Isolation and characterization of type VIII collagen synthesized by cultured rabbit corneal endothelial cells. A conventional structure replaces the interrupted-helix model

COL8A1 overexpression promotes glioma cell growth by activating focal adhesion kinase signaling cascade

We explored expression and biological roles of collagen type VIII alpha-1 chain (COL8A1) in glioma. Bioinformatics analyses unveiled COL8A1 overexpression within glioma tissues correlates with adverse clinical outcomes of patients. COL8A1 overexpression was also detected in local glioma tissues and various glioma cells. In primary and immortalized glioma cells, COL8A1 shRNA or knockout (KO) reduced cell viability, proliferation and mobility, disrupted cell cycle, and prompted apoptosis. While COL8A1 overexpression augmented the malignant behaviors in glioma cells. COL8A1 shRNA or KO in primary glioma cells decreased phosphorylation of FAK and downstream targets Akt and Erk1/2. Conversely, elevating COL8A1 expression increased their phosphorylations. In vivo experiments confirmed growth inhibition of patient-derived glioma xenografts within the mouse brain following COL8A1 KO. Hindered proliferation, lowered phosphorylation levels of FAK, Akt, and Erk1/2, as well as increased apoptosis were observed within the COL8A1 KO intracranial glioma xenografts. Thus, COL8A1 overexpression promotes glioma cell growth.

In Vitro Profiling of the Extracellular Matrix and Integrins Expressed by Human Corneal Endothelial Cells Cultured on Silk Fibroin-Based Matrices

Developing a scaffold for culturing human corneal endothelial (HCE) cells is crucial as an alternative cell therapeutic approach to bridge the growing gap between the demand and availability of healthy donor corneas for transplantation. Silk films are promising substrates for the culture of these cells; however, their tensile strength is several-fold greater than the native basement membrane which can possibly influence the dynamics of cell-matrix interaction and the extracellular matrix (ECM) secreted by the cells in long-term culture. In our current study, we assessed the secretion of ECM and the expression of integrins by the HCE cells on Philosamia ricini (PR) and Antheraea assamensis (AA) silk films and fibronectin-collagen (FNC)-coated plastic dishes to understand the cell-ECM interaction in long-term culture. The expression of ECM proteins (collagens 1, 4, 8, and 12, laminin, and fibronectin) on silk was comparable to that on the native tissue. The thicknesses of collagen 8 and laminin at 30 days on both PR (4.78 ± 0.55 and 5.53 ± 0.51 μm, respectively) and AA (4.66 ± 0.72 and 5.71 ± 0.61 μm, respectively) were comparable with those of the native tissue (4.4 ± 0.63 and 5.28 ± 0.72 μm, respectively). The integrin expression by the cells on the silk films was also comparable to that on the native tissue, except for α3 whose fluorescence intensity was significantly higher on PR (p ≤ 0.01) and AA (p ≤ 0.001), compared to that on the native tissue. This study shows that the higher tensile strength of the silk films does not alter the ECM secretion or cell phenotype in long-term culture, confirming the suitability of using this material for engineering the HCE cells for transplantation.

New developments in corneal endothelial cell replacement

Corneal transplantation is currently the most effective treatment to restore corneal clarity in patients with endothelial disorders. Endothelial transplantation, either by Descemet membrane endothelial keratoplasty (DMEK) or by Descemet stripping (automated) endothelial keratoplasty (DS(A)EK), is a surgical approach that replaces diseased Descemet membrane and endothelium with tissue from a healthy donor eye. Its application, however, is limited by the availability of healthy donor tissue. To increase the pool of endothelial grafts, research has focused on developing new treatment options as alternatives to conventional corneal transplantation. These treatment options can be considered as either 'surgery-based', that is tissue-efficient modifications of the current techniques (e.g. Descemet stripping only (DSO)/Descemetorhexis without endothelial keratoplasty (DWEK) and Quarter-DMEK), or 'cell-based' approaches, which rely on in vitro expansion of human corneal endothelial cells (hCEC) (i.e. cultured corneal endothelial cell sheet transplantation and cell injection). In this review, we will focus on the most recent developments in the field of the 'cell-based' approaches. Starting with the description of aspects involved in the isolation of hCEC from donor tissue, we then describe the different natural and bioengineered carriers currently used in endothelial cell sheet transplantation, and finally, we discuss the current 'state of the art' in novel therapeutic approaches such as endothelial cell injection.

