two

critical and competing responsibilities: one structural, theother metabolic. In the first case, the skeleton protects vitalorgans of the cranial and abdominal cavities; it encloses theblood-forming elements of the bone marrow, and facil-itates locomotion. Second, the skeleton serves as a mineralreservoir that contains 99% of the bodys total calcium,85% of its phosphorus, and 66% of its magnesium keyelements for the function of the organism.

Bone is a composite material comprised of a mineralizedmatrix interspersed with an organic scaffold consistingprimarily of collagen. There are three different cell types

found in the bone tissue: osteoblasts, which form bone,osteocytes, which maintain bone, and osteoclasts, whichare multinucleated giant cells responsible for the resorp-tion of bone. As a structure, bone possesses severalremarkable attributes, with high tensile and compressivestrength similar in magnitude to that of cast iron, while, atthe same time, being a relatively lightweight material, andthus efficient during movement.

Bone, as an idealized structural material, can readily beappreciated by considering the forces and impact involved

during the gallop of a thoroughbred racehorse. The canobone (third metacarpal, towards the foot of the front limbwith a cross-sectional area of approximately 2.5 cm omineralized tissue, successfully holds up an animaweighing 500 kg and travelling at about 50 km per houYet, with all its strength, hardness and resilience, bone isdynamic living material, constantly being renewed anreconstructed throughout the lifetime of the individuaIndeed, bone is the only tissue in the body that has thcapacity to heal without forming a scar. Bone even has thability to adapt its structure relative to its functionademands, exemplifying the paradigm of form followfunction. In the positive direction, this plasticity

evidenced by the 40% more bone in the professional tennplayers serving arm compared with the arm that simplthrows the ballintothe air. Onthe negative side, there is thdrastic osteoporosis that results from long-term spacflight, creating a significant hurdle to extended exploratioof space.

The structural success of skeletal morphology can bexamined effectively at various levels: by its gross anatomand functional responsibility, by its ultrastructural morphology (cortical or cancellous), by its microscopiorganization (lamellar or woven) and on the basis of itdevelopment and repair, processes achieved througendochondral ossification or intramembranous ossifica

tion.

Macroscopic Structure of Bone

On the basis of their gross appearance, bones are typicallclassified as either long or flat. Long bones include thbones of the axial skeleton (e.g. tibia, femur, radius, ulnand humerus); flat bones include the skull bones pluthe sternum, scapula and pelvis. Each long bone can b

Article Contents

Introductory article

. Bone

. Macroscopic Structure of Bone

. Microscopic Organization

. Bone Cells

. Composition of Bone Matrix

. Development of the Skeleton

. Modelling and Remodelling

. Mechanical Properties of Bone

. Cartilage

. Ligament

. Tendon

. Summary

ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net


2/12

divided into three distinct anatomical areas. The regionat the end of a long bone is called the epiphysis. Themetaphysis is the area just beneath the epiphysis, wherethe bone becomes more slender. The long shaft of thebone is the diaphysis.

Characteristic of all bones are a dense outer sheet ofcortical bone (alternatively referred to as cortical bone)

and a central medullary cavity, which is filled with eitherred or yellow bone marrow. The marrow cavity isinterrupted throughout its length, particularly in theepiphyseal and metaphyseal regions, by a reticular net-work of trabecular bone (alternatively referred to ascancellous or spongy bone). These internal trabeculae actas well-banded reinforcement rods, or internal scaff-olding, to buttress the outer compact bone as it becomesmore slender at the articulating surfaces. Despite thedifferences in their appearance, under the microscopecortical and trabecular bone can be seen to have the samebasic histological structure (Figure 1).

Surrounding every compact bone is an osteogenic

connective tissue membrane, the periosteum, whichconsists of two distinct layers. The inner layer, adjacentto the actual bone surface, consists of osteoblasts andtheir precursors, as well as a rich microvasculature.The outer layer, which is more fibrous, gives rise tothe Sharpey fibres that penetrate the bone matrix, bindingthe periosteum to bone and aiding in anchoringtendons, ligaments and muscles to the bone. A singlelayer of bone cells, a syncitium referred to as theendosteum, covers the internal surface of bone. Theendosteum, considerably thinner than the periosteum,physically separates the bone surface from the bonemarrow within. The principal functions of periosteum

and endosteum are nutrition of osseous tissue andprovision of a continuous supply of new osteoblasts forrepair or growth of bone.

Microscopic Organization

At the microscopic level, bone can be generalized into twspecific morphologies: the disorganized, hypercellulawoven bone, and the highly organized, relatively hypocelular, lamellar bone. Woven bone is the first bone tissue temerge in embryonic development as well as the fir

mineralizing tissues to appear in repair process such afracture healing. Woven bone has an irregular, disorganized pattern of collagen orientation and osteocytdistribution, indicative of the rapid manner in which it ilaid down. While woven bone is characteristic of embryonic development and early stages of repair; it is also founin the healthy adult skeleton at ligament and tendoinsertions. Woven bone also is evident in specific diseasstates which affect local bone sites such as osteogenisarcoma or metastatic cancerous lesions. It serves a criticawound-healing role by rapidly filling osseous defects, callus that provides the initial continuity for fractures anosteotomy segments. Within a few weeks of bein

deposited, woven bone is most often reabsorbed anreplaced during remodelling by lamellar bone. It is thihighly orchestrated process of repair and remodelling thaallows bone fractures to repair without leaving a scar.

Lamellar, or mature, bone characteristically showcollagen fibres arranged in lamellae (37 mm thick) thaare parallel to one another or concentrically organizearound a vascular canal. The whole complex of concentrilamellae, or plates, of bone that surround a canal containing blood vessels, nerves and loose connective tissue, icalled an osteon (Figure 2). Osteons are typically 200300mm in diameter, consisting of up to 20 lamellar plateWithin these mineralized plates are lacunae, small lake

that house osteocytes, the cells residing within the bontissue. To provide added tensile strength, each lamellconsists of collagen fibres organized such that they arparallel to one another. Importantly, the orientation of thcollagen in adjacent lamellar plates is distinct from that oits neighbour, adding tremendous strength to the composite material, a strategy similar to that used in plywood tadd rigidity. The arrangement of the collagen matri(Figure 3) ultimately determines the orientation of the bonmineral crystals. Mineralization proceeds and extends ovethe collagen matrix, with the long axis of the hydroxyapatite crystal parallel to the collagen fibre, a process that alsadds strength to the tissue.

