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Home , Collagen, Type II collagen, Type IV collagen, Type I collagen, Vitreous body

Eye (1987) 1, 175-183

Structure, Function and of the of the

ALLEN 1. BAILEY Bristol

I. Introduci:ion twelve genetically distinct types of in The collagenous tissues of the eye have mammalian tissues. They have all been charac­ received considerable attention since, 'apart terised biochemically and their precise from the importance ofthe itself, the eye location in complex tissues has been deter­ is composed of several highly specialised mined by immunohistochemical techniques. tissues which possess distinct collagenous The collagens identifiedto date have been clas­ . It is, in fact, a vivid example of the sified by several criteria, length of molecule, biological diversity of collagenous tissues. This molecular weight, flexibility of the molecule, diversity has recently been shown to be due to and by ultimate supramolecular . the existence of a whole family of collagen mol­ Employing the latter classification one can ecules that are capable of aggregating in dif­ consider three groups: the fibrous collagens, ferent ways to produce a variety of collagenous the non-fibrouscollagens and the filamentous structures.l At the present time there are about collagens. The variations in structure of these aggregates is shown in Figure 1.

FffiROUS COLLAGENS Fibrous collagens. These collagens are revealed in the as thick TypeI,n,JIl fibreswith a characteristic axial repeat pattern of 67 nm. The diameter of the fibresvaries con­ siderably, from 200 nm in and to NON - FIBROUS COLLAGENS about 25 nm in the . The size distri­

Type IV bution within a particular may be uni­ form, for example the fibresof the cornea vary from 25 to 30 nm, or may be highly variable as in the skin, where they can vary from 20 to 200nm. This group of collagens is comprised of the genetic Types I, II and III collagens. Type I is the major collagen of skin, tendon and , and is the most abundant of all the collagens. FILAMENTOUS COLLAGENS Type II is the major collagen of , and Type VI Type III of fetal and vascular tissue.

Non-fibrous collagens. In contrast to the fibrous collagens, this group forms the non­ fibrous basement membranes, which even in Fig. 1. Classification of collagen types into fibrous, the electron microscope appear homoge­ non-fibrous and filamentous collagen, and the corre­ sponding supramolecular structures. neous. They are thin membranes separating

Correspondence to: Allen J. Bailey, AFRC Institute of Food Research-Bristol, Langford, Bristol BS18 7DY. 176 ALLEN J. BAILEY the fibrous stromal tissue from the cells, and distance apart. The fibres are arranged in vary in thickness from 25 nm in and layers of parallel sheets with each layer at right glomeruli to 200 nm in the . At the angles to the adjacent layer. present time the only collagen in this group is It is generally agreed that the major molecu­ Type IV collagen and has been identifiedin all lar species of collagen in cornea is Type I, basement membranes so far examined. although there is some disagreement on the presence of other collagens (Table I). Types II, Filamentous collagens. The collagens of this III and V have been reported. Type III has group form loosely aggregated fibreswith little variably been reported as totally absent, less or no periodicity. They can be subdivided into than 1 per cent, and as much as 20 per cent.2 pericellular and collagens. This group Some of the differences could be due to species includes Types VI, VII, IX and X. and age of the tissue examined, whilst others could be due to the technique employed in the Collagenous structures of the eye analysis. On balance, Type III is probably not The connective tissues of the eye can be present and Type V is between 5-10 per cent. divided into specifictypes for ease of descrip­ Type II is only present at the embryonic stage. tion of their respective collagens: Biochemical studies on show (1) cornea-is a transparent tissue containing that it possesses a higher level of fine collagen fibres of uniform diameter of the residues than Type I in with a high degree of spatial organisation other tissues. It has been suggested that the (2) -is an opaque tissue containing extent of glycosylation may control the size of thick interwoven collagen fibres the fibresin the cornea, but this is probably too (3) -is a polysaccharide gel simplistic an explanation. containing small amounts of finecollagen Age-related studies have shown that the fibres amount of Type V increases during maturation (4) lens capsule-has an apparently amor­ from 5 to 10 per cent whilst Type III decreases phous structure from 2 per cent in the embryonic eye to zero in (5) -the retinal pigmented epithelium the adult.3 This shift in collagen types was is a typical thin basement membrane determined from pepsin digests. More (6) -highly vascularised tissue of the accurate data could have been obtained by . analysis of CNBr cleaved . Using monoclonal antibodies to follow the II. Fibrous Collagens of the Cornea, Sclera developmental changes in location of Type V, and Vitreous Linsenmayer et al. 4 found the fibresto be pres­ ent in a masked form, treatment with acetic (a) Cornea acid to swell the fibresbeing necessary to reveal The human cornea contains 90 per cent col­ the antibody sites. Examining 4 day embryo to lagen by dry weight yet, unlike other 1 day post-hatching the Type V in the cornea collagenous tissues, it is transparent. This fea­ appears after the sixth day of development ture is primarily due to the precise packing of when the primary stroma swells and is invaded collagen fibres of uniform diameter at a fixed by mesenchymally derived . Fluo-