Basement membranes in the cornea and other organs that commonly develop fibrosis

Basement membranes are thin connective tissue structures composed of organ-specific assemblages of collagens, laminins, proteoglycan-like perlecan, nidogens, and other components. Traditionally, basement membranes are thought of as structures which primarily function to anchor epithelial, endothelial, or parenchymal cells to underlying connective tissues. While this role is important, other functions such as the modulation of growth factors and cytokines that regulate cell proliferation, migration, differentiation, and fibrosis are equally important. An example of this is the critical role of both the epithelial basement membrane and Descemet's basement membrane in the cornea in modulating myofibroblast development and fibrosis, as well as myofibroblast apoptosis and the resolution of fibrosis. This article compares the ultrastructure and functions of key basement membranes in several organs to illustrate the variability and importance of these structures in organs that commonly develop fibrosis.

Change in long-spacing collagen in Descemet's membrane of diabetic Goto-Kakizaki rats and its suppression by antidiabetic agents

We examined changes in the ultrastructure and localization of major extracellular matrix components, including 5 types of collagen (type I, III, IV, VI, and VIII), laminin, fibronectin, and heparan sulfate proteoglycan in Descemet's membrane of the cornea of diabetic GK rats. In the cornea of diabetic GK rats, more long-spacing collagen fibrils were observed in Descemet's membrane than in the membrane of the nondiabetic Wistar rats. Both GK and Wistar rats showed an age-dependent increase in the density of the long-spacing collagen. Immunoelectron microscopy showed that type VIII collagen was localized in the internodal region of the long-spacing collagen, which was not labelled by any of the other antibodies used. The antidiabetic agents nateglinide and glibenclamide significantly suppressed the formation of the long-spacing collagen in the diabetic rats. Long-spacing collagen would thus be a useful indicator for studying diabetic changes in the cornea and the effect of antidiabetic agents.

Expression and Supramolecular Assembly of Recombinant α1(VIII) and α2(VIII) Collagen Homotrimers

Collagen VIII is an extracellular matrix macromolecule comprising two polypeptide chains, alpha1(VIII) and alpha2(VIII), that can form homotrimers in vitro and in vivo. Here, recombinant collagen VIII was expressed to study its supramolecular assembly following secretion. Cells transfected with alpha1(VIII) or alpha2(VIII) assembled and secreted homotrimers that were stable in denaturing conditions and had a molecular mass of approximately 180 kDa on SDS-PAGE gels. Co-transfection with prolyl 4-hydroxylase generated homotrimers with stable pepsin-resistant triple-helical domains. Size fractionation of native recombinant collagen VIII molecules expressed with or without prolyl 4-hydroxylase identified urea-sensitive high molecular mass assemblies eluting in the void volume of a Superose 6HR 10/30 column and urea-resistant assemblies of approximately 700 kDa, all of which were composed of homotrimers. Immunofluorescence analysis highlighted the extracellular deposition of recombinant alpha1(VIII)(3), alpha2(VIII)(3), and co-expressed alpha1(VIII)(3)/alpha2(VIII)(3). Microscopy analysis of recombinant collagen VIII identified rod-like molecules of 134 nm in length that assembled into angular arrays with branching angles of approximately 114 degrees and extensive networks. Based on these data, we propose a model of collagen VIII assembly in which four homotrimers form a tetrahedron stabilized by central interacting C-terminal NC1 trimers. Tetrahedrons may then act as building blocks of three-dimensional hexagonal lattices generated by secondary interactions involving terminal and helical sequences.

Type VIII collagen: heterotrimeric chain association

Two chains, alpha1(VIII) and alpha2(VIII), have been described for type VIII collagen. Early work suggested that these chains were present in a 2:1 ratio, although recent work has shown that homotrimers can form and predominate in some tissues. In order to address the question of whether the alpha1(VIII) and alpha2(VIII) chains could co-polymerise we made a shortened alpha1(VIII) chain and expressed this with full length alpha2(VIII) chain in an in vitro translation system supplemented with semi-permeabilised cells. Heterotrimers containing either two or one alpha2(VIII) were evident. Interestingly, a point mutation in the NC1 domain of the alpha1(VIII) chain abrogated trimer formation. In addition we were able to demonstrate chain association of the alpha1(X) chain of type X collagen with the shortened alpha1(VIII) chain. Variations in chain association were seen when altered ratios of message were used. These results demonstrate the importance of the NC1 domain in chain association and suggest that gene expression regulates the composition and function of type VIII collagen by varying chain composition.

