Epithelial collagens and glycosaminoglycans in the embryonic cornea. Macromolecular order and morphogenesis in the basement membrane

Cutaneous tissue repair: basic biologic considerations. I

Wound repair of the integument is reviewed in the context of new developments in cell biology and biochemistry. Injury of the skin and concomitant blood vessel disruption lead to extravasation of blood constituents, followed by platelet aggregation and blood clotting. These events initiate inflammation and set the stage for repair processes. The macrophage plays a pivotal role in the transition between wound inflammation and repair (granulation tissue formation), since this cell both scavenges tissue debris and releases a plethora of biologically active substances that include growth factors. Although concrete evidence is lacking, growth factors are probably at least partially responsible for the angiogenesis and fibroplasia (granulation tissue) that gradually fill the wound void. If the epidermal barrier is disrupted during injury, reepithelialization begins within 24 hours and proceeds first over the margin of residual dermis and subsequently over granulation tissue. The signals for angiogenesis, fibroplasia, neomatrix formation, and reepithelialization in wound repair are not known, but a number of possibilities are discussed. Matrix remodeling is the last stage of wound repair and gradually increases the scar tensile strength to 70% to 80% of normal skin.

Histochemical studies on pseudocysts in adenoid cystic carcinoma of the human salivary gland

Pseudocysts are unique structures found in adenoid cystic carcinomata of human salivary glands. They were studied in 13 such cases by histochemical and immunohistochemical means. The pseudocysts contained an abundance of mucoid materials which reacted strongly with both Alcian Blue and dialysed iron ferrocyanide. The mucoid material was digested with chondroitinase ABC and heparitinase, but was resistant to Streptomyces hyaluronidase. The inner surfaces of the pseudocysts were strongly reactive for laminin, whereas the interface between the tumour cell nests and the outer stromal area was intensely reactive for fibronectin. Numerous fibronectin-reactive fibrils and blood coagulation factor XIII (F-XIII)-positive cells were distributed extensively in the outer stromal area. The F-XIII-positive cells were also found within some pseudocysts. The results obtained in the present study have shown that the pseudocysts represent a peculiar structure consisting of basement membrane components; laminin, fibronectin, heparan sulphate and chondroitin sulphate.

The retention and ultrastructural appearances of various extracellular matrix molecules incorporated into three-dimensional hydrated collagen lattices

Artificial extracellular matrices composed of collagen, glycosaminoglycans (GAG), proteoglycans (PG), plasma fibronectin (FN), and a hyaluronate-binding protein (HABP) have been prepared that morphologically resemble embryonic extracellular matrices in vivo at the light and electron microscope level. The effect of each of the above matrix molecules on the structure and "self-assembly" of these artificial matrices was delineated. (1) Matrix components assembled in vitro morphologically resemble their counterparts in vivo, for the most part. Scanning and transmission electron microscopy indicate that under our assembly and fixation conditions, collagen forms striated fibrils that are 125 nm in diameter, FN forms 30- to 60-nm granules, chondroitin sulfate proteoglycan (CSPG) forms 27- to 37-nm granules, chondroitin sulfate (CS) assembles into 100- to 250-nm spheres, and hyaluronate (HA) appears either as granular mats when fixed with cetylpyridinium chloride (CPC) or as 1.5- to 3-nm microfibrils when preserved with ruthenium red plus tannic acid. These molecules are known to assume the same configurations in embryonic matrices when the same preservation techniques are used with the exception of FN, which generally forms fibrillar arrays. (2) Addition of various matrix molecules can radically change the appearance of the collage gels. HA greatly expands the volume of the gel and increases the space between collagen fibrils. CSPG at low concentrations (less than 1 mg/ml) and CS at high concentrations (greater than 20 mg/ml) bundle the collagen fibrils into twisted ropes. (3) A variety of assays were used to examine binding between various matrix components and retention of these components in the hydrated collagen lattices. These assays included solid-phase binding assays, negative staining of spread mixtures of matrix components, cryostat sections of unfixed mixtures of matrix components, and retention of radiolabeled matrix molecules in fixed and washed gels. A number of these binding interactions may play a role in the assembly and stabilization of the matrix. (a) HA, CSPG, and FN bind to collagen. CS appears to only weakly bind to collagen, if at all. (b) FN promotes the increased retention of HA, CSPG, and to a very small degrees, CS, in collagen gels. Conversely, the GAG increase the retention of 3H-FN in the gels. Furthermore, FN binds to HA, CS, and CSPG as demonstrated by solid surface binding assays and morphological criteria. The increased retention of GAG and CSPG by the addition of FN may be due to both stabilization of binding to the collagen and trapping of matrix complexes within the gel. (c) HA binds to both CS and CSPG.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatial interrelationships between proteoglycans and extracellular matrix proteins in cell cultures