Table I Major collagen types of the eye

Fibrous Non-fibrous Filamentous

Sclera Type I, Type III (-10%) Cornea Type I, Type V (-10%) Type VI Vitreous Type II Type IX Lens capsule Type IV Descemet's membrane Type IV Type VIII Bowman's membrane Type IV Retinal pigment epithelium Type IV THE COLLAGENS OF THE EYE 177 rescence appears throughout the cornea and it compressibility of the hyaluronate gel when has been suggested that hybrid fibrilsof Type I exposed to external . and Type V are formed. In this way Type V may In the electron microscope the characteristic exert an influenceon the precise fibrildiameter striations of the collagen fibre are difficult to of the corneal collagen. Alternatively, the observe in detail unless the fibres are pre­ absence of Type III, which normally co-dis­ treated with typsin. Biochemical analysis has tributes with Type I, may contribute to the shown that the vitreous collagen is Type II, latter's ability to form uniform narrow fibres. although minor modifications have been reported.7 The origin of the Type II of the vitreous lies (b) Sclera in the early embryonic stages when it is derived The sclera protects the intraocular contents from the neural retina which secretes Type II, from injury and the function of the collagen in and some Type V, into the vitreous body. It is the sclera is obviously structural. The strength not clear which cells synthesise vitreous col­ and resilience of the sclera is imparted by close lagen, indeed it is thought that different cells interlacing ofthe collagen fibreswhich account synthesise the collagen at different stages of for 80 per cent of the dry weight. In contrast to development. the cornea, the fibresare like tendon and skin and vary in diameter from 30 flm to 300 flm within a single fibre bundle. The elasticity of Structure and stabilisation o/fibrous collagens The biosynthesis of collagen has been exten­ the sclera is increased by the presence of a small sively reviewed8,9 and will not be dealt with proportion of fibres. here, but the subsequent extracellular Sclera contains predominantly Type I col­ aggregation of the molecules is relevant to a lagen (�90 per cent), and a small amount of discussion of the different connective tissues of Type III ( � 10 per cent). In the avian eye Type the eye and the effect of age on these tissues. II is also present in the cartilaginous scleral The fibrous collagens are initially syn­ support ring.5 thesised as procollagens possessing large N­ The biological function of Type III is and C-terminal globular peptides, These unknown although it is widely distributed in globular domains are enzymatically removed other tissues of the body. It has been suggested during secretion and fibrillogenesis, thereby that increases in the proportion of Type III allowing lateral association of the long triple impart greater plasticity to the tissue, for helical molecules to form precisely banded example in fetal tissues and the vascular .The assembly is directed by acidic and system. Certainly, when the ratio is disturbed basic groups of the amino acids, and stabilised in heritable disorders in favour of Type III, the by the hydrophobic groups present along the tissues such as skin and become more molecule, to form an end-overlap and quarter­ flexibleand the sclera becomes translucent. 6 stagger alignment of the molecules. 10 Changes in the proportions of the collagen Further stabilisation of the fibresto impart types occur with ageing and may result in struc­ tensile properties to the tissue occurs through tural changes which could playa role in the formation of intermolecular covalent and glaucoma. bonds. Specific in the residual N- and C-terminal non-helical regions are (c) Vitreous humour enzymatically oxidised through the E-amino The central part of the vitreous shows a three­ groups to reactive aldehydes. The precise dimensional network of finecollagen fibresof alignment of the molecules in the fibreensure about 7-13 nm in diameter. The collagen is this reactive aldehyde is opposite the E-amino embedd�din a hyaluronate gel and contributes group of a hydroxylysine in an adjacent mole­ to the maintenance of the , cule. The resulting reaction produces stable acting as a shock absorber mitigating against covalent aldimine or keto-imine bonds the effect of body movements in the eye. The throughout the fibre(Fig. 2) . The difference in fine collagen fibres have the functions of sta­ the two cross-links probably arises from the bilising the shape of the gel and reducing the different levels of in the 178 ALLEN J. BAILEY (a) (b) y CH2 A Ho- _CH2-CH2 T T I I -< ) ,�; I

