The presence of EC collagen and type IV collagen in bovine Descemet's membranes

Focus on seed cells: stem cells in 3D bioprinting of corneal grafts

Corneal opacity is one of the leading causes of severe vision impairment. Corneal transplantation is the dominant therapy for irreversible corneal blindness. However, there is a worldwide shortage of donor grafts and consequently an urgent demand for alternatives. Three-dimensional (3D) bioprinting is an innovative additive manufacturing technology for high-resolution distribution of bioink to construct human tissues. The technology has shown great promise in the field of bone, cartilage and skin tissue construction. 3D bioprinting allows precise structural construction and functional cell printing, which makes it possible to print personalized full-thickness or lamellar corneal layers. Seed cells play an important role in producing corneal biological functions. And stem cells are potential seed cells for corneal tissue construction. In this review, the basic anatomy and physiology of the natural human cornea and the grafts for keratoplasties are introduced. Then, the applications of 3D bioprinting techniques and bioinks for corneal tissue construction and their interaction with seed cells are reviewed, and both the application and promising future of stem cells in corneal tissue engineering is discussed. Finally, the development trends requirements and challenges of using stem cells as seed cells in corneal graft construction are summarized, and future development directions are suggested.

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.

Descemet's membrane development, structure, function and regeneration

Basement membranes are layers of extracellular matrix which anchor the epithelium or endothelium to connective tissues in most organs. Descemet's membrane- which is the basement membrane for the corneal endothelium- is a dense, thick, relatively transparent and cell-free matrix that separates the posterior corneal stroma from the underlying endothelium. It was historically named Descemet's membrane after Jean Descemet, a French physician, but it is also known as the posterior limiting elastic lamina, lamina elastica posterior, and membrane of Demours. Normal Descemet's membrane ultrastructure in humans has been shown to consist of an interfacial matrix that attaches to the overlying corneal stroma, an anterior banded layer and a posterior non-banded layer-upon which corneal endothelial cells attach. These layers have been shown to have unique composition and morphology, and to contribute to corneal homeostasis and clarity, participate in the control of corneal hydration and to modulate TGF-β-induced posterior corneal fibrosis. Pathophysiological alterations of Descemet's membrane are noted in ocular diseases such as Fuchs' dystrophy, bullous keratopathy, keratoconus, primary congenital glaucoma (Haab's striae), as well as in systemic conditions. Unrepaired extensive damage to Descemet's membrane results in severe corneal opacity and vision loss due to stromal fibrosis, which may require penetrating keratoplasty to restore corneal transparency. The purpose of this article is to highlight the current understanding of Descemet's membrane structure, function and potential for regeneration.

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.

Ex Vivo Functionality of 3D Bioprinted Corneal Endothelium Engineered with Ribonuclease 5-Overexpressing Human Corneal Endothelial Cells

Human corneal endothelial cells (HCECs) are scarcely proliferative in vivo. The cultured HCECs engineered to overexpress ribonuclease (RNase) 5 (R5-HCECs) are prepared after transient transfection with RNase 5 plasmid vector. As candidate targets of R5-HCECs for enhancement of cellular proliferation and survival of R5-HCECs, programmed cell death protein 4 is inhibited, and cyclin D1 and cyclin E1 are activated. The cultured R5-HCECs and control HCECs on lyophilized amniotic membrane (AM) are deposited as a carrier by extrusion-based 3D bioprinting to prepare transplantable RNase 5 vector-transfected HCECs-laden AM graft (R5-Graft) and the control HCECs-laden AM graft (Ct-Graft), respectively. The ready-to-use R5-Graft shows clearer basolateral expression of Na⁺ -K⁺ ATPase pump and higher cell confluency than Ct-Graft. From 2 weeks after graft transplantation, both R5-Graft and Ct-Graft start restoring clarity of the rabbit corneas, and their central corneal edema are much less than those in the control group at 3 and 4 weeks. The ex vivo expression of corneal endothelial phenotypical markers is clear in R5-Grafs rather than in Ct-Grafts at 4 weeks. In conclusion, the fabricated corneal endothelium with cultured HCECs easily survives and functions as corneal endothelium in vivo. Furthermore, the use of the cultured HCECs engineered to overexpress RNase 5 (R5-HCECs) may be an option to obtain higher graft cellularity and to enhance the function of transplanted grafts.