In addition to concentric lamellae, two other type olamellae are recognized. Circumferential lamellae enclosthe entire bone, forming its outer perimeter. Interstitialamellae are interspersed between adjacent concentrilamellae and fillthe spaces between them. They are actuallfragments of preexisting concentric lamellae, or remnantof the early stages of woven bone.

An osteon that has formed de novo is recognized as primary osteon. If an osteon is established within thcortex via bone resorption, a process that succeeds i

Figure 1 Bone can be categorized into two morphological components:cortical andcancellous bone. Thedensecortical bone envelopesthe entire

structure,while cancellousor trabecularboneis typicallyfound towardsthe ends of the bone. The internal spaces of bone are filled with marrow.Reprinted, with permission, from Lynch SE, Genco RJ and Marx RE (1999)

Tissue Engineering: Applications in Maxillofacial Surgery and Periodontics.Quintessence.

Comparative Skeletal Structure

2 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net


3/12

replacing preexisting bone tissue, it is referred to as asecondary osteon, or haversian system. The bulk of bonetissue within the adult human is haversian, as it has been

remodelled and replaced over the lifetime of the skeleton.In the adult skeleton, when bone tissue needs to be

repaired or replaced, it undergoes a processof remodelling.Bone remodelling is an intricate, interdependent, highlyorchestrated process thatbegins by the removal of tissue byosteoclasts and the tightly coupled replacement of tissue byosteoblasts. In the adult skeleton, approximately 5% of theskeleton is constantly involved in bone remodelling, or theturnover of bone tissue.

Bone Cells

Bone is composed, in essence, of four different cell types:osteoblasts, osteoclasts, osteocytes and the bone-liningcells. Osteoblasts, osteocytes and bone-lining cells origi-nate from local osteoprogenitor cells, whereas osteoclastsarise from the fusion of mononuclear precursors, whichoriginate in the various haemopoietic tissues.

Osteoprogenitor cells develop from embryonic me-senchymal stem cells and are able to proliferate anddifferentiate into chondroblasts(cartilage-forming cells) or

osteoblasts, dependent on local factors such as cytokinesThese osteoprogenitor cells persist throughout postnatalife and arefound on or near all of the free surfaces of bonesuch as the inner layer of the periosteum or the endosteumOsteoprogenitor cells are most active during the growtand development of bones but are reactivated in adult lifin the repair of bone fractures, and when adaptive o

remodelling processes stimulate the need for bone formation. Osteoblasts and osteocytes are thought to bincapable of division. Thus, as the population of osteoblasts depletes, they are replaced by proliferation andifferentiation of the osteoprogenitor cells.

Osteoblasts

Osteoblasts (Figure 3) are mononucleated cells, derivefrom the mesenchyme, that synthesize both collagenouand noncollagenous bone proteins, which all togethemakes up the organic matrix of bone called the osteoid

Embryonic undifferentiated mesenchymal cells can givrise to cartilage, bone, muscle cells or adipocytedependent on the influence of hormones and growtfactors. Osteoblasts are connected to one another viextended cell processes that are interconnected via gajunctions, establishing a single continuous blanket of cellon the bone surface, all of which are in direct communication with neighbouring cells.

The active osteoblast is a plump cuboidal cell with aeccentrically placed or polarized nucleus, which resideaway from that part of the cell nearest to the mineralizesurface. The cytoplastic elements of the osteoblast includabundant endoplasmic reticulum, a well developed Golg

body, and numerous free ribosomes that are responsiblfor the basophilia seen in sections stained with haematoxylin and eosin.

In bone remodelling, osteoblasts are transiently active asites of reformation of bone. These features help tdistinguish osteoblasts from the mesenchymal precursor(preosteoblasts), which are also found on bone surface(Figure 4). These bone-lining cells are flattened spindleshaped cells with oval nuclei and few organelles. Bonelining cells cover bone surfaces, particularly in the aduskeleton, that are quiescent in terms of bone formation anresorption. In certain circumstances, such as when exposeto growth factors or other anabolic stimuli, theyproliferat

and differentiate into osteoblasts.The osteoblastic lineage comprises several cells a

different stages of differentiation, including osteoprogentor cells, preosteoblasts and osteoblasts according to thprogressive expression of markers of the osteoblasphenotype. Preosteoblasts express markers such as alkaline phosphatase, osteopontin and collagen type I, whereamature postmitotic osteoblasts express osteocalcin.

A temporal sequence of expression of genes encodinosteoblast phenotype markers has defined three distinc

Figure 2 Diagram depictinga section of thecorticalshaft of a long bone,showing the arrangement of the lamellae in the osteons, the interstitial

lamellae, and the outer and inner circumferential lamellae. The outersurfaceis protectedby theperiosteum, whilethe innersurfaceis covered bythe endosteum. Within the cortical shell, the branching out of thebuttressing trabeculae can be seen. Reprinted, with permission, from

Bloom W and Fawcett DW (1986) A Textbook of Histology, 11th edn.Saunders.




4/12

periods: a growth period (proliferation), a period of matrix

development and a mineralization period. A period ofactive proliferation is reflected by mitotic activity withexpression of cell cycle and cell growth-regulated genes(e.g. histone, c-myc, c-fos and AP-1 (alkaline phosphatase)activity). During this proliferation period, and fundamen-tal to development of the bone cell phenotype, severalgenes associated with formation of the extracellular matrix(type I collagen, fibronectin and transforming growthfactor (TGF) b) are actively expressed, and these proteinsare ultimately key to maintaining the intricate balancebetween bone formation and resorption.

In bone, type I collagen, a heterotrimer of one a2 (I) andtwo a1 (I) chains,makes up about 90% of the total organic

matrix. It is the collagen, with its organization, thatprovides a great deal of the tensile strength of bone.Therefore, even subtle variations in the composition orcontent of the collagen can have devastating effects. Forexample, mutations in the gene coding for the a1 (I) chain,as well as mutations in the gene coding for the a2 (I) chain,can cause osteogenesis imperfecta, an affliction that makesbone very susceptible to fracture.

Immediately after the downregulation of osteoblastproliferation reflected by the decline in deoxyribonucleic

acid (DNA) synthesis and histone gene expression, th

expression of alkaline phosphatase, a protein associatewith the bone cell phenotype, increases greater than 10fold. During this period the extracellular matrix undergoea series of modifications in composition and organizatiothat facilitate the mineralization of the matrix. The onset omineralization is also signalled by the induction of severaother genes, such as osteopontin and osteocalcin.