Aldimine Keto - imine

Hydroxy - pyridinoline

Fig. 2. Chemistry of the collagen cross-links: (a) cross-links present in immature collagen. the aldimine and keto-imine divalent cross-links (b) trivalent hydroxypyridinoline cross-link present in mature tissue tissue such that the N- and C-terminal lysyl with age is illustrated by the high proportion of residue can be or hydroxylysine. The the keto-imine in embryonic corneal collagen physiological significance of the two different and a changeover to only the aldimine in the types of cross-link has not been established. young (Fig. 2) . Tissue specificityacross This polymerisation of the molecules within many species is demonstrated by the predomi­ the fibre imparts the mechanical properties nance of the aldimine in dermal collagen, the necessary for collagen to act as a supporting equal proportion of the two in Achilles tendon, tissue.11 and exclusively the keto-imine in cartilage col­ The relative proportion of the keto-imine lagen. Within these tissues a number of dif­ and aldimine cross-links varies with the age ferent types of collagen may be present but and ofthe collagenous tissue. Variation they will all possess the same type of cross-link. Cross-linking therefore appears to be tissue specificrather than collagen type specific. The divalent cross-links are crucial to the production of high tensile properties of the immature fibre. However, the mechanical mature -- /" stability of the fibre continues to increase during maturation, resulting in a fibreless sus­ ceptible to swelling and degradation (Table II). At the same time the proportion of the divalent cross-linking decreases. These

o aldimine and keto-imine divalent cross-links

Fig. 2(c) variation in the proportion of the different have now been shown to undergo further reac­ cross-links with the age of the corneal collagen. tion to form stable multivalent cross-links THE COLLAGENS OF THE EYE 179

Table II Effects of ageing on collagen fibres valent cross-links would result in providing a network throughout the fibre and would Increase in fibre size account for the increased stability of mature Increase in tensile strength 12 Increase in thermally or chemically induced collagen. isometric tension The nature of the mature cross-link has not Decrease in yet been established. Several proposals have Decreased susceptibility to proteolytic degradation been made, and 3-hydroxypyridinoline has Decreased ability to swell received considerable attention. This cross­ Decrease in reducible and increase in mature cross­ link is formed from hydroxylysine aldehydes links and is found in cartilage and bone, but is not present when the cross-link precursor is lysine r aldehyde in tissue such as cornea and skin. The thereby p oviding further stability to the fibre. proposal that pyridinoline is the major cross­ For this type of interaction to occur the mole­ link in some tissues means that two different cules must be in register rather than quarter­ mechanisms must be operating in the matura­ staggered. We suggested therefore that the tion of collagen since other tissues do not pos­ polymerisation of collagen occurs in two sess pyridinoline.13 On the other hand, stages. The first stage involves end-overlap Barnard et at. 14 have isolated a complex puta­ head-to-tail cross-linking to form a long poly­ tive cross-link (Compound M) derived from meric , and the second stage the trans­ lysine which is present in both mature bone and verse cross-linking through interaction of the mature skin. The characterisation of this cross­ reducible cross-links of molecules in register link has not been completed, but it is suggested (Fig. 3). The formation of these stable multi- that the presence of this compound in all mature tissues indicates a common mechanism

