Structure and biosynthesis of basement membranes

Immunhistochemical demonstration of different collagen types in the normal epiphyseal plate and in benign and malignant tumors of bone and cartilage

Several benign and malignant tumors of bone and cartilage were examined by means of type-specific collagen antibodies in connection with indirect immunofluorescence technique in order to determine wether there is a positive correlation between cell morphology and gene expression as refered to the synthesis of tissue- or cell-specific collagen. In general benign bone and cartilage tumors show the collagen type corresponding to the original maternal tissue. In malignant osteogenic tumors a strong positive correlation was found between morphologic differentiation of osteosarcoma cells and tissue specific collagen synthesarcomas. Unrelated to the grade of differentiation and the type of malignant tumor, collagen type III could be demonstrated in all tumors investigated, occurring rather from vascular stroma than from the tumor cell itself.

Isolation and characterization of rat tubular basement membrane

Tubules were isolated by the passage of coarsely homogenized kidney parenchyma through a series of stainless steel sieves of varying pore sizes. The isolated tubules, which were free of glomeruli or any contaminating tissue, based on light microscopic examination, served as starting material for the isolation of tubular basement membrane (TBM). The tubules were sonically disrupted, and the subsequent isolation procedure was conceptually similar to that of the isolation of glomerular basement membrane (GBM). The chemical composition of TBM was compared to that of GBM and collagen. 3-hydroxyproline, a marker for basement membrane protein, was present in higher amount in TBM (10 residues per 1,000 amino acid residues) than in GBM (7 residues per 1,000). Glycine and hydroxylysine content was also higher in TBM (255 and 27 residues per 1,000, respectively, in contrast to 204 and 18 residues in GBM). The total carbohydrate content was 7.6 mg per 100 mg of dry weight (neutral sugars, 6.3; GlcN, 0.9; GalN, 0.1; and sialic acid, 0.3). By electron microscopic examination, the isolated TBM was similar to GBM, except for greater variability in diameter and serrated appearance of one surface of the tubular basement membrane fragments. The method is suitable for the isolation of TBM in sufficient quantity for subsequent chemical analyses and immunologic studies.

Electron-microscopic histochemistry of the most peripheral retinal vessels

The reaction of Thiéry (periodic acid-thiocarbohydrazide-silver proteinate) has been applied for histochemical analysis of the structures composing the halo of the most peripheral human retinal vessels. It is suggested that glycoproteins lie in the ground substance of the halo as well as well as in the basement membranes of endothelial cells, pericytes, and Müller cells. Glycogen particles were found in endothelial cells, in pericytes and especially in Müller cells.

Mechanisms of the puromycin-induced defects in the transglomerular passage of water and macromolecules

To investigate the mechanism(s) of increased filtration of serum proteins after glomerular injury, polydisperse samples of uncharged [(3)H]dextran (D) or anionic [(3)H]dextran sulfate (DS) were infused into 14 control and 16 puromycin aminonucleoside- (PAN) treated Munich-Wistar rats. Fractional clearances of D or DS ranging in radius from 18 to 42A were determined in these rats, together with direct measurements of the forces governing the glomerular filtration rate of water. Whole kidney and single nephron glomerular filtration rates were approximately 40% lower in PAN-treated rats, relative to controls, due mainly to a marked reduction in the glomerular capillary ultrafiltration coefficient and, to a lesser extent, to a small reduction in glomerular plasma flow rate as well. In PAN-treated rats, as in normal controls, inulin was found to permeate the glomerular capillary wall without measurable restriction, and both D and DS were shown to be neither secreted nor reabsorbed. Fractional clearances of uncharged D were reduced after PAN administration, falling significantly for effective D radii from 22 to 38A. Utilizing a theory based on macromolecular transport through pores, these results indicate that in PAN-treated rats, effective pore radius is the same as in controls, approximately 44A. In PAN nephrosis, however, the ratio of total pore surface area/pore length, a measure of pore density, is reduced to approximately one-third that of control, due very likely to a reduction in filtration surface area. In contrast to the results with uncharged D, fractional clearances of DS were found to increase after PAN administration for all DS radii studied. These results with D and DS suggest that proteinuria in PAN nephrosis is due, not to an increase in effective pore radius or number of pores, but rather to a diminution of the electrostatic barrier function of the glomerular capillary wall, thereby allowing increased passage of polyanions such as DS and albumin.