Mechanisms of maturation and ageing of collagen

The deleterious age-related changes in collagen that manifest in the stiffening of the joints, the vascular system and the renal and retinal capillaries are primarily due to the intermolecular cross-linking of the collagen molecules within the tissues. The formation of cross-links was elegantly demonstrated by Verzar over 40 years ago but the nature and mechanisms are only now being unravelled. Cross-linking involves two different mechanisms, one a precise enzymically controlled cross-linking during development and maturation and the other an adventitious non-enzymic mechanism following maturation of the tissue. It is this additional non-enzymic cross-linking, known as glycation, involving reaction with glucose and subsequent oxidation products of the complex, that is the major cause of dysfunction of collagenous tissues in old age. The process is accelerated in diabetic subjects due to the higher levels of glucose. The effect of glycation on cell-matrix interactions is now being studied and may be shown to be an equally important aspect of ageing of collagen. An understanding of these mechanisms is now leading to the development of inhibitors of glycation and compounds capable of cleaving the cross-links, thus alleviating the devastating effects of ageing.

The alpha1(VIII) and alpha2(VIII) chains of type VIII collagen can form stable homotrimeric molecules

Type VIII collagen is a short chain collagen. Two chains have been described, alpha1(VIII) and alpha2(VIII), but the chain composition of type VIII collagen is far from resolved. To address this question, we have expressed full-length alpha1(VIII) and alpha2(VIII) chains in an in vitro translation system supplemented with semipermeabilized cells. Both chains gave a translation product of approximately 80 kDa that could be shown to produce a chymotrypsin/trypsin-resistant product of approximately 60 kDa, indicating that both chains could form homotrimers. Hydroxylation of proline residues was a prerequisite for stable trimer formation. The melting temperature for the alpha1(VIII) homotrimer was 45 degreesC, whereas that for alpha2(VIII) was 42 degreesC. The ability of both chains of type VIII collagen to form stable triple helices suggests that there may be different forms of this collagen and that cells may modulate the chain composition in response to different biological conditions.

Type VIII collagen

Type VIII collagen is a product of endothelial cells, keratinocytes, mast cells, microvascular endothelial cells and some tumour cells. It is also present in a variety of extracellular matrices as diverse as sclera, skin and glomerulus. Type VIII molecules have a proposed chain composition of [alpha 1(VIII)2 alpha 2(VIII)]. While the function of collagen type VIII is uncertain recent work has highlighted the importance of this collagen in the vasculature. Particularly significant may be its up-regulation in smooth muscle cell migration and potential role in maintaining the smooth muscle cell phenotype. It is interesting to speculate that this collagen may provide a substratum for a variety of cells and facilitate movement of endothelial cells in angiogenesis, smooth muscle cells in intimal invasion and myofibroblasts in fibrotic conditions.

Generation of type VIII collagen-specific antibodies

We generated a specific polyclonal antibody against the alpha 1 chain of type VIII collagen. The antibody detects type VIII collagen and is definitely free of crossreactivities with the closely related type X collagen. The antibody was generated against a dodecamer peptide chosen to satisfy the following requirements: (a) maximal homology between collagen type VIII molecules from different species; (b) maximal antigenicity as predicted by algorithms from Emini et al. (J. Virol. (1985) 55, 836). Hoop and Woods (Proc. Natl. Acad. Sci. USA (1981) 78, 3824), and Karplus and Schulz (Naturwissenschaften (1985) 72, 212); and (c) maximal specificity, i.e. absence of this sequence in all other proteins known so far. All three requirements were satisfied for a sequence fragment of 12 amino acids (100-111) in the alpha 1(VIII) NC2 domain. This peptide was produced synthetically. Polyclonal antibodies were raised in rabbits and affinity purified on a peptide column. The antibody was tested in a quantitative EIA, immunoblots and in immunocytochemistry and found to be well-suited for all three types of application. The antibody did not crossreact with type X collagen and other extracellular matrix molecules in the EIA. In immunoblots of affinity-purified extracts of the Descemet membrane, a major source of type VIII collagen, the antibody detected several known forms of type VIII collagen. In immunocytochemistry the antibody stained endothelial and astrocytoma cells in monolayer cultures, and cells and extracellular matrix in cryosections of the human Ewing sarcoma, arterial vessels and chicken embryonic heart, whereas the chicken tibiotarsus remained negative. This distribution of immunoreactivity corresponds to the distribution of type VIII but not that of type X collagen. In conclusion this antibody may serve as a highly specific and sensitive tool for investigating the appearance and regulation of type VIII collagen.