Altered expression of matrix metalloproteinases and their tissue inhibitors as possible contributors to corneal droplet formation in climatic droplet keratopathy

PURPOSE

Climatic droplet keratopathy (CDK) is an acquired corneal disease characterized by progressive scarring of the cornea. In several corneal diseases, matrix metalloproteinases (MMPs) are upregulated during the degradation of epithelial and stromal tissues. We investigated the levels, degree of activation and molecular forms of MMP-2, MMP-9, MMP-8 and MMP-13 and their tissue inhibitors TIMP-1 and TIMP-2 in tear fluid of patients with CDK.

METHODS

Seventeen CDK patients and 10 controls living in Argentine Patagonia received a complete eye examination, and MMPs and TIMP-1/2 were determined by immunofluorometric assay (IFMA), gelatin zymography and quantitative Western immunoblot analysis in tear samples.

RESULTS

The MMPs were detected mostly in their latent forms. The levels of MMP-9 and MMP-2 were found to be significantly elevated in CDK patients, whereas latent and active MMP-8 levels were significantly enhanced in controls. There was no significant difference in the level of MMP-13. TIMPs were found as part of complexes, and the TIMP-1 levels were significantly lower in patients than controls.

CONCLUSION

Elevated MMP-2 and MMP-9 levels have been implicated in the failure of corneal re-epithelialization, and enhanced MMP-2 and MMP-9 levels in CDK patients suggest that these MMPs may play a role in corneal scarring in CDK. Elevated levels of MMP-8 suggest a defensive role for this MMP in inflammatory reactions associated with recurring corneal traumas. Decreased expression of TIMP-1 in CDK patients suggest deficient antiproteolytic shield likely to render the corneas of CDK patients vulnerable to enhanced MMPs. Overall, these data suggest a mechanistic link between MMPs and TIMP-1 level in cornea and tears with corneal scarring in CDK.

Essential role for the alpha 1 chain of type VIII collagen in zebrafish notochord formation

Several zebrafish mutants identified in large-scale forward genetic screens exhibit notochord distortion. We now report the cloning and further characterization of one such mutant, gulliver(m208) (gul(m208)). The notochord defect in gul(m208) mutants is exacerbated under conditions of copper depletion or lysyl oxidase cuproenzyme inhibition that are without a notochord effect on wild-type embryos. The gul(m208) phenotype results from a missense mutation in the gene encoding Col8a1, a lysyl oxidase substrate, and morpholino knockdown of col8a1 recapitulates the notochord distortion observed in gul(m208) mutants. Of interest, the amino acid mutated in gul(m208) Col8a1 is highly conserved, and the equivalent substitution in a closely related human protein, COL10A1, causes Schmid metaphyseal chondrodysplasia. Taken together, the data identify a new protein essential for notochord morphogenesis, extend our understanding of gene-nutrient interactions in early development, and suggest that human mutations in COL8A1 may cause structural birth defects.

Development of the corneal stroma, and the collagen-proteoglycan associations that help define its structure and function

The cornea of the eye is a unique, transparent connective tissue. It is comprised predominantly of collagen fibrils, remarkably uniform in diameter and regularly spaced, organized into an intricate lamellar array. Its establishment involves a precisely controlled sequence of developmental events in which the embryonic cornea undergoes major structural transformations that ultimately determine tissue form and function. In this article, we will review corneal developmental dynamics from a structural perspective, consider the roles and interrelationships of collagens and proteoglycans, and comment on contemporary concepts and current challenges pertinent to developmental processes that result in an optically clear, mature cornea.

Advanced glycation end products in Descemet's membrane and their effect on corneal endothelial cell

PURPOSE

[corrected] The purpose of this study was to evaluate the effect of advanced glycation end products (AGEs) in Descemet's membrane on the attachment and spreading of the corneal endothelial cells.

METHODS

An anti-AGEs monoclonal antibody (6D12), which recognizes a N(epsilon)-carboxymethyl lysine (CML)-protein adduct as an epitope, was used for immunohistochemistry and enzyme-linked immunosorbent assay (ELISA). Fresh bovine Descemet's membrane was incubated for 4 weeks in the buffered solution with 500 mM of glucose-6-phosphate (G-6-P). In the incubated Descemet's membrane, the immunohistochemical localization of CML was examined. Type I collagen-, type IV collagen-, fibronectin-, or laminin-coated 96-well plates were glycated by G-6-P. The amount of CML was determined by ELISA using 6D12. Cultured bovine corneal endothelial cells were seeded onto glycated or non-glycated extracellular matrix (ECM) in 96-well plates and allowed to attach for 3 hours. The number and the surface area of the attached cells were examined.