The ultrastructural histochemistry of the basement membranes of the exocrine pancreas

The basement membranes in the exocrine pancreas were examined by routine electron microscopy, by fixation for electron microscopy with a glutaraldehyde-tannic acid solution and by staining for ultrastructural demonstration of mucosubstances with the dialyzed iron, high iron diamine, ruthenium red and Concanavalin A-horseradish peroxidase procedures. The basement membranes are considered from morphologic and histochemical observations to consist of an inner lamina lucida, an intermediate lamina densa and an outer lamina diffusa. Sulfated mucosubstance was found in the lamina diffusa of the basement membrane of all epithelial cells and capillary endothelial cells but was encountered in the lamina lucida of the duct cells exclusively. Bridging structures, presumably polypeptides, were also seen connecting the lamina densa and the basal plasma membrane in specimens fixed with the glutaraldehydetannic acid solution. The findings demonstrated that the histochemical and morphological qualities of the basement membrane are uniform of a given cell type but differ considerably among the several cell types in the pancreas.

A preliminary characterization of human cementum collagen

Human teeth were used to obtain cementum. Collagen could not be significantly solubilized from the cementum by salt and acetic acid extraction or by pepsin digestion. CNBr digestion (86%) of cementum and subsequent carboxymethyl cellulose chromatography suggests that human cementum consists of type I collagen only as identified by amino acid and hexose analyses.

Composition of the glomerular basement membrane in the congenital nephrotic syndrome of the Finnish type

The composition of the glomerular basement membrane (GBM) was studied in three patients with the congenital nephrotic syndrome of the Finnish type (CNF). A decrease was found in the relative amounts of amino acids characteristic of collagen, i.e. 3-hydroxyproline, 4-hydroxyproline, hydroxylysine and glycine. The decrease of hydroxyproline was also apparent in the ratio of 4-hydroxyproline to proline and in direct assays of 4-hydroxyproline in whole glomeruli. The ratio of 3-hydroxyproline to 4-hydroxyproline was decreased by about 40%. Slight increases were found in the amounts of some amino acids. No significant change was found in the glucose content of the GBM whereas the galactose content was slightly decreased. The results suggest a decrease in the relative amount of the collagen component in the GBM of CNF patients, but a decrease in the relative amount of 3-hydroxyproline indicates that other changes also exist.

[The function of kidney membranes]

Plasma membranes were isolated from rat kidney and their transport properties for sodium, calcium, protons, phosphate, glucose, lactate, and phenylalanine were investigated. The results demonstrate that the luminal plasma membrane, which is in contact with the primary urine, and the contraluminal plasma membrane, which is in contact with the interstitial fluid, differ in their content of ion-sensitive ATPases and sodium-cotransport systems. This allows conclusions on the mechanism of renal transport in which luminal and contraluminal membranes are arranged functionally in series.

Collagen synthesis as a marker for cell type in mouse 3T3 lines

Collagen synthesis was applied as a test of the fibroblastic or endothelial origin of the 3T3 mouse lines. Swiss/3T3 lines and Balb/3T3 (clone A31) were compared with respect to the amounts and types of collagen synthesized. All lines symthesized collagen at relatively high and similar rates. For all lines, 75-90% of the collagen synthesized was type I, and the remainder was type III. There was no evidence for synthesis of type IV collagen. By all these parameters, Swiss and Balb/3T3 lines resemble cultured fibroblasts rather than cultured vascular endothelia.

Basal lamina of embryonic salivary epithelia. Production by the epithelium and role in maintaining lobular morphology

The role of the basal lamina in maintaining the normal morphology of mouse embryo submandibular epithelia was assessed by examining its production as well as the cellular and organ culture changes associated with its removal and replacement. The lamina was removed from epithelia isolated free of mesenchyme by brief treatment with testicular hyaluronidase in the absence of calcium. The treatment causes rounding-up of the cells, loss of cellular cohesion, appearance of microvilli, and changes in the organization of cytoskeletal structures. The lamina is not removed and the cellular alterations do not occur in the absence of hyaluronidase in calcium-free medium or when both enzyme and calcium are present, possibly because digestion of chondroitin sulfate, a component of the lamina, is inhibited by calcium. Within 2 h after treatment, in the absence of mesenchyme or biological substrata, the epithelia deposits a new lamina, which is identical by several criteria to the preexisting lamina, and reverses the cellular alterations. Epithelia treated with hyaluronidase lose lobular morphology during culture with mesenchyme. Delaying culture with mesenchyme, to allow restoration of the lamina and of normal cellular architecture, prevents the loss of lobular morphology. The results indicate that the basal lamina imposes morphologic stability on the epithelium, while the mesenchyme apparently affects processes involved in changes in morphology, possibly by selective degradation of the basal lamina.