NORMAL CROSS-LINKING of maturation.

(a) IMMATURE (I ) Both the corneal and scleral collagens are initially stabilised in embryonic tissue by the

-. -'- -,- �--,I______keto and aldimine cross-links, and sub­ --'------sequently only the aidimine bond in the post­ �- -. ��I �___ --'- , natal tissue. In the mature tissue the reducible (I ____ --'---- cross-links disappear and the major cross-link

(b) MATURE (») appears to be compound M (Fig. 2) . No pyridinoline could be identified in these tissues. On the other hand, the collagen fibresof the vitreous body are stabilised by the keto-imine bond and 3-hydroxypyridinoline in the mature eye, similar to previous studies on the stabilisa­ tion of Type II collagen in young and mature

(e) GLYCOSYlATION (t) cartilage collagen. However, the proportion of cross-links is much smaller and the extent of maturation apparently slower in the vitreous IS • compared to cartilage. It is unlikely that the maturation and slight -,-I__ IL­ t t increase in the proportion of the collagen fibres • with age dramatically affects the properties of the vitreous gel, liquefaction more likely being Fig. 3. Location of the collagen cross-links: due to a modification in the synthesis of the (a) head-to-tail cross-linking in immature tissues (b) additional transverse cross-linking fo rmed fr om polysaccharide gel. the divalent cross-links to produce interfilament III. Non-Fibrous Basement Membrane multivalent cross-links Collagens of the Eye (c) random cross-links within and between filaments fo rmed through further reaction of the glucosyl (a) The lens capsule lysines. The lens is surrounded by a collagenous base- 180 ALLEN 1. BAILEY ment membrane, which because of its collagen under conditions favouring fibroblas­ accessibility has been extensively studied as an tic morphology of the cells. This change may be example of basement membrane collagen. It is important in defects of the retina, particularly an elastic membrane and appears to be com­ . pletely homogeneous. Early studies on the Turkeson et al.21 have recently demon­ composition, X-ray diffraction and strated by immunofluorescenttechniques that the use of antibodies indicated the presence of Type IV and the other normal components of collagen in the lens capsule. Despite the trans­ basement membrane, , , parency and apparently amorphous nature of , heparan sulphate and the membrane in the electron microscope, are indeed components of the intact retinal intaet collagen molecules have been isolated pigment epithelium and established this mem­ and this collagen is referred to as Type IV. brane as a typical basement membrane.