It is likely that differentiation along the osteoblalineage involves osteoblast-specific transcription factor(OSFs) that have yet to be identified. OSF2 is a cis-actinelement in the promoter of themouse osteocalcin gene thabinds a factor present only in nuclear extracts oosteoblasts and confers osteoblast-specific activity. Severa

studies have indicated that Osf2/Cbfa1 is encoded by thgene for Cbfa1. Mutation in OSF2/CBFA1 canresultinthabnormal skeletogenesis seen in cleidocranial dysplasiand transgenic mice who have Cbfal knocked out arunable to ossify their cartilaginous skeletons.

In addition to secreting several matrix componentincluding type I collagen, proteoglycans, osteocalcinosteonectin and osteopontin, osteoblasts also producgrowth factors that have important autocrine and paracrine effects on bone growth and remodelling (Figure 4

Figure3 Bone may be categorized into threemicrostructuralcomponents: (1) bone cells, whichinclude osteoblasts,osteocytesand osteoclasts(stainewith a modified Goldner trichrome stain); (2) an organic matrix consisting of collagenous and noncollagenous factors, such as the bone morphogenetproteins (the mineralizedmatrixhas been removed andcells have been coloured green to distinguish them from theorganic framework);(3) an inorgan

component consistingprimarily of calcium andphosphate; this componenthas beenstylizedas an arrayof hexagonalcrystals.Reprinted, withpermissiofrom Lynch SE, Genco RJ and Marx RE (1999) Tissue Engineering: Applications in Maxillofacial Surgery and Periodontics. Quintessence.




5/12

They also have surface receptors for a variety of hormones,vitamins and cytokines, which influence their activity. Asthe osteoblasts produce osteoid, they often becomeengulfed in their own matrix and differentiate intoosteocytes.

Osteocytes

The osteocyte (Figure 3) is a mature osteoblast entrappedwithin the bone matrix, and is now believed to be largelyresponsible for maintaining the viability of the tissue. Notonly can osteocytes sense alterations in physical orchemical conditions (and thus orchestrate the recruitmentof osteoblasts and osteoclasts), it has been recentlydemonstrated that these cells can both synthesize andresorb matrix, thus modifying their own immediatephysical and chemical environment.

Osteocytes reside in lacunae within the matrix and are11by far the most abundant cells of mature bone. As

osteoblasts become entombed in their own matrix anddifferentiate into osteocytes, the cells diminish in size andlose many parts of their cytoplasmic organelles. Withineach osteon, osteocytes are connected to each other vialong, slender, branched cytoplasmic processes and gapjunctions. These cytoplasmic extensions pass through anetwork of catacombs, called canaliculi, which connectadjacent lacunae. These interconnecting canaliculi areideal pathways for chemical, electrical and stress generatedfluid communication through the dense bone matrix.

Osteocytes express cellcell channels called connexinthrough which small molecules such as second messengercan pass, suggesting that networks of osteocytes are iintricate communication with one another.

Considering the skeletons ability to adapt to changes iits functional environment, it is important to consider th

means by which the bone tissue can perceive and responto mechanical signals. Given their preponderance in bonand their intricate three-dimensional network, osteocyteare most probably the key mechanosensor element in boneLoading of bone results in strain, or deformation, in thmatrix. This deformation may evoke cellular responseeither directly or indirectly via fluid shear stress produceby increased fluid flow in the lacunocanalicular system oelectrical strain potential. Osteocytes may orchestrate thoverall remodelling response by secreting key factors, sucas prostaglandins, nitric oxide and insulin-like growtfactors (IGFs). These, and certainly other yet unknowfactors, activate the bone remodelling system of osteo

blasts and osteoclasts, and thus demonstrate the balancbetween bone forming and resorbing cells.

Recent evidence suggests that osteocytes can metabolcally manipulate their environment more or less independent of surface resorption and accretion. This ability iimportant to the cellular regulation of calcium exchangMost of the bone crystals, buried away from the endosteaand periosteal bone surfaces, appear to be unavailable teffect the necessary mineral exchange with extracellulafluid, making it difficult to explain the immediate exchang

Macrophage

Monocyticprecursor CFU-M

Hematopoeticstem cell(CFU-GM)

Osteoclast

Resorption pit

Osteoclast precursors

Osteoblasts

Osteoblastprecursors

Bone and bone matrix

+ Resorption

BMPs

Growth factorsInterleukins

+ Formation

RANKL

MCSFInterleukins

Figure 4 Osteoclasts (red) and osteoblasts (dark green) interact through cytokines released into the bone micromilieu. Macrophages secrete

macrophage colony-stimulating factor (MCSF), various interleukins and tumour necrosis factor, all of which promote osteoclast differentiation fromhaematopoietic stem cells, from the colony forming unit for granulocyte-macrophages (CFU-GM) and the CFU-M (CFU for macrophage) to terminal

osteoclast phenotype. Osteoblasts interact by expressing factors which affect osteoclasts, mainly RANKL and MCSF, as well as factors affecting bonemineralization and progression of theirown phenotype, such as insulin-likegrowthfactors and basicfibroblast growth factors. Importantly, disusewill als

upregulate osteoclast activity, while increases in mechanical factors will elevate bone formation.




6/12

of bone mineral with the extracellular fluid. There is,however, a vast surface area on the Haversian canal andlacunar walls, and an even larger area on the canalicularwalls,where bone mineral exchange with extracellular fluidcan takeplace. Nevertheless,when major reorganization ofbone tissue is called for, the bone resorption is performedby the osteoclast.

Osteoclasts

Osteoclasts (see Figure 3) are the major resorptive cells ofbone and are characterized by their large size and multiplenuclei. They are specialized monocyte/macrophage familymembers that differentiate from haematopoietic precur-sors. Whereas monocytes are mononuclear cells, osteo-clasts form from the fusion of monocytes. Terminaldifferentiation in this lineage is characterized by theacquisition of mature phenotypic markers, such as thecalcitonin receptor, tartrate-resistant acid phosphatase

(TRAP), morphological conversion into large multinu-cleated cells, and the capacity to form resorption lacunaeon bone.