RESULTS

Immunoreactivity to CML was detected in Descemet's membrane incubated in the buffered solution containing G-6-P. Glycation of fibronectin and laminin decreased the number and the surface area of the attached corneal endothelial cells. Aminoguanidine in the incubation mixture inhibited CML formation of ECM components and increased the number and the surface area of the attached corneal endothelial cells in a dose-dependent manner.

CONCLUSIONS

AGE formation on fibronectin and laminin attenuated the attachment and spreading of the corneal endothelial cells. AGEs' formation in Descemet's membrane may be responsible for the corneal endothelial cell loss with aging and corneal endothelial abnormalities in diabetic patients

Keratan sulphate in the trabecular meshwork and cornea

PURPOSE

A study was made of the distribution of keratan sulphate in the human anterior chamber.

METHODS

The monoclonal antibody, 5-D-4, was used in immuno-electron microscopy to visualise keratan sulphate distribution in the anterior chamber of 16 normal eyes, 7 Fuchs' dystrophy corneas, and a macular dystrophy cornea.

RESULTS

Keratan sulphate was detected in normal human aqueous humour and also on the surface of trabecular cells in the uveal meshwork. Normal corneal stroma showed an increase in keratan sulphate labelling from anterior to posterior, with marked labelling in the posterior region of Descemet's membrane. The apical surface of the corneal endothelium labelled positively, but showed considerable variation in the level of labelling from cell to cell. The macular dystrophy cornea had the classic histopathological features of a type I case, including a highly abnormal Descemet's membrane. No keratan sulphate was detected in the macular dystrophy patient's corneal stroma or serum. The Fuchs' endothelial dystrophy corneas showed a normal distribution of keratan sulphate labelling in the stroma. The Fuchs' endothelial cells labelled for keratan sulphate but were highly abnormal in appearance, often exhibiting long filopodia and appearing to be actively migrating.

CONCLUSIONS

This work has shown that keratan sulphate has a much wider distribution than was previously believed. The detection of keratan sulphate on the trabecular and endothelial cell surfaces also suggests a possible role for this molecule in cell adhesion and/or migration.

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.

Increased levels of type VIII collagen in human brain tumours compared to normal brain tissue and non-neoplastic cerebral disorders

The expression of type VIII collagen was examined in the normal and diseased human brain. Focal immunoreactivity was seen in histologically abnormal vessels of all four angiomas and 40 of 52 brain tumours (gliomas, meningiomas and schwannomas). An extended staining pattern, as well as a punctate distribution, was frequently observed in affected vessels. Staining was not apparent in nine normal brains and in 15 pathologic brains showing various cerebrovascular abnormalities, including Alzheimer's, Leigh's and Wernicke's diseases. Immunoblotting of glioblastomas revealed two bands at 56 kD and 67 kD which were also present at low levels in normal frontal cortex. The extracellular distribution of type VIII collagen was different from that of the other collagen types which have been described in brain and resembles patterns of expression described for certain tissues during mammalian embryogenesis (Kapoor et al., 1988). Our results provide additional evidence for the participation of type VIII collagen in some types of angiogenesis.

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.

The spatial organization of Descemet's membrane-associated type IV collagen in the avian cornea

The organization of type IV collagen in the unconventional basement membrane of the corneal endothelium (Descemet's membrane) was investigated in developing chicken embryos using anti-collagen mAbs. Both immunofluorescence histochemistry and immunoelectron microscopy were performed. In mature embryos (greater than 15 d of development), the type IV collagen of Descemet's membrane was present as an array of discrete aggregates of amorphous material at the interface between Descemet's membrane and the posterior corneal stroma. Immunoreactivity for type IV collagen was also observed in the posterior corneal stroma as irregular plaques of material with a morphology similar to that of the Descemet's membrane-associated aggregates. This arrangement of Descemet's membrane-associated type IV collagen developed from a subendothelial mat of type IV collagen-containing material. This mat, in which type IV collagen-specific immunoreactivity was always discontinuous, first appeared at the time a confluent endothelium was established, well before the onset of Descemet's membrane formation. Immunoelectron microscopy of mature corneas revealed that the characteristic nodal matrix of Descemet's membrane itself was unreactive for type IV collagen, but was penetrated at intervals by projections of type IV collagen-containing material. These projections frequently appeared to contact cell processes from the underlying corneal endothelium. This spatial arrangement of type IV collagen suggests that it serves to suture the corneal endothelium/Descemet's membrane to the dense interfacial matrix of the posterior stroma.