The human cutaneous basement membrane--anchoring fibril complex: preparation and ultrastructure

The human cutaneous basement membrane (CBM) has been isolated together with the anchoring fibrils (AF) of the dermis. These structures are removed from the dermis as a single complex, the CBM-AF complex. The epidermis is first removed from the dermis by immersion of whole, defatted skin in cold 2 n NaSCN for 5 to 6 hr. Subsequently, removal of the CBM-AF complex is effected by focusing high-intensity ultrasonic energy against the dermal surface. Purification of the preparation is achieved by low-speed sedimentation. All stages of the process were monitored by electron microscopy, which demonstrated that the morphology of the CBM-AF complex was well preserved and that this complex represents the principal mass component in the system. In addition, our report describes an ultrasound-induced variation in collagen fibril morphology, which we have called 'disordered collagen.' This altered type of fibril lacks periodicity or recognizable banding and has a frayed appearance. Finally, the experiments provide the basis for an analysis of the relative strengths of certain bonds at the dermal--epidermal junction. The strongest of these appears to be the bond between basement membrane and anchoring fibril.

Prolyl 3-hydroxylase: partial characterization of the enzyme from rat kidney cortex

The formation of 3-hydroxyproline was studied with crude rat kidney cortex extract as a source of enzyme and chick embryo tendon protocollagen and procollagen or cartilage protocollagen as a substrate. Synthesis of 3-hydroxyproline was observed with all these substrates and the formation of 3-hydroxyproline ranged up to seven residues per pro-alpha-chain. The highest rate of 3-hydroxylation took place at 20 degrees C and the reaction required Fe2+, O2,2-oxoglutarate and ascorbate. The formation of 3-hydroxyproline was affected by chain length and the conformation of the substrate, in that longer polypeptide chains proved better substrates, while the native triple-helical conformation of protocollagen or procollagen completely prevented the reaction. Formation of 3-hydroxyproline with tendon procollagen as a substrate was not inhibited by antiserum to prolyl 4-hydroxylase or by poly(L-proline) when these substances were used in concentrations which clearly inhibited 4-hydroxyproline formation with tendon protocollagen as a substrate. Furthermore, pure prolyl 4-hydroxylase did not synthesize any 3-hydroxyproline under conditions in which the crude rat kidney cortex enzyme would readily do so. The data thus strongly suggest that prolyl 3-hydroxylase and prolyl 4-hydroxylase are separate enzymes.

Effect of diabetes and insulin on rat renal glomerular protocollagen hydroxylase activities

The effect of diabetes and insulin on the activities of both prolyl hydroxylase (trivial name; proline,2-oxoglutarate dioxygenase, EC 1.14.11.2) and lysyl hydroxylase (trivial name; lysine,2-oxoglutarate dioxygenase, EC 1.14.11.4) in isolated rat renal glomeruli was determined. Three groups of experimental animals were used: age-matched controls, streptozotocin-diabetic, and insulin-treated streptozotocin-diabetic. Using 14C-labeled lysine or proline hydroxylase substrate prepared from chick embryo tibiae, glomerular 17 000 X g supernatant enzyme was incubated in a complete hydroxylating system for 60 and 120 min Lysyl hydroxylase activity was significantly increased in diabetic preparations, but prolyl hydroxylase activity did not differ from control. Administration of insulin to streptozotocin-injected animals completely restored glomerular lysyl hydroxylase to normal levels. The results suggest that the specific elevation of lysyl hydroxylase relates to the biochemical changes contributory to diabetic nephropathy, and that insulin may reverse this process.

A murine tumor producing a matrix of basement membrane

We have studied a murine tumor previously classified as a poorly differentiated chondrosarcoma. Although the cells in this tumor are surrounded by large quantities of extracellular matrix material, the matrix fails to react with stains specific for the sulfated glycosaminoglycans present in normal cartilage. Here we show at the ultrastructural level that the tumor matrix is a homogeneous, nonfibrillar material, resembling basement membrane. Neither the proteoglycan matrix granules nor collagen fibrils characteristic of cartilage are present in the tumor matrix. Amino acid analyses of whole tumor tissue, enzyme-solubilized tumor components, and the protein extracted from lathyritic tumors confirmed that the tumor matrix is a basement membrane collagen. The collagenous protein extracted from the tumor by nonenzymatic means contains three unique polypeptides larger than the alpha-chain components of the other types of collagen. These studies indicate that the tumor is not a type of chondrosarcoma but a basement membrane producing tumor.