An osteoclast is a strongly polarized cell with a paucityof rough endoplasmic reticulum, a moderate number ofribosomes, numerous smooth vesicles, and well developedmitochondria. These cells occupy shallow concavities,called Howship lacunae, which are created by the erosiveaction of the osteoclast on the underlying bone. Osteoclaststypically attach only to fully mineralized bone matrix, andare not able to resorb bone that is not fully mineralized.The osteoclast has three different domains: ruffled border,sealing zone and smooth basolateral border. The ruffled

border is the central, highly infolded area of the plasmamembrane where secretion for bone resorption takes place.Osteoclasts appearing some distance from the surface ofbone do not have ruffled bordersand are called inactive orresting osteoclasts. The sealing zone is a microfilament-rich, organelle-free area of the plasma membrane thatsurrounds the ruffled border and serves as the point ofanchoring of the osteoclast to the underlying bone matrixachieved by integrin attachment. This sealing zone createsa closed subosteoclastic compartment, or pocket, betweenthe cell and bone that permits a confined space subject toresorption. Osteoclasts not only attach to the bone: theycan move along the surface, thus increasing their range of

action. During the movement of the osteoclast, the sealmust remain intact in order for resorption to continue.

Once activated, osteoclasts resorb bone by isolating anarea of bone under the region of cell attachment. Theosteoclasts then actively reduce the pH of the localenvironment by production of hydrogen ions through thecarbonic anhydrase system. The lowered pH increases thesolubility of the apatite crystals, in essence dissolving themineral. After the mineral is removed, the organiccomponents of the matrix are hydrolysed through acidic

proteolytic digestion. The reconstruction of bone requireseven to ten generations of osteoblasts to follow and fill thresorption space. Absence of osteoclasts, or a populatioof dysfunctional osteoclasts, leads to osteopetrosis, omarble bone disease, which can be fatal in childhoodDevoid of osteoclasts, the development of bone througendochondral ossification fails to create a marrow cavity

thereby eliminating the space necessary for the productioof all haematopoietic elements.

Many factors are involved in regulating osteoclarecruitment from marrow precursors. Two factors predominate: MCSF and RANKL. MCSF is necessary tsupport proliferation, early differentiation and survival othe monocytic progenitors of osteoclasts. Loss of MCSaction engenders deficiencies in both osteoclast anmacrophage recruitment, as well as differentiation. MCSalone is unable to generate osteoclasts in in vitro culturesystems; further progression into the osteoclast lineagrequires the presence of RANLK which binds to RANKreceptors on the monocytic precursors, inducing lat

differentiation and fusion. Expression of RANKL on thsurfaceof bone stromaland osteoblast cells is controlledbhormones that regulate calcium balance, such as PTH anvitamin D. Increased PTH causes increased expression oRANKL, leading to increased osteoclastogenesiRANKL can also be expressed on lymphocytes and mabe responsible for hypercalcaemia associated with haematopathologic diseases. Bone cells also secrete a proteicalled osteoprotegerin (OPG), which serves as a decoreceptor for RANKL, preventing its action on monocytiprogenitors. Overexpression of OPG is thus associatewith decreased bone turnover. Many other factors arcapable of regulation of osteoclast formation, including c

fos, a ubiquitous nuclear transcription factor whosabsence prevents late osteoclast differentiation, and TNFawhich can support osteoclastogenesis even in the absencof RANKL.

Systemic factors such as vitamin D, parathyroihormone and tumour necrosis factor can enhance thdevelopment of osteoclasts from haematopoietic progentor cells in the presence of stromal elements from bonelargely by up-regulating bone expression of RANKL, bualso affecting cytokines such as the interleukins. Togethethe osteoblasts, osteoclasts and osteocytes work togetheto lay down, maintain and remodel the bone matrix.

Composition of Bone Matrix

The composite structure of bone allows it to withstancompressive and tensile stresses, as well as bending antorsional moments. The inorganic phase of bone, withydroxyapatite crystals arrayed in a protein matrixprovides the ability to resist compression. Individuacalcium phosphate crystals of multiple sizes are imbedde




7/12

in and around the fibrils of the collagen type I lattice.Hydroxyapatite crystal, while effectively resisting com-pressive loads, has a poor ability to withstand tensile loads.As in concrete, a material that excels at resisting compres-sion but is poor in resisting tension, tensile elements (e.g.steel reinforcing rods) are added to create a compositematerial that can cope with complex loading environ-

ments. As discussed, in the case of bone, this tensilestrength arises from collagen fibrils organized intolamellae.

The collagen orientation between adjacent lamellae canrotate by as much as 908, permitting the tissue to resistforces and moments acting from several different direc-tions, much like the added strength in plywood realized bythe distinct orientation of the fibres in each specific ply.While the ultrastructural organization is, to a certainextent, defined by the genome, the functional environmentalso contributes to the distribution of lamellae, as well asthe osteons that house them. This directed deposition ofcollagen adds to the anisotropy of the bone. Given that

more than 80% of functional strains are due to bending(and thus a high percentage of strain is tensile), thestructural quality of the bone may ultimately be deter-mined by the quality of the collagen and the organizationof the microarchitecture. Recent studies have shown thatcollagen itself deteriorates with age, and undoubtedlycontributes to the declining material properties of theskeleton.

Before its calcification, newly synthesized bone matrix isessentially completely organic and is called osteoid.Collagen type I is the predominant organic component inbone, accounting for approximately 94% of the unminer-alized matrix. Collagen fibres are embedded in a ground

substance which is rich in proteoglycans, chondroitinsulfate, keratan sulfate and hyaluronic acid. Other non-collagenous proteins most often found in bone are found inosteoid, and account for approximately 4% of its weight.These include glycoproteins and phosphoproteins such asosteonectin; sialoproteins, which are predominantly os-teopontin; bone Gla protein, also called BGP (bonegamma-carboxyglutamic acid-containing protein) or os-teocalcin; and bone morphogenetic protein. Extracts ofbone also include enzymes, hormones, growth factors andother metabolites essential for bone metabolism. It is eventhought that many of these molecules serve as couplingfactors, which, when released from the bone matrix during

osteoclastic resorption, serve to stimulate recruitment anddifferentiation of osteoblasts.

The inorganic component of bone (see Figure 3) iscomposed principally of a calcium phosphate mineralanalogous to crystalline calcium hydroxyapatite, Ca10(-PO4)6(OH)2. This crystallite structure is available for theexchange of ions. Thus magnesium and sodium cansubstitute in the calcium position, fluoride and chloridein the hydroxyl position, and carbonate in both thehydroxyl and phosphate positions. These small amounts

of impurities in hydroxyapatite may alter certain physicaproperties of the crystal, such as solubility. For examplefluoride substitution decreases the solubility of the crystalites, whereas carbonate increases it. Many investigatorbelieve that the impurities of the crystalline matrix arcritical to enabling the osteoclast to resorb the tissue.