(b) Descemet's membrane Structure and stabilisation of Type IV This membrane, located under the corneal collagen stroma, is usually referred to as a basement The non-fibrillar nature of basement mem­ membrane based on light microscope studies, brane clearly indicates a different structural but the electron microscope reveals a more assembly of the Type IV collagen molecules. highly organised structure. The structure The molecule itself is longer (400 nm) than the appears to consist of rigidly orientated fibrilsin fibrouscollagens and retains its large globular hexagonal array about 100 nm long with nodes C-terminal after secretion. Again, of 27 nm in diameter at either end. X-ray unlike the fibrous collagens, the crucial triple diffraction meridional spacing of 2.86 A con­ helix sequence of Gly-X-Y- is interrupted at a firmed the presence of triple helical collagen number of sites along the molecule. This and the 11.5 A equatorial spacing indicates results in a much more flexible triple helical lateral association of the molecules. molecule. Furthermore, the charge profile Biochemical analysis indicated the presence along the molecule is completely different of Type IV collagenl6 and about 10 per cent from the fibrous collagens. Because of these Type V. On the other hand, Schittny et al.17 unique properties Type IV molecules self­ failed to detect Type IV using immunofluores­ assemble to form a tetramer by anti-parallel cent techniques. Davison3 also reported sig­ end-overlap of the N-terminal domains.22 It nificantamounts of Type I. More recently the has been further proposed that these tetramers presence of a high proportion of Type VIII col­ aggregate through the C-terminal globular lagen has been reported.18.19 In the light of regions to form a non-fibrillar open structure these results a more detailed analysis of the (Fig. 1), which has been referred to as a relative proportion of the various collagen 'chicken wire net' .23 types needs to be carried out. The single molecule framework may not be correct, indeed its obvious fragility would sug­ (c) Retina gest that the molecules are associated in some The delicate light-sensitive retina is supported form of lateral organisation. Yurchenco and by a structure known as Bruch's membrane. Furthmayr24 have proposed models involving This is in effect two basement membranes, the lateral association but without the necessary pigment epithelial membrane on the retinal experimental evidence. More recently Bar­ side and the end'1thelial membrane on the nard et al.25 have obtained evidence of loose choroidal side, with loose in lateral association of about four molecules by between. X-ray diffraction of stretched lens capsules. Cloned colonies of retinal pigment epithelial Despite the novel network structure, analy­ cells from chick embryos have been shown to sis of the cross-links revealed the same keto­ synthesise Type IV collagen in culture, imine bonds in intact lens capsule and follow­ although the conditions appear to be critical in ing in vitro culture26 to those stabilising the respect of the products synthesised.20 For fibrous collagens. The cross-links are located example, these cells synthesise Type I and III in the anti-parallel aligned N-terminal regions. THE COLLAGENS OF THE EYE 181

Further analysis indicated that the globular the human cornea. Indirect immuno­ regions may be similarly cross-linked, and the fluorescence demonstrated that this collagen presence of cross-links along the molecule sug­ was distributed throughout the corneal gested the possibility of lateral association of stroma. Whether the Type VI is intimately the molecules in the structureY associated with the Type I fibre or is in the The age-related changes occur rapidly, the between these fibresis as keto-imine being barely detectable in the lens yet unknown. In the aorta the Type VI fibrils capsules from a 2-3 week old calf, suggesting appear to be located between the striated col­ maturation of this structure occurs as in the lagen Type I fibres. It is possible that these fibrous collagens. Analysis for the mature Type VI fibrilscould act as spacers for the pre­ cross-link, pyridinoline, failed to reveal its cisely aligned Type I fibresof the cornea. presence despite its precursor keto-imine Type VI forms loosely packed filamentswith cross-link. The mature cross-link, compound a repeat period of about 100 nm31 produced by M, was however detected in significant quan­ a unique macromolecular organisation (Fig. 1) tities. To achieve this second stage reaction of of the Type VI molecules.32 the reducible cross-links would require the The cross-linking of Type VI fibresappears sheets in the chicken wire conformation to be to be different from other collagens. Analysis in precise register, such that the chains cross­ for the known reducible or mature cross-links linked by the keto-imine overlap similar failed to reveal any significant amounts. regions of chains in the next layer.27 This Further, the ready extraction of Type VI from second stage polymerisation would greatly various tissues suggests that the fibresare not stabilise the basement membrane. cross-linked other than through disulphide Descemet's membrane, like lens capsule, bonds. contains the keto-imine cross-link but not the pyridinoline cross-link. The mature cross-link Type VIII. This collagen has only been identi­ (compound M) was also found to be present. fiedin vivo in Descemet's membrane where it Heathcote et al. 28 reported a surprising finding has been reported to be a major constituent that Descemet's membrane contained and may therefore play a role in the unique and isodesmosine, normally con­ structure of this basement membrane. finedto elastin, as the major cross-linking com­ More recently, Type VIn has been reported pounds. It remains to be established which of to be synthesised by corneal endothelial cells. 33 the different collagen types present in the Although the structure has not yet been estab­ membrane possess these cross-links. lished completely it is believed to consist of These age-related changes could have a sig­ three different a-chains of 61 K with non-heli­ nificanteffect on the physico-chemical proper­ cal domains at each end. ties of these membranes.29 In the case of the The identification of polymers of the lens capsule this could contribute to pres­ a-chains indicated the presence of cross-links byopia and cataracts. Research on cataracts but their nature has not yet been determined. generally concentrates on the crystallins, but a Initial studies indicate that they are different change in the diffusion of metabolites through from the reducible cross-links of the fibrous the lens could trigger changes in the crystallins. collagens.