The evolution of early fibromuscular lesions hemodynamically induced in the dog renal artery. I. Light and transmission electron microscopy

In view of the important roles of arterial intimal fibromuscular lesions as precursors of atherosclerotic plaque and occlusive lesions in arterial reconstructions, a model has been developed for the rapid hemodynamic induction of these lesions by anastomosis of the dog right renal artery to the inferior vena cava. Light and transmission electron microscopic observations were made on the arterial shunt after periods of rapid flow ranging form 10 minutes to 2 hours to identify initial factor(s) and evolutionary mechanisms in the etiology of the lesions. The sequence of events included aberrations in ruthenium red staining of the endothelial luminal membrane at 10 minutes, multilayered thickening of the subendothelial basement membrane (BM) at 15 minutes, and initial reorientation and migration of smooth muscle cells (SMC) into the intima along with the appearance of areas of degeneration of the internal elastic lamina (IEL) at 30 minutes. The endothelial cells were still intact in some areas overlying the SMC migration and IEL degeneration, but they were separating from the surface in other such areas. As subendothelium became exposed, some platelet adherence was noted. By 2 hours, the entire wall reaction was fully developed. Initial observations indicate that in the evolution of this hemodynamically induced lesion visible alteration in the endothelial cells is not prerequisite to degeneration of the underlying IEL and reorientation and migration of medial SMC.

Fetal membrane collagens: identification of two new collagen alpha chains

Human fetal membranes contain two new genetically distinct collagen polypeptide chains which are subunits of one (or two) new molecular species of collagen. These new polypeptide chains, which we have tentatively named alphaA and alphaB, have been directly compared with the polypeptide chain subunits of Types I, II, and III human collagen and Type IV collagen from bovine lens capsule. Both alphaA and alphaB exhibit characteristic profiles on carboxymethyl-cellulose chromatography and sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The distribution of methionine residues along both new chains is different from known collagen chains as manifest by distinctly different cyanogen bromide peptide profiles on carboxymethyl-cellulose chromatography and/or sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Both alphaA and alphaB exhibit contents of amino acids and glycine typical of collagens, and comparison with the observed and reported compositions of collagen chains of Types I-IV collagens reveals notable differences, particularly in the content of alanine, leucine, isoleucine, and the basic amino acids, lysine, hydroxylysine, and arginine. The new collagen species containing both alphaA and alphaB may be separated in the native (triple-helical) state from other native collagen species by differential salt precipitation. The observations that both chains coprecipitate in the same narrow NaCl range, and that the ratio of alphaA:alphaB is constant, suggest the possibility of a single new species of collagen with a subunit structure alphaA [alphaB]2.

Lipids associated with bovine kidney glomerular basement membranes

1. After incubation of bovine glomeruli with D-[U-14C]glucose, about 21% of the total radioactivity is found in lipid extracts of glomerular basement membranes. 2. The concentration of lipids in glomerular basement membranes (4.3% of dry wt.) is lower than in the residual glomerular particles (10.8% of dry wt.). The concentrations of neutral lipids (13.9%), phospholipids (46.7%) and cholesterol (37.9%) in the total lipid extract of the glomerular basement membranes, however, differ from those in the residual glomerular particles (15.6, 54.0 and 30.9% respectively). Though residual glomerular particles show a higher lipid content, the radioactivity in this fraction only amounts to 38% of that found in the glomerular basement membranes. 3. The specific radioactivity of total glomerular basement-membrane lipids (12 600 d.p.m./mg) is about 4 times as high as that of the glomerular basement membranes. The specific radioactivities of the individual lipid components, however, differ. The highest values are found for phosphatidylcholine and triacylglycerols. The largest proportion of the radioactivity is found in the glycerol of the glycerides. The radioactivity in the fatty acids is much less and does not differ significantly in the various classes of lipids. 4. G.1.c. of methyl esters of the fatty acids does not reveal a clear difference between the fatty acid compositions of glomerular basement membranes and residual glomerular particles. 5. Treatment of glomerular basement-membrane preparations with ultrasound, the generally used procedure for glomerular basement-membrane preparations, drastically decreases the lipid content of glomerular basement membranes. 6. It is concluded that lipids are associated with the basement membranes. Further, the comparatively high radioactive labelling suggests that glomerular basement-membrane lipids may be an interesting class of substances for further pathological studies.