Development of the Skeleton

Skeletal development requires the exquisite coordinatioof many distinctive cell types and the orchestration of thecellular growth, differentiation, apoptosis, production oextracellular matrix and remodelling. Formation of thskeleton (ossification) occurs by either a direct (intramembranous) or an indirect (endochondral) process. Intramembranous ossification occurs during embryondevelopment by the direct transformation of mesenchymacells into osteoblasts. The first event of this process is th

migration of undifferentiated mesenchymal cells into aarea destined to become bone. Mesenchymal cells condense, the surrounding tissue becomes vascularized, ancells differentiate directly into osteoblasts. Intramembranous ossification begins in the centre of the mesenchymacell concentration. The osteoblasts then synthesize thosteoid in which hydroxyapatite crystals are formed bcalcium and phosphate ion enrichment. Collagen molecules secreted by the osteoblasts during embryonic bondeposition polymerize extracellularly to form numerourandomly oriented fibrils throughout the trabeculae. Thearly intramembranous bone, in which the collagen fibrerun in all directions, is described as woven bon

Intramembranous ossification occurs primarily in thneurocranium (which forms a protective case around thbrain) and viscerocranium (which forms the skeleton of thface) and part of the clavicle.

In endochondral ossification, the condensed embryonimesenchyme transforms first into a cartilage anlage (population of cells that constitutes the beginning of tissue), which reflects both the position and form of theventual bone at that site. During mouse embryondevelopment the undifferentiated cells initially expresmesenchymal matrix (e.g. collagen types I and III) bugradually, cells in the central region dramatically changtheir phenotype and become large and round. As thes

prechondrocytes differentiate further, their cytoplasmvolume, endoplasmic reticulum and Golgi complex increase and they switch from the production of mesenchymal matrix to the production of cartilaginous matrix (e.gcollagen types II, IX, XI and matrix Gla protein) anseveral other matrix proteins. These chondrocytes theundergo a programme of differentiation, which includehypertrophy, expression of type X collagen and decreaseexpression of type II collagen. The cartilage anlage growrapidly through appositional and interstitial growth, an




8/12

resembles the future bone in shape, but, as it grows in size,the cartilage anlage becomes too big to support thechondrocytes purely through diffusion, and the hyper-trophic cells begin to die off and ossify.

Through endochondral ossification, cells from theperichondrium differentiate into osteoblasts and begin todeposit a thin layer of bone, appearing as a collar, around

the cartilage (Figure 5). Concurrently, mononuclear cells(monocytes) differentiate into chondroclasts, which beginto break down the ossifying cartilage. From the perichon-

drium, the invasion of vascular channels begins, a keprocess called angiogenesis. As the hypertrophic chondrocytes continue to die through apoptosis, the osteoblasbrought in by the blood vessels start the process odepositing bone tissue just as if bone were forming on thsurface of adult bone. The development of bone througendochondral ossification, therefore, allows the rapi

expansion of a bone template, through both interstitiaand appositional growth. This swelling could not occur ithe tissue was originally bone, as the mineralized matrirestricts growth only to surfaces. The replacement of thdying cartilage with mineralizing bone, however, allowskeleton to fulfil its responsibilities as a calcium reservoand a weight-bearing structure.

Mineralization of bone tissue

The mineralization of bone begins approximately 101

days after the organic osteoid matrix has been laid downAt this point,mineral increases almost immediately to 70%of the ultimate content, whereas deposition of the fina30% takes several months. While the actual process omineralization is not completely understood, it is believethat two mechanisms are involved. The first involves structure called the matrix vesicle, and the second heterogeneous nucleation.

Matrix vesicles are small, round, extracellular lipidbilaminar bound organelles, which bud from hypertrophichondrocytes or osteoblasts undergoing the process oapoptosis as well as from cell processes originating fromthe plasma membrane. There is a definite polarity to th

vesicles, with mineralization occurring in a predictable anorganized way adjacent to the requisite phosphatases othe inner leaflet of the membrane. The matrix vesiclecontain high levels of alkaline phosphatase, adenosintriphosphatase (ATPase), inorganic pyrophosphatase, 5nucleotidase and ATP-pyrophosphohydrolase, in additioto phospholipids (especially phosphatidylserine), whichave a strong affinity for calcium ions. It is believed thathese ions accumulate in the matrix vesicle because of theaffinity for the phospholipids and a membrane-bouncalcium pump. At a point of supersaturation, nucleation othe mineral begins.

Alkaline phosphatase, a biosynthetic product of osteo

blasts, is present in very high concentrations durindevelopment and osteoid production. The regulatory rolof this disulfide-linked dimer is not known, but its presencmay increase the local concentration of P (phosphate) anthereby facilitate hydroxyapatite deposition. Increasinthe concentration of P in the micromilieu exceeds the locasolubility product and catalyses deposition along the inneleaflet of the vesicle. Following this accretion, thdestruction of the vesicles membrane has been attributeto an increasing concentration of lysophospholipids withi

Figure 5 Development of a long bone as shown in longitudinal sections(AJ), and in cross-sections A, B, C and D. Pale blue is cartilage; purple,

calcified cartilage;black,bone; red,arteries.A, The original cartilagemodel

of thebone; B, a periosteal collarof bone appears beforeany calcification ofcartilage occurs; C, cartilage begins to calcify; D, vascular mesenchyme

enters the calcified cartilage and divides it into two zones of ossification (Eand F); G, blood vessels and mesenchyme penetrate the epiphysealcartilage and the epiphyseal ossification centre develops within it; H, a

similar ossification centredevelopsin thelowerepiphysealcartilage; as theboneceases to growin length, thelower epiphyseal platedisappears first(I)

and then the upper epiphyseal plate (J). The marrow cavity then becomescontinuousthroughoutthe lengthof thebone, andthebloodvessels of thediaphysis, metaphyses and epiphyses intercommunicate. Reprinted, with

permission, from Bloom W andFawcettDW (1986) A Textbook of Histology,11th edn. Saunders.




9/12

the matrix vesicles, which suggests that vesicles areprogrammed to self-destruct.