Cytological and immunocytochemical approaches to the study of corneal endothelial wound repair

The vertebrate corneal endothelium represents a unique model system for investigating many cellular aspects of wound repair within an organized tissue in situ. The tissue exists as a cell monolayer that resides upon its own natural basement membrane that can be prepared as a flat mount to observe the entire cell population. Thus, it readily avails itself to many cytological and immunocytochemical methods at both the light microscopic and ultrastructural levels. In addition, the tissue is easily explanted into organ culture where further investigations can be carried out. These techniques have enabled investigators to use many approaches to explore function and changes in response to injury. In vivo, the endothelium acts as a transport tissue to actively pump Na+ and bicarbonate ions from the corneal stroma into the aqueous humor to control corneal transparency. Physiological findings indicate that fluid diffuses back into the stroma, across the endothelium, and thus hydration is said to be controlled by a pump-leak mechanism. Ultrastructural investigations, some employing horseradish peroxidase and lanthanum, have established the morphological basis for this mechanism as apical focal junctions that are not the classical tight junctions and do not constitute a complete zona occludens. Along with these apical focal junctions are gap junctions that appear identical to their counterparts in other cell types. Cytochemical studies localized both Na+K(+)-ATPase and carbonic anhydrase, the main pump enzymes associated with corneal hydration, to the lateral plasma membranes. Corneal endothelial cells of noninjured tissue do not traverse the cell cycle and are considered to be in the "Go" phase of the cell cycle as determined by microfluorometric analysis with DNA binding dyes such as auramin O and pararosaniline-Feulgen. However, injury can initiate cell cycle transverse and histochemical and cytological methods have been used to understand the tissue's response. Classical histochemical studies revealed that increased staining was observed for metabolic (NADase and NADPase) and lysosomal enzymes in cells bordering the wound area. The use of radiolabelled agents has further lead to an understanding of the endothelial wound response. Autoradiographic analyses of 3H-actinomycin D incorporation indicated that injury initiates changes in chromatin leading to increased binding levels of the drug in cells surrounding the wound. This change suggests that those cells undergo heightened macromolecular synthesis and this was confirmed by examining 3H-uridine and 3H-thymidine incorporation. The major mechanism involved in corneal endothelial repair is cell migration. Cytochemical and immunocytochemical investigations have allowed investigators an opportunity to gain some insight into changes that occur during this cellular process.(ABSTRACT TRUNCATED AT 400 WORDS)

Wound healing in response to keratorefractive surgery

Over one million Americans have undergone refractive keratoplasty since the introduction of radial keratotomy into the United States in 1978. There are now a number of alternative techniques available for reshaping the corneal surface to alter ocular refractive errors. Numerous technologic advances in the past decade now enable us to perform these procedures in a safer and more reliable fashion. The ability to control precisely the refractive outcome, however, continues to elude us and appears to be limited, in part, by interindividual variability in the wound healing response. Presently, we review the corneal wound healing response to various keratorefractive approaches and suggest some interventional strategies which might enable us to modulate more precisely our refractive results.

Angiogenic oligosaccharides of hyaluronan enhance the production of collagens by endothelial cells

The present study demonstrates a relationship between angiogenic oligosaccharides of hyaluronan (HA) and the production of collagens during the process of angiogenesis in vivo and in vitro. The addition of angiogenic oligosaccharides of HA to the chorioallantoic membrane of the chick embryo induced a deposition of collagen fibrils. The treatment of sub-confluent cultures of bovine aortic endothelial cells with the same oligosaccharides (1 microgram/ml) increased the uptake of [3H]proline by approximately 60%. SDS-polyacrylamide gel electrophoresis of treated cultures demonstrated the enhanced synthesis of type I and type VIII collagens. The production of type VIII collagen was confirmed by western blotting and immunocytochemistry using antibodies to sheep and bovine type VIII collagen. Type VIII collagen is a short chain collagen that has a high degree of homology to cartilage-specific type X collagen. The biological functions of type VIII and type X collagens are unknown. We have suggested that the two collagens play a role in the process of angiogenesis.