Permselectivity of the glomerular capillary wall. Studies of experimental glomerulonephritis in the rat using dextran sulfate

To determine whether the increased filtration of serum proteins after glomerular injury is the consequence of altered electrostatic properties of the glomerular capillary wall, we measured fractional clearances of the anionic polymer, dextran sulfate, in nine Munich-Wistar rats in the early autologous phase of nephrotoxic serum nephritis (NSN). In agreement with previous studied from this laboratory, whole kidney and single nephron glomerular filtration rates were normal in NSN rats despite histological evidence of glomerular injury, and despite a marked reduction in the glomerular capillary ultrafiltration coefficient to approximately one-third of normal. In the companion study (9), it was shown that in NSN rats the mean fractional clearances of neutral dextrans over the range of effective molecular radii from 18 to 42 A were reduced, compared to normla. In contrast, in the present study the mean fractional clearances for dextran sulfate over the same range of molecular radii were significantly greater than those found previously for normal Munich-Wistar rats. The fractional clearance of dextran sulfate molecules of the same molecular radius as serum albumin (approximately 36 A) was increased markedly, from 0.015 +/- 0.005 (SEM) in nonnephritic controls to 0.24 +/- 0.03 in NSN (P less than 0.001). The sialoprotein content of glomeruli, estimated by the colloidal iron reaction, was reduced in NSN rats as compared to normal controls. It is concluded that the abnormal filtration of anionic serum proteins, such as albumin, seen in glomerulopathies is, at least in part, the consequence of loss of fixed negative charges from the glomerular capillary wall.

Molecular forms of acetylcholinesterase from Torpedo californica: their relationship to synaptic membranes

The 16S and 8S forms of acetylcholinesterase (AchE), which are composed of an elongated tail structure in addition to the more globular catalytic subunits, were extracted and purified from membranes from Torpedo californica electric organs. Their subunit compositions and quaternary structures were compared with 11S lytic enzyme which is derived from collagenase or trypsin treatment of the membranes and devoid of the tail unit. Upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the absence of reducing agent, appreciable populations of monomeric through tetrameric species are observed for the 11S form. Under the same conditions, the 16S form yields only monomer and dimer in addition to a higher molecular weight species. If complete reduction is effected, only the 80,000 molecular weight monomer is dominant for both the 11S and 16S forms. Cross-linking of the 11S form by dimethyl suberimidate followed by reduction yields monomer through tetramer in descending frequency, while the 16S form again shows a high molecular weight species. A comparison of the composition of the 11S and 16S forms reveals that the latter has an increased glycine content, and 1.1 and 0.3 mol % hydroxyproline and hydroxylysine, respectively. Collagenases that have been purified to homogencity and are devoid of amidase and caseinolytic activity, but active against native collagen, will convert 16S acetylcholinesterase to the 11S form. Thus, composition and substrate behavior of the 16S enzyme are indicative of the tail unit containing a collagen-like sequence. A membrane fraction enriched in acetylcholinesterase and components of basement membrane can be separated from the major portion of the membrane protein. The 16S but not the 11S form reassociates selectively with this membrane fraction. These findings reveal distinct similarities between the tail unit of acetylcholinesterase and basement membrane components and suggest a primary association of AchE with the basement membrane.

The resolution of Ascaris cuticle collagen into three chain types

Reduced and methylated collagen from Ascaris lumbricoides cuticle was resolved into three major components by chromatography on phosphocellulose. The components have similar molecular weights of about 52,000 by sedimentation equilbrium and molecular sieve chromatography, but they have different amino acid compositions. Since they do not appear to be stoichiometrically related, they apparently represent chains from collagens of more than one type. All three chains contain about 27 residue % glycine, 36 residues of proline, and 17 residues of methylcysteine, suggesting that the collagens can be maximally about 80% triple helical and are extensively disulfide cross-linked in the native state. Two minor components from the cuticle are apparently derived from one of the major chains by cleavage in a single region to give two-third and one-third fragments.