Following dissolution of the matrix vesicle membrane,the hydroxyapatite crystals are exposed to the extravesi-cular environment, where additional mineral accretes tothe newly formed crystal. The crystal is then believed tomove chemotactically toward and to bind preferentially at

the gap between collagen fibrils, precipitated by the nestingosteonectin and fibronectin. Mineralization proceeds andextends over the collagen matrix, with the long axis of thehydroxyapatite crystal parallel to the collagen fibre.

In the second mechanism, apatite crystallites aredeposited in the gap zones at the ends of collagenmolecules, where the mineral first appears. Initially, thesegaps are filled with proteoglycans, which bind to calcium.The proteoglycans are removed enzymatically, leavingbehind the calcium. At next step, phosphoproteins bind tothe collagen. Dephosphorylation of the phosphoproteins(as a result of alkaline phosphatase activity) provides theadditional phosphate ions for nucleation and crystal

growth.

Modelling and Remodelling

Both trabecular and cortical bone grow, change andturnover through two fundamentally distinct mechanisms:modelling and remodelling. Bone modelling typicallyrefers solely to the deposition of new bone without boneresorption. Bone remodelling, in contrast, is a specific,coupled sequence of resorption and formation events thatreplaces previously existing bone. As this constant turn-

over and replenishment of damaged tissue is critical toretaining bone strength, it is clear that disruption of theseprocesses through age and disease will ultimately compro-mise the structural viability of the skeleton.

The mechanism for internal remodelling of denscompact bone is via axially oriented cuttingfillincones. The cuttingfilling cone has a front of osteoclastthat bore through the bone, followed by a tail oosteoblasts that coordinately lay down osteoid and begithe formation of the new secondary osteon (Figure 6A group of osteoblasts, osteocytes and osteoclasts tha

are linked and participate in remodelling (activationresorption and formation) of a discrete area of bonis called a bone modelling unit (BMU). Traumatic osurgical wounds usually results in an intense, bulocalized, modelling and remodelling response. Thprocess, referred to as a regional acceleratory phenomeno(RAP), is an important aspect of the postoperativhealing process. In contrast, the slow but persistenremodelling of adult bone involves approximately 5%of skeleton at any given time, progressing througactivation of a resting site, resorption of the bone, reversaof the process finishing with formation to fill in thpocket of excavated bone (ARRF).

Both modelling and remodelling are controlled ban interaction of metabolic and mechanical signalBone modelling is patterned by temporal expressioof structural genes followed by the integrated biomechanical control of functional applied loads. Howevehormones and other metabolic agents have a stronsecondary influence, particularly during growth anadvanced ageing. Paracrine and autocrine mechanismsuch as local growth factors and prostaglandins, arcapable of temporarily overriding the mechanicacontrol mechanism during wound healing. Remodellinresponds to metabolic mediators such as parathyroihormone and oestrogen, primarily by varying the rat

of bone turnover. Certainly, a combination of mechanicasignals, such as duration, magnitude and distribution oloads, also influence remodelling and the form anfunction of bone.

Osteoblastsform new bone

Resorptioncavity

Advancedfilling cone

Completedsecondary osteon

2 1

Figure 6 The cuttingfilling cone has a head of osteoclasts that cut through the bone, and a tail of osteoblasts that form a new secondary osteon. Th

velocity through boneis determined by measuring between two tetracycline labels (1 and 2) administered 1 weekapart. Reprinted, withpermission, froGraber T and Vanarsdall RL (2000) Orthodontics: Current Principles and Techniques, 3rd edn. Mosby.




10/12

Mechanical Properties of Bone

Bone, as an organ, needs to be both stiff (to resistdeformation) and tough (to prevent crack propagation).However, there is a compromise between these twoobjectives, as they are attained through a balance betweenthe resiliency to crack propagation provided by collagen

and the resiliency to deformation provided by mineral. Fornormal tensile or compressive loading, the stiffness of thematerial, or elastic modulus, for human haversian bone isapproximately 17.0 GPa (gigapascals, or 109 pascals) in thelongitudinal direction, 11.5 GPa in the transverse directionand 3.3 GPa in shear. The degree of mineralization (youngbone) or porosity (old bone) compromises the stiffness ofthe bone and thereby lowers the elastic modulus. However,the effective modulus of the bone can compensate fordecreased stiffness through changes in morphology, suchas the periosteal expansion seen with ageing.

Bone tissue has the capacity to adapt to its functionalenvironment. Theterm adaptive bone remodelling refers to

changes in bone mass and geometry in response to analteration in the bones mechanical environment. The mostfamiliar examples are increased bone mass with certainkinds of exercise (such as jumping or tournament-leveltennis playing) and bone loss following disuse (such as bedrest or space flight-induced disuse).

Although vertebrate design and function are diverse, atthe level of small volumes of tissue all loads and bendingmomentsresolve into strain.Strain, a dimensionless unit ofchange in length divided by its original length, is used inbone physiology as 102 6 strain, or microstrain. The yieldstrain of bone, or the degree of deformation at which thebone does not recover elastically, is approximately 7000

microstrain; that is, a 0.7% change in length causesirreversible damage to the tissue. Ultimate strain in bone,or the degree of deformation at which the material actuallyfractures, is 15 000 microstrain.

Strain generated in long bones by vigorous activityrarely exceed peaks of 3000 microstrain, indicating a safetyfactor to failure of approximately 23-fold. The similarityin peak strain magnitude of 20003500 microstrain hasbeen called dynamic strain similarity, and suggests thatskeletalmorphology and locomotion character combine toelicit a very specific and perhaps beneficial levelof strain. Itis also important to emphasize that the great majority ofstrain elicited in bone is due to bending (4 85%).

Therefore, assumption that skeletal elements are subjectsolely to compression must be approached with caution.

Although the nature of this structurefunction relation-ship is only poorly understood, it has been proposed thatbone remodelling is continually influenced by the level anddistribution of the functional strains within the bone.Alteration in bone mass, turnover and internal replace-ment are sensitive to changes in the magnitude, distribu-tion and rate of strain generatedwithin the bone tissue. Forexample, static load applied continuously on bone

produces an effect on remodelling activity that is ndifferent from that produced by disuse. For a loadinregimen to be anabolic, it must be dynamic in nature; statloads do not influence bone morphology. In parallel witstudies trying to identify pharmacological agents to inhibthe bone loss that follows the menopauseor ageingprocesseveral investigators are attempting to define exercis

studies or mechanical interventions that will serve as stronanabolic agents to resist osteoporosis.