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.

Characterization of the collagen in the hexagonal lattice of Descemet's membrane: its relation to type VIII collagen

To investigate the nature of the hexagonal lattice structure in Descemet's membrane, monoclonal antibodies were raised against a homogenate of bovine Descemet's membranes. They were screened by immunofluorescence microscopy to obtain antibodies that label Descement's membrane. Some monoclonal antibodies labeled both Descemet's membrane and fine filaments within the stroma. In electron microscopy, with immunogold labeling on a critical point dried specimen, the antibodies labeled the hexagonal lattices and long-spacing structures produced by the bovine corneal endothelial cells in culture; 6A2 antibodies labeled the nodes of the lattice and 9H3 antibodies labeled the sides of the lattice. These antibodies also labeled the hexagonal lattice of Descemet's membrane in situ in ultrathin frozen sectioning. In immunofluorescence, these antibodies stained the sclera, choroid, and optic nerve sheath and its septum. They also labeled the dura mater of the spinal cord, and the perichondrium of the tracheal cartilage. In immunoblotting, the antibodies recognized 64-kD collagenous peptides both in tissue culture and in Descemet's membrane in vivo. They also recognized 50-kD pepsin-resistant fragments from Descemet's membranes that are related to type VIII collagen. However, they did not react either in immunoblotting or in immunoprecipitation with medium of subconfluent cultures from which type VIII collagen had been obtained. The results are discussed with reference to the nature of type VIII collagen, which is currently under dispute. This lattice collagen may be a member of a novel class of long-spacing fibrils.

Immunolocalization of type VIII collagen in vascular tissue

Type VIII collagen was first detected in the culture medium of aortic endothelial cells. Subsequently its synthesis by a number of other cell lines was demonstrated. Its presence in vascular tissue is reported here. It is a component of sheep aorta and carotid artery but could not be demonstrated in the jugular vein. It is principally localized in the subendothelial region but this can only be demonstrated after pretreatment of the tissue with proteases. Thus type VIII collagen is a constituent of blood vessels.

Type VIII collagen has a restricted distribution in specialized extracellular matrices

A pepsin-resistant triple helical domain (chain 50,000 Mr) of type VIII collagen was isolated from bovine corneal Descemet's membrane and used as an immunogen for the production of mAbs. An antibody was selected for biochemical and tissue immunofluorescence studies which reacted both with Descemet's membrane and with type VIII collagen 50,000-Mr polypeptides by competition ELISA and immunoblotting. This antibody exhibited no crossreactivity with collagen types I-VI by competition ELISA. The mAb specifically precipitated a high molecular mass component of type VIII collagen (EC2, of chain 125,000 Mr) from the culture medium of subconfluent bovine corneal endothelial cells metabolically labeled for 24 h. In contrast, confluent cells in the presence of FCS and isotope for 7 d secreted a collagenous component of chain 60,000 Mr that did not react with the anti-type VIII collagen IgG. Type VIII collagen therefore appears to be synthesized as a discontinuous triple helical molecule with a predominant chain 125,000 Mr by subconfluent, proliferating cells in culture. Immunofluorescence studies with the mAb showed that type VIII collagen was deposited as fibrils in the extracellular matrix of corneal endothelial cells. In the fetal calf, type VIII collagen was absent from basement membranes and was found in a limited number of tissues. In addition to the linear staining pattern observed in the Descemet's membrane, type VIII collagen was found in highly fibrillar arrays in the ocular sclera, in the meninges surrounding brain, spinal cord, and optic nerve, and in periosteum and perichondrium. Fine fibrils were evident in the white matter of spinal cord, whereas a more generalized staining was apparent in the matrices of cartilage and bone. Despite attempts to unmask the epitope, type VIII collagen was not found in aorta, kidney, lung, liver, skin, and ligament. We conclude that this unusual collagen is a component of certain specialized extracellular matrices, several of which are derived from the neural crest.