Partial purification and characterization of collagen galactosyltransferase from chick embryos

Collagen galactosyltransferase was purified 50-150-fold from chick-embryo extract. The tissue homogenate was prepared in the presence of Triton X-100, since the addition of the detergent doubled the enzyme activity in the homogenate and the extract. Three species of the enzyme activity with different molecular weights were recovered on gel filtration, the mol.wts. being about 450000, 200000 and 50000. Collagen galactosyltransferase activity was strongly inhibited by p-mercuribenzoate, and stimulated by the addition of dithiothreitol to the incubation system. Studies on substrate requirements indicated that denatured citrate-soluble collagen is a more effective substrate than gelatinized insoluble collagen, as judged from their Km values. Experiments on three peptide fractions prepared from citrate-soluble collagen indicated that a fraction with an average mol.wt. of 500-600 contained peptides large enough to meet a minimun requirement for interaction with the enzyme. However, longer peptides were clearly better substrates. When native and heat-denatured citrate-soluble collagens were compared as substrates, practically no synthesis of galactosylhydroxylysine was found with native collagen. This finding suggests that the triple-helical conformation of collagen prevents the galactosylation of hydroxylysine residues.

Light and electron microscopic observations on hydrocortisone-induced cleft palate in hamsters

Palatal histogenesis in hydrocortisone-treated hamster fetuses was studied by both light and electron microscopy. At an early stage in the hydrocortisone-affected fetuses, when the palatal shelves hung vertically on either side of the tongue, necortic changes could be seen in some of the basal epithelial cells which lay adjacent to the fragmented basal lamina. The normal looking cells lay on an intact basal lamina and were attached to the contiguous necrotic cells by desmosomes. With horizontal reorientation of the palatal shelves and their approach to the midline, cellular necrosis and fragmentation of the basal lamina increased. When compared with normal cells, the hydrocortisone-affected ones were seen to be lighter, to contain fewer ribosomes and no lysosomes. At a later stage, when midline palatal fusion was lacking, the epithelium underwent stratification and keratinization while the necrotic debris was removed by mesenchymal macrophages. It appears that the normal process of protein synthesis is inhibited following hydrocortisone administration and that this, in turn, during palatogenesis, disrupts normal cellular differentiation and the integrity of the basal lamina, which are associated with the production of a cleft palate.

Defects in the biochemistry of collagen in diseases of connective tissue

The collagens are the major structural glycoproteins of connective tissues. A unique primary structure and a multiplicity of post-translational modification reactions are required for normal fibrillogenesis. The post-translational modifications include hydroxylation of prolyl and lysyl residues, glycosylation, folding of the molecule into triple-helical conformation, proteolytic conversion of precursor procollagen to collagen, and oxidative deamination of certain lysyl and hydroxylysyl residues. Any defect in the normal mechanisms responsible for the synthesis and secretion of collagen molecules or the deposition of these molecules into extracellular fibers could result in abnormal fibrillogenesis; such defects could result in a connective tissue disease. Recently, defects in the regulation of the types of collagen synthesized and in the enzymes involved in the post-translational modifications have been found in heritable diseases of connective tissue. Thus far, the primary heritable disorders of collagen metabolism in man include lysyl hydroxylase deficiency in Ehlers-Danlos syndrome type VI, p-collagen peptidase deficency in Ehlers-Danlos syndrome type VII, decreased synthesis of type III collagen in Ehlers-Danlos syndrome type IV, lysyl oxidase deficency in S-linked cutis laxa and Ehlers-Danlos syndrome type V, and decreased synthesis of type I collagen in osteogenesis imperfecta.

Quantitative and comparative ultrastructure of the vertebrate cornea. I. Urodele Amphibia

The cornea of the urodele amphibian Triturus c. cristatus was studied ultrastructurally in order to provide the basis for a comparison among corneas throughout the vertebrate phylum. The cornea of this salamander consists of relatively thick epithelium and basement membrane and thin Descemet's membrane, unlike the mammalian corneas. The outermost epithelial cells contain Ruthenium Red stainable extracellular filaments and intracellular vesicles which are thought to play a role in the process of lubricating the corneal surface. Occluding junctions have been observed in the apical region of the superficial epithelial cells and are considered as barriers to the intercellular passage of material. A thin substantia propria (stroma) consists of about 40 collagenous highly organized lamellae. The thicknesses of the basement membrane, Descemet's membrane and the epithelium are believed to represent the primitive situation in the process of corneal evolution.