Cartilage

Cartilage is a special type of connective tissue which has stiff and firm, but not hard, extracellular matrix. Icontrast to bone, cartilage is neither vascular nor calcifiedCartilage provides three basic functions. It gives flexiblsupport in appropriate anatomical places (the nasal tip, ealobe, thoracic cage, tracheal rings); it is a pressure-toleranextremely low friction tissue located in specific skeleta

areas where direct pressure and articulation occur (e.gsurfaces of bones); and it functions as a site of rapid growtin conjunction with many bones undergoing elongation.

Cartilage consists of cells called chondrocytes, and aextensive extracellular matrix composed of fibres anground substance. Chondrocytessynthesize and secrete thextracellular matrix, and the cells themselves are located imatrix cavities, called lacunae, just as in bone. It is thextracellular matrix with its embedded fibres that not onlgives cartilageits resiliency, but also allowsit to bearweighand to achieve considerable tensile strength. Collagenhyaluronic acid, proteoglycans and small amounts oseveral glycoproteins are the principal macromolecule

present in all types of cartilage matrix. Cartilage has nvasculature, lymphatic network or innervation of its ownChondrocytes are nourished by diffusion of oxygen annutrition through the matrix from blood vessels located isurrounding connective tissues. Because diffusion is thonly source for oxygen and nutrition, the maximum widtof the cartilage is limited. However, vessels may pasthrough cartilage without supplying it directly.

Cartilage, like bone, derives from the mesenchyme. Thfirst modification observed is the rounding up of thmesenchymal cells, which retract their extensions, multiplrapidly, and form mesenchymal condensations. The celformed by this direct differentiation of mesenchymal cell

are called chondroblasts. Synthesis and deposition of thmatrix then begin to separate the chondroblasts from onanother. The differentiation of cartilage takes place fromthe centre outward; therefore, the more central cells havcharacteristics of chondrocytes, while the peripheral celare typical chondroblasts. The growth of cartilage attributable to two processes: interstitial growth, resultinfrom the mitotic division of preexisting chondrocytes, anappositional growth, resulting from the differentiation operichondrial cells.




11/12

Like bone, cartilage is sensitive to mechanical stimuli.The synthesis of the extracellular matrix and maintenanceof the tissue benefits from strain, promoting diffusion andeliciting electrical potential. However, too much strain canbe damaging to the cartilage, causing degeneration andleading to osteoarthritis. Based on the amount of fibre andground substance, and the type of fibres, three varieties of

cartilage are distinguished: hyaline cartilage, elasticcartilage and fibrocartilage.

Hyaline cartilage

Hyaline cartilage is the most common form of cartilage. Asdiscussed, it forms the majority of the temporary skeleton,as the cartilage anlage, until it is gradually replaced bybone. Further, the epiphyseal plate, located between themetaphysis and the epiphysis of growing long bones, iscomposed of hyaline cartilage, which is responsible for thelongitudinal growth of bone. In adults, hyaline cartilage isfound in the nasal septum, larynx, trachea and bronchi of

the respiratory system, on the articulating surfaces ofbones (articular cartilage) and on the ventral ends of theribs where they attach to the sternum. A definingcharacteristic of adult skeleton is that the growth platehas been mineralized, preventing further growth of longbones.

The extracellular matrix of hyaline cartilage containsprimarily type II collagen, but several other types are alsopresent. Type XI collagen forms a core for the type IIcollagen fibril, and contributes to the control of fibrilgrowth. Type IX collagen, which is really a form of aproteoglycans molecule, is found periodically along thetype II collagen fibril and is covalently cross-linked to it.

Collagen types VI, X, XII and XIV have also beendiscovered in cartilage. Cartilage proteoglycans containchondroitin 4-sulfate, chondroitin 6-sulfate and keratansulfate, covalently linked to core proteins. These coreproteins are in turn the attachment points for glycosami-noglycans (GAGs). Most of these proteoglycans arenoncovalently associated with long molecules of hyaluro-nic acid, forming proteoglycan aggregates (aggrecan) thatinteract with collagen. The high content of water bound tothe negative charges of GAGs serve, in essence, as a shockabsorber that is of great functional importance, especiallyin articular cartilages. In fact, it is believed that age-relateddegeneration of GAGs contributes to the progressive

destruction of cartilage in the sense that it loses itsresiliency to impact-loading. In contrast to bone, oncedamaged, cartilage is not able to repair itself unfortu-nately this is why so many athletes require surgery in mid-life. The amount of strain that causes cartilage to fail is notknown, but it is believed that this tissue is rarely strainedmore than 25%. Another important component ofcartilage matrix is the glycoprotein chondronectin, amacromolecule that promotes the adherence of chondro-cytes to matrix collagen.

Except in the articular cartilage of joints, all hyalincartilage is covered by a layer of dense connective tissuethe perichondrium, which is essential for the growth anmaintenance of cartilage. It is rich in collagen type I fibreand contains numerous chondroblasts that easily differentiate into chondrocytes.

Elastic cartilage and fibrocartilage

Elastic cartilage is typically found in regions requiring flexible form of support: the auricle of the ear, the walls othe external auditory canals and eustachian tubes, thepiglottis, and the cuneiform cartilage in the larynxBasically, elastic cartilage is identical to hyaline cartilagexcept that, in addition to collagen fibrils, it contains aabundant network of fine elastin fibres.

Fibrocartilage has characteristics intermediate betweethose of dense connective tissue and hyaline cartilage. It ifound in intervertebral discs, in attachments of certailigaments to the cartilaginous surface of bones, and in th

pubic symphysis. Chondrocytes occur singly or in rowbetween large bundles of collagenous fibres. Fibrocartilagmatrix contains a great number of coarse type I collagefibres, contributing to the elasticity, toughness, resiliencand strength of the structure.

Ligament

Ligaments are tough bands of fibrous connective tissuthat tether bones together and support organs. Ligamentoriginate and insert on bone. From a skeletal point of view

their principal function is to maintain correct bone anjoint geometry. The teres ligament helps to contain thfemoral head in the pelvis, while ligaments such as thcruciates and collaterals bind the bones around the knetogether. Ligaments, together with their associated joincapsules (composed of similar material), are often referreto as passive joint stabilizers, and together with tharticular contours determine a joints range of motion. Asecond function of ligamentous tissue is proprioceptionProprioceptive receptors in ligament permit the sensing ojoint position, the monitoring of ligament tension anintegrity, and the initiation of protective reflexes.