IV. Filamentous Collagen of the Eye Few studies have been carried out on the pres­ Type IX. This collagen was initially reported by ence of the filamentouscollagen of the eye, but Duance et al. 34 to be closely associated with the it is almost certain that a number of these new pericellular area of of cartilage, collagens will be found associated with the but has now been reported to be associated other major collagen types in the eye. with Type II collagen in other tissues. The vitreous contains about three times more Type Type VI. Recent studies by Zimmermann et IX than cartilage.35 The cross-linking of Type al.30 have identified Type VI collagen as a IX occurs through the same mechanism as the majorcomponentoftheextracellularmatrixof fibrous collagen, at least in cartilage, the 182 ALLEN J. BAILEY changes in vitreous having not yet been molecular level, at the supramolecular struc­ reported. ture, and at the morphological level, the age­ related changes in the properties of the col­ V. Non-Enzymic Glycosylation lagen proceed by a similar mechanism. This In diabetic subjects the collagen has been mechanism involves the enzymic oxidation of reported to undergo 'accelerated ageing' as specific lysines and the subsequent non­ evidenced by its increase in mechanical stab­ enzymic interaction of these groups due to the ility and resistance to . It has been sug­ highly organised nature of the molecular gested that this could occur by enhancement aggregates. The secondary reaction of non­ of normal cross-linking. However, we have enzymatic glycosylation, although slow in shown this mechanism does not occur, and a normal subjects, may also play arole in stabilis­ proposed alternative is through the accumula­ ing the collagen fibrein older tissues. The for­ tion of moieties. mation of all these different multivalent cross­ It has been known for some time that glucose links stabilises the collagen, increasing its reacts with the E-NHz groups of lysine in pro­ mechanical strength and resistance to swelling, teins, and that this reaction is particularly and enzymic attack. These age-related important in collagen because of its long bio­ changes in the properties of collagenous logical half-life. The initial reaction results in tissues, initially essential for optimal function, the formation of glycosyl-lysine through an could if continued cease to be beneficial. aldimine bond but this is subsequently stabil­ Excessive cross-linking could result in dele­ ised by the Amadori rearrangement. 36.37 The terious effects on the properties of the fibres, presence of a keto group following this re­ possibly causing inflexibility of the fibre, a arrangement provides the potential for reac­ change in its interaction with other com­ tion with further lysine residues, and thereby ponents of the extracellular matrix, and resis­ form an intermolecular cross-link. In vitro tance to the enzymes involved in the normal studies have demonstrated that new covalent metabolism of the tissue. These changes could cross-links derived from lysine and glucose are be particularly important in affecting the filtra­ indeed formed when collagen is incubated with tion properties of basement membranes. For glucose over long periods of time.3R These example, they could be important in cataract cross-links would be formed as interhelical formation by affecting the diffusionof specific bonds and therefore have a considerable sta­ metabolites through the lens capsule resulting bilising effect on the fibril even if only a few in a change in the properties of the lens com­ such bonds formed. The effect of 'accelerated ponents, particularly the crystallins. Increas­ ageing' is dramatic in diabetics, but similar ing stiffness of the lens due to cross-linking of reactions are occurring slowly in normal col­ the collagen could also be a component in pres­ lagenous tissues, and may therefore be impor­ byopia. Similar changes are occurring in the tant in determining age-related changes. The retinal basement membranes and could also effect may be of little consequence to the thick lead to retinal disorders. We have learned a lot fibrous tissues such as the sclera, but may be about the detailed mechanism involved but important in changing the charge profileof the now need to relate these changes to specific fibresin precisely organised tissues such as the property changes in the ageing collagenous cornea. The effect could be even more dra­ tissues. Disorders of the vision are dramatic matic in the more metabolically sensitive and a greater knowledge of the normal ageing tissues such as basement membrane of the eye, process is essential to help understand and per­ particularly the capillaries. However, until the haps alleviate some of the aberrations. nature and extent of these possible cross-links has been elucidated the effect of non-enzyma­ References tic glycosylation ageing can only be guessed at. 1 Martin GR, Timp\ R, Muller PK, Kuhn K: The genet­ ically distinct collagens. Trends in Biochem. 1985; 10: 285-7. VI. Conclusions 2 Freeman IL: The eye. In: Weiss JB, Jayson MIV, eds. It is clear that, despite the difference in struc­ Collagen in health and disease. Edinburgh: ture of the collagenous tissue of the eye, at the Churchill Livingstone 1982; 388--403. THE COLLAGENS OF THE EYE 183