Comparative studies of collagens in normal and keratoconus corneas

In this paper we present strong evidence that the aberrations in keratoconus corneas are not directly related to alterations in collagen composition and distribution. This conclusion is based on comparative studies of collagen types I, III, IV, V and the recently described collagen types VI and VII in keratoconus and normal corneas. The data are derived from biochemical analysis of collagen fractions sequentially extracted with pepsin and sodium-dodecylsulphate, from amino acid analysis of hydrolysates of entire corneal tissues as well as from immunoblotting of the extracted collagens with specific antibodies. These antibodies were also used to examine the distribution of the collagens in immunofluorescence experiments on corneal sections. The yields of the collagen extractions were demonstrated to be age dependent but were not altered in keratoconus samples. Apart from one case associated with osteogenesis imperfecta type I, comparative studies of keratoconus and normal corneas showed no differences in collagen composition of the extracts. This was confirmed by amino acid analysis of tissue-hydrolysates. The distributions of collagen types I, III, IV, V, VI and VII in corneal sections were in general unchanged in keratoconus corneas, the only differences being in scar tissues observed in the Bowman layer of some keratoconus samples.

Modulation of rabbit keratocyte production of collagen, sulfated glycosaminoglycans and fibronectin by retinol and retinoic acid

This study elucidates the biochemical response of rabbit corneal keratocytes (fibroblasts) to retinol and retinoic acid in their production of collagen, fibronectin, sulfated glycosaminoglycans, collagenase, and [3H]thymidine incorporation. The morphologic appearance of cultured keratocytes was not altered by retinoid treatment. Collagen production and [3H]thymidine incorporation demonstrated a parallel decline in response to retinoids. Collagen type was unaffected as was collagenase activity. Retinoids had minimal effect on cell layer-associated 35S-labeled glycosaminoglycans, however medium-soluble 35S-glycosaminoglycans were increased. The most dramatic effect was in fibronectin synthesis which was increased 2-3-fold. These data demonstrate that rabbit keratocytes alter their biosynthesis of extracellular matrices in response to retinoids. This may be significant in corneal pathology, since the delicate balance of these extracellular macromolecules is responsible for corneal integrity and stability.

The collagens of the developing bovine cornea

The morphology of the developing bovine eye has been examined and the collagens in fetal bovine eyes from three months' gestation to maturity have been solubilized by pepsin treatment and analyzed to determine the ratios of the predominant types of collagen. The type I collagen decreased, while the type V collagen increased with age. Type III collagen comprised less than 1% of all the corneas, except for the three-month fetal calf. The anterior to posterior thickness of the paraffin-embedded fetal calf cornea increased from the third to the seventh month, decreased from the seventh month to birth, and then increased after birth. Descemet's membrane increased in thickness with age. Analysis of dissected regions of the calf cornea showed a uniform distribution of the collagen populations from the center to the limbus (89% type I, 10% type V and less than 1% type III collagen) and uniformity through the depth of the stroma, except that type III was concentrated around Bowman's layer, and type IV in Descement's membrane. The localization of the different collagens was consistent with the immunofluorescent staining studies with anticollagen antibodies, but the ratios of the intensities of the fluorescence did not correspond to the quantitative analyses. These results are concordant with other studies that have shown that antibody binding may be masked or diminished in certain tissues and therefore immunofluorescence cannot be used reliably for quantitative measurements.

Characterization of the Descemet's membrane/posterior collagenous layer isolated from Fuchs' endothelial dystrophy corneas

The combined Descemet's membrane (DM) and posterior collagenous layer (PCL) of Fuchs' endothelial dystrophy corneas were isolated and characterized by biochemical and immunofluorescence methods. The amino acid composition of the Fuchs' DM-PCL was similar to age-matched normal Descemet's membranes (DM). As determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and 125I two-dimensional peptide mapping, normal DM and Fuchs' DM-PCL contained the same collagen types [type IV and endothelial cell (EC) collagen], but a slight discrepancy was seen in the electrophoretic mobility of some collagen chains. Immunofluorescence staining localized fibrinogen/fibrin to Fuchs' DM-PCL but not to normal DM. These data suggest that the appearance of 110 nm banded material in sheets and fusiform bundles characteristic of Fuchs' PCL is not due to the presence of a new (abnormal) collagen type but may represent altered assembly of collagen molecules, and that the fibrinolytic system may play a role in the degenerative process of Fuchs' endothelial dystrophy.