Ligament cells are called fibroblasts, which also deriv

from mesenchymal stem cells. They are generally orientelongitudinally along the length of the ligament bodyFibroblasts are responsible for synthesizing and degradinthe ligament matrix in response to various stimuli. Thmatrix comprises virtually the entire body of the ligamenIt consists of water, collagen, proteoglycans, fibronectinelastin, actin and a few other glycoproteins. Water makeup approximately two-thirds of the wet weight of ligament. Collagen (primarily type I with some type IIIcomprises approximately 7080% of the dry weight o


1ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net


12/12

ligament, andelastin a further1015%. In contrast to bonebeing subject to strains of 0.3% (3000 microstrain),ligaments can stretch up to 5% (50000 microstrain) beforethey are damaged.

Tendon

Tendons are anatomically and functionally discrete, verycomplex, dense collagenous structures that connectmuscles to bones. Collagen molecules combine to formordered units of microfibrils, subfibrils and fibrils. Theseunits are arranged in closely packed, highly ordered,parallel bundles that are oriented in a distinct longitudinalpattern, with proteoglycans and glycoproteins in associa-tion with water incorporated in a matrix, binding the fibrilstogether.

Tendons contain a relatively poor blood supply and fewcells, making the tissue relatively hypometabolic. Tendonscontain a small number of nociceptive and proprioceptivenerves, particularly near their attachment sites. One ofthese specialized sensory endings in tendon is called theGolgi tendon organ, which senses tension and providesfeedback control that alters muscle activity. Golgi tendonorgans normally have a somewhat steady rate of impulsetransmission to the spinal cord, one that is proportional tothe degree of muscle tension. However, if the tension issuddenly increased, an intense response of the tendonorgan results in a reflex inhibitory effect on the muscle thatpresumably prevents the development of potentiallydestructive excessive tension.

Energy storage is another important function of tendon.The distinctly kinked morphology of the collagen withinthe tendon is believed to facilitate the elastic storage andrecoveryof mechanical strain energy, much like a spring. Ina runner, the tendons of the lower limb muscles arestretched when the ball of the foot strikes the ground, andthe kinetic energy is stored in the elastic components of thetissue. At lift-off, the recoil of the tendon to its originallength helps to push the foot off the ground at thebeginning of the next stride. Unlikemusclecontraction,theelastic energy of tendon recoil requires no energy input.

The composition of tendon is similar to that of ligament.Collagen comprises as much as 85% of tendon dry weight;of this 95% is type I and 5%type III and/or type V.Unlikeligament, elastin constitutes less than 3% of tendon dryweight.

Summary

A brief overview of the biology, physiology and morphol-ogy of bone and connective tissues, as well as the local andsystemic factors that influence their growth, development,maintenance and repair, has been provided. The process ofcell proliferation and differentiation, matrix synthesis and

mineralization, growth and adaptation, all contribute tthis elaborate balance of modelling and remodellingSystemic conditions such as ageing or the menopausmay interfere with any number of these processeultimately contributing to the aetiology of diseases sucas osteoporosis. Thus, caution must be exercised wheconsidering interventions that influence a single aspect o

this elaborate balance (e.g. calcitonin disruption oosteoclast activity) which may, in turn, interrupt the abilitof remodelling to function appropriately. Hopefully, it clear that the osteoblast,osteocyte and osteoclast togetheandnot alone,are responsiblefor the skeletons success asdynamic mineral reservoir and structural material extraordinaire.

Further Reading

Alsina M, Guise T and Roodman GD (1996) Cytokine regulation

bone cell differentiation. Vitamins and Hormones 52: 6398.

Baron R, Ravesloot JH, Neef L et al. (1993) Cellular and molecul

biology of the osteoclast. In: Noda M (ed.) Cellular and MoleculaBiology of Bone, pp. 445495. San Diego: Academic Press.

Bilezikian JP, Raisz LG and Rodan GA (eds) (1996) Principles of Bon

Biology. San Diego: Academic Press.

Buckwalter JA, Einhorn TA and Sheldon RS (eds) (2000)Orthopaed

BasicScience. Rosemont, Illinois:AmericanAcademy of Orthopaed

Surgeons.

Cowin SC (1989) Bone Mechanics. Boca Raton, FL: CRC Press.

Ducy P and Karsenty G (1998) Genetic control of cell differentiation

the skeleton. Current Opinion in Cell Biology 10: 614619.

FergusonCM,MiclauT, HuD, AlpernE andHelms JA(1998) Commo

molecular pathways in skeletal morphogenesis and repair. Annals

the New York Academy of Sciences 857: 3342.

Hall BK andMiyake T (1992)The membranous skeleton:the role of c

condensations in vertebrate skeletogenesis.Anatomy and Embryolog

186: 107124.Lian B and Stein GS (1992) Concepts of osteoblast growth a

differentiation: basis for modulation of bone cell development an

tissue formation. Critical Reviews in Oral Biology and Medicine 3(3

269305.

Martin RB,BurrDB andSharkey NA (1998)Skeletal Tissue Mechanic

New York: Springer.

Niiweide PJ, Ajubi NE and Aarden EM (1998) Biology of osteocyte

Advances in Organ Biology 5B: 529542.

Owen TA, Aronow M, Shalhoub V, Lian JB and Stein GS (199

Progressive development of the rat osteoblast phenotypein vitro

reciprocal relationships in expression of genes associated wi

osteoblast proliferation and differentiation during formation of t

bone extracellular matrix. Journal of Cellular Physiology 143: 420

430.

Reddi AH (1994) Bone and cartilage differentiation. Current Opinion Genetics and Development 4(5): 737744.

Rubin CT (1984) Skeletal strain and the functional strain significance

bone architecture. Calcified Tissue International36: S11S18.

Rubin CT and Lanyon LE (1984) Dynamic strain similarity

vertebrates: an alternative to limb bone scaling.Journal of Theoretic

Biology 107: 321327.

Rubin CT and Rubin J (2000) Biology, physiology and morphology

bone. In: Harris E, Ruddy S and Sledge C (eds) Kellys Textbook o

Rheumatology, 6th edn, pp. 16111634. Philadelphia, PA: W

Saunders.


12 ENCYCLOPEDIA OF LIFE SCIENCES / 2001 M ill P bli h L d N P bli hi G / l

Documents

comparative skeletal structure.pdf