3 Lee RE, Davison PF:Collagen composition and turn­ heparan sulphate in chick retinal pigment epi­ over in ocular tissues of the rabbit. Exp. Eye Res. thelial basement membrane. 1. Histochem. & 1981; 32: 737-45. Cytochem. 1985; 33: 668-71. 4 Linsenmeyer TF, Fitch 1M, Gross 1, Mayne R: Are 22 Kiihn K, Wiedemann H, Timpl R, Risteli 1, the collagen fibrils in the avian cornea composed Dieringer H, Voss T, Glanville R: Macro­ of two different collagen types? Ann. NY Acad. molecular structure of basement membrane col­ Sci. 1985; 460: 232-45. lagen. FEBS Lett. 1981;125: 123-8. 5 Trelstad RL, Kang AH: Collagen heterogeneity in 23 Timpl R, Wiedemann H, van Delden V, Furthmayr the avian eye, lens, vitreous body, cornea and H, KiihnK: A network model for the organisation sclera. Exp. Eye Res. 1974; 18: 395-406. of Type IV collagen in basement membrane. Eur. 6 Prockop OJ, Kivirikko KI: Heritable diseases of col­ J. Biochem. 1981; 120: 203-11. lagen. New Engl. f. Med. 1984; 311:37 6--86. 24 Yurchenco PD, Furthmayr H: Self-assembly of base­ 7 Swann DA, Sotman S: The chemical composition of ment membrane collagen. Biochemistry 1984;23: bovine vitreous humour collagen fibres. Biochem. 1839-50. 1. 1980;185: 545-54. 25 Barnard K, Gathercole LJ, Bailey AJ: Basement 3 Prockop DJ, Kivirikko KI, Tudermann L, Gunzman Membrane collagen--evidence for a novel NA: The biosynthesis of collagen and its dis­ molecular packing. FEBS Lett 1987; 212: 49-52. orders. New Engl. f. Med. 1979; 301: 13-23, 26 Heathcote JG. Bailey Al. Grant ME:Studies on the 77-85. assembly of lens capsule. Biochem. J. 1980; 9 Bailey AJ, Etherington DJ: Metabolism of collagen 190: 229-37. and elastin. In: Neuberger A, ed. Comprehensive 27 Bailey AJ, Sims TJ, Light ND: Crosslinking in Type Biochemistry Vol. 19B. Amsterdam: Elsevier IV collagen. Biochem. f. 1984; 218: 713-23. 1980; 299-460. 2" Heathcote JG, Eyre DR, Gross J: Mature bovine to Piez KA: Molecular and aggregate structure of col­ Descemet's membrane contains desmosine and lagen. In: Piez KA, Reddi AH, eds. Extracellular isodesmosine. Biochem. Biophys. Res. Commun. matrix biochemistry. New York: Elsevier 1984; 1982; 108: 1588-94. 1-39. 29 Fisher RF: The structure and function of basement 11 Bailey Al, Robins SP, Balian G: Biological signifi­ membrane lens capsule in relation to diabetes and cance of the intermolecular crosslinks of collagen. cataract. Trans. ophthalmol. Soc. UK. 1985; 104: Nature 1974; 251: 105-9. 755-9. 