Cleavage of type VIII collagen by human neutrophil elastase

In this report, the susceptibility of type VIII collagen to human neutrophil elastase is compared to other extracellular matrix components. Type X collagen is degraded to specific fragments at a substrate to enzyme ratio of 5:1 after 20 h at room temperature, but type VIII collagen is almost completely degraded after only 4 h incubation at a substrate to enzyme ratio of 50:1 and partly degraded after only 15 min. Laminin, merosin and types I, III, IV and V collagen exhibit no susceptibility to neutrophil elastase under the latter conditions, while fibronectin is degraded.

alpha 1(VIII)-collagen gene transcripts encode a short-chain collagen polypeptide and are expressed by various epithelial, endothelial and mesenchymal cells in newborn mouse tissues

Type-VIII collagen is a major constituent of Descemet's membrane and contains two genetically distinct alpha chains, alpha 1(VIII) and alpha 2(VIII). We have previously cloned the human alpha 1(VIII) and alpha 2(VIII) genes and determined that they are located on chromosomes 3 and 1, respectively. Comparison of the alpha 1(VIII) and alpha 2(VIII) genes with the alpha 1(X)-collagen gene showed that the structure of the three genes and the sequence of their collagen polypeptides were strikingly similar. Therefore, we have grouped the three genes in a common subclass of collagens which we have named the short-chain collagens because of the relatively small size of their triple-helical domains. In the present study, we have isolated and characterized a mouse gene fragment encoding the entire mouse alpha 1(VIII)-collagen chain and determined the complete primary structure of the polypeptide chain. The size of mouse alpha 1(VIII)-collagen mRNA, as estimated by Northern-blot analysis, was 4.2 kb, compared with 2.8 kb previously reported for the corresponding rabbit mRNA. By cloning and sequencing of four overlapping cDNA, we demonstrate that this larger size of mouse alpha 1(VIII) mRNA is due to a larger 3' untranslated region in the mouse transcript. Using the gene fragment as a probe, we performed Northern-blot hybridization analysis of RNA prepared from newborn mice and demonstrated that alpha 1(VIII) collagen mRNA is expressed at high levels in the calvarium, eye and skin. In situ hybridization revealed that alpha 1(VIII) RNA is present in skin keratinocytes, corneal epithelial and endothelial cells, lens epithelial cells, as well as mesenchymal cells surrounding cartilage and calvarial bone and in the meninges surrounding the brain.

Identification and partial characterisation of a low Mr collagen synthesised by bovine retinal pericytes. Apparent relationship to type X collagen

Bovine retinal pericytes (BRP) in culture synthesise a low Mr collagenous polypeptide which appears similar, but not identical, to bovine type X collagen and which we have called 'BRP collagen'. This polypeptide displays the following characteristics: (i) it is sensitive to digestion by bacterial collagenase and is resistant to pepsin digestion; (ii) it has an apparent Mr of 45 kDa (pepsinised form); (iii) it is recognised by specific antibodies to type X collagen using immunoblotting; (iv) it is present in the cell layer/matrix but not in the medium of pericyte cultures; and (v) it is not disulphide-bonded into higher Mr multimers. The latter two properties distinguish BRP collagen from bovine type X collagen. We have recently shown that pericytes calcify in vitro. We now report that this calcification is associated with an increased synthesis of BRP collagen.

Synthesis of type VIII collagen by epithelial cells of human gingiva

The composition of the extracellular matrix produced by epithelial cells in contact with metabolically inert substrata was studied with antibodies specific for type VIII collagen. Type VIII collagen was present in the cell layer of cultured gingival epithelial cells, and at the epithelium-substratum interface of an explant culture model for junctional epithelium. Samples of the epithelial attachment apparatus (EAA) in vivo were collected by removing the junctional cells directly attached to the tooth (DAT cells) and the associated EAA matrix. The amount of material collected was sufficient for biochemical analysis of both intra- and extracellular components of the junctional cell-EAA complex. Immunological examination of the samples revealed that type VIII collagen was associated with epithelial cells forming the EAA in vivo. We suggest that this collagen type functions in the extracellular space as an attachment-promoting factor for gingival epithelial cells at the tooth surface.