12 Bailey AJ, Light ND, Atkins EDT: Chemical cross­ 10 Zimmermann DR. Triieb B, Winterhalter KH. linking restrictions in models for the molecular Witmer R, Fischer RW: Type VI collagen is a organization of the collagen fibre. Nature 1980; major component of the human cornea. FEBS 288: 408-10. Lett. 1986; 197: 55-8. 13 Eyre DR, Paz MA, Gallop PM: Crosslinks in col­ 31 Bruns RR: Beaded filamentsand long-spacing fibrils: lagen and elastin. Ann. Rev. Biochem. 1984; 53: relation to type VI collagen. 1. Ultrastruct. Res. 717-48. 1984; 89:136--45. 14 BarnardK, Light ND, SimsTJ, Bailey AJ:Chemistry 32 Furthmayr H, Weidemann H. Timpl R, Odermatt E, of collagen crosslinks. Biochem. 1. (in press). Engel J: Electron microscopical approach to a 15 Snowden JM, Eyre DR, Swann DA: Age-related structural model of interna collagen. Biochem. J. changes in the thermal stability and crosslinking of 1983; 211: 303-11. vitreous, articular cartilage and tendon collagen. 33 Benya P A, PadillaSR: Isolation and characterization Biochem. Biophys. Acta. 1982; 706: 153-7. of Type VIII collagen systems by cultured rabbit 16 Kefalides NA: Biology and chemistry of basement corneal endothelial cells. 1. Bioi. Chem. 1986; membranes. New York: Academic Press 1978. 261: 4160-9. 17 Schittny lC, Dziadek M, Timpl R. EngeIJ: Localiza­ 34 Duance VC, Shimokomaki M, Bailey AJ: Immu­ tion of Type IV in basement membrane. Bioi. Bio­ nofluorescencelocalization of Type M collagen in chem. Hoppe-Seyler 1985; 366: 846--7. articular cartilage. Biosci. Reports 1982;2: 223-7. 18 Labermeier U, Kenney MC:The presence of EC col­ 35 AyadS, WeissJB:Anewlookatthevitreous-humour lagen and Type IV collagen in bovine Descemet's collagen. Biochem. 1. 1984; 218: 835-40. membrane. Biochem. Biophys. Res. Commun. 36 Robins SP, Bailey AJ: Age-related changes in col­ 1983; 116: 619-25. lagen. Biochem. Biophys. Res. Commun. 1972; 19 Kapoor R, Bornstein P, Sage HE:Type VIII collagen 48: 76--84. from Descemet's membrane. Biochemistry 1986; 37 LePape A, Muh J-P, Bailey AJ: Characterization of 25: 3930---7. N-glycosylated Type I collagen in streptozotocin 20 Newsome D, KenyonK: Collagen production in vitro induced diabetes. Biochem. J. 1981; 197: 405-12. by the retinal pigmented epithelium of the chick 38 Kent M1C, Light ND, Bailey AJ: Evidence for embryo. Develop. Bioi. 1973; 32: 387-96. glucose-mediated covalent cross-linking of col­ 21 Turkeson K, Aubin lE, Sodek J, Kalnins VI: lagen after glycosylation in vivo. Biochem. J. Localisation of Type IV collagen, fibronectin, and 1985;225:745-52.