Expression of type VIII collagen during morphogenesis of the chicken and mouse heart

The expression of type VIII collagen is restricted, in adult mammals, to specialized extracellular matrices and to a select subset of blood vessels. We have examined the distribution of type VIII collagen in sequential stages of mouse and chicken embryos and found a temporal and spatially restricted pattern of expression during cardiogenesis. Type VIII collagen was first detected by immunocytochemistry on Day 11 in the developing mouse embryo and at stage 19 in the chicken embryo. The distribution of this protein was rapidly modulated during cardiac morphogenesis. Initially (Day 11 in the mouse embryo), type VIII collagen was associated with cardiac myoblasts. From Days 15 to 18, the immunoreactive component was progressively diminished in the myocardium; however, this collagen was observed in the subendocardial layer of the atrioventricular canal and later in the cardiac jelly (or the myocardial basement membrane, an area associated with the formation of cardiac valves). On Day 17, type VIII collagen was also detected in the subendothelium (intima) and tunica media of large vessels. Neonatal and adult hearts contained low to undetectable levels of type VIII collagen. The presence of type VIII collagen was confirmed by immunoblot analysis of heart extracts at different stages of development. A major 185-kDa component, as well as polypeptides of 68 and 15 kDa, reacted with anti-type VIII collagen IgG. Exposure of heart extracts to hyaluronidase or reducing agent eliminated immunoreactivity of the 185-kDa component but not that of the 68- and 15-kDa polypeptides. Type VIII collagen therefore appears to be associated with a hyaluronidase-sensitive component of the extracellular matrix during a temporally restricted stage of embryonic cardiogenesis. The contribution of this collagen to cardiac morphogenesis might reside, in part, in its ability to influence the differentiation of the myocardium and formation of the cardiac valves.

The primary structure of a triple-helical domain of collagen type VIII from bovine Descemet's membrane

We have isolated and sequenced a fragment of 469 amino acid residues from bovine type VIII collagen. The sequence was composed of a series of Gly-X-Y repeats which was interrupted 8 times by short imperfections. The number and relative location of these interruptions were similar to those of chicken alpha 1(X) and rabbit alpha 1(VIII) chain triple-helical domains. Comparison to published N-terminal sequences to two triple-helical fragments of bovine type VIII collagen and to the cDNA derived sequence of the rabbit alpha 1(VIII) chain showed that this fragment was the triple-helical domain of a second type VIII collagen chain which we designate alpha 2(VIII).

Characteristics and in vivo occurrence of type VIII collagen

Type VIII collagen was isolated from bovine Descemet's membranes by pepsin treatment and salt fractionation, as described by Kapoor et al. [(1986) Biochemistry 25, 3930-3937]. Contaminating type IV collagen was removed by ion-exchange chromatography. Purified type VIII collagen consisted of two different polypeptide chains and, compared to the fiber forming collagens, showed a higher thermal stability. Corresponding fractions isolated from pepsinized human Ewing's sarcoma and fetal calf aorta reacted immunologically with a protein of similar molecular mass. After extraction of Descemet's membranes with guanidine hydrochloride, a peptide of about 60 kDa was obtained. This seems to be the tissue form of type VIII collagen.

Type VIII collagen in murine development. Association with capillary formation in vitro

Bovine endothelial and human astrocytoma cells, and a limited number of other normal and malignant cells, synthesize three chains that have been identified as type VIII collagen (180 kDa, 125 kDa, and 100 kDa). Digestion with pepsin converts these forms to major fragments of 65 kD (based on globular protein standards). In this study we have examined the structure and distribution of type VIII collagen in developing mice by immunohistological and immunoblotting techniques. Temporal and tissue-specific expression was observed in embryonic heart, cranial mesenchyme, and placental capillaries. Western blotting of embryonic and neonatal tissues showed major species of 125 and 65 kDa in the brain, placenta, heart, lung, and thymus. The predominant band in pepsin-treated tissues was 60-70 kDa, with additional forms of 250 and 150 kDa in neonatal heart and lung. Type VIII collagen was also synthesized by endothelial cells, forming capillary tubes in vitro. We suggest that type VIII collagen functions in cellular organization and differentiation, and that its various forms reflect not only tissue-specific processing but the presence of several related chains.