Immunolabelling of cultured cells for chondroitin-sulfate proteoglycan (CSPG), in conjunction with antibodies to fibronectin, collagen and laminin, revealed the spatial interrelationships between the different matrix components. CSPG was organized in two major forms. Fibronectin-independent dotted patterns of CSPG were detected on the substrate and cell surfaces at early stages after plating. At later stages, however, significant overlapping was found between the two extracellular matrix components. Immunoelectron microscopic examination indicated that the CSPG was organized as granules of varying sizes which were associated with the cell surface, the substrate, or with the periphery of the fibronectin network.

Distribution and characterization of anionic sites in trophoblast and capillary basal laminas of human placental villi

The distribution of anionic sites was studied in the trophoblastic and fetal capillary basal laminas of developing human placental villi with the cationic stain ruthenium red. At 7-12 weeks of gestation the trophoblastic basal lamina (TBL) contained ruthenium red-positive granules in a quasi-regular array throughout the lamina densa or sometimes concentrated at the interstitial surface of the lamina densa. The capillary basal lamina (CBL) (and anionic sites) were not present at this age. Anionic sites were also associated with collagen or reticular fibrils. At term, the TBL was largely devoid of anionic sites except for some distributed along its interstitial surface. The CBL was present in later gestation and sometimes had arrays of anionic sites. In order to characterize the anionic sites, minced pieces of villi were incubated in the presence or absence of either chondroitinase ABC, heparitinase, neuraminidase, or Streptomyces hyaluronidase in appropriate buffer systems. Incubation of early villi with heparitinase resulted in the disappearance of the TBL-associated sites. Chondroitinase ABC appeared to reduce staining of collagen-associated sites. In term villi, heparitinase removed those few sites still associated with the TBL but did not affect sites associated with the CBL or collagen. Chondroitinase ABC resulted in the disappearance of all anionic sites. In later gestation, a number of developmentally important macromolecules are transported across the trophoblast and enter the fetal capillaries. We conclude that the absence of an array of polyanionic sites from the term placenta TBL and the reduction in the amount of extracellular matrix intervening between the trophoblast and capillaries are adaptations to enhance the exchange of macromolecules across the placenta.

Distribution and characterization of anionic sites in the basal lamina of developing human amniotic epithelium

We have studied the distribution of anionic sites in the basal lamina of developing human amniotic epithelium by using the cationic stain ruthenium red. Amnions at 7-12 weeks of gestation and at term contained ruthenium red-positive granules in a quasi-regular array on both the cellular and interstitial sides of the lamina densa. In order to characterize the anionic sites, small pieces of amnion were incubated in the presence or absence of either chondroitinase ABC, neuraminidase, Streptomyces hyaluronidase, or heparitinase in appropriate buffer systems. Incubation in the presence of heparitinase resulted in the complete disappearance of the basal lamina-associated granules, but other enzymes tested had no demonstrable effect on these granules. We conclude that the anionic sites associated with amnion basal lamina, and demonstrable with ruthenium red, consist of glycosaminoglycans rich in heparan sulfate, probably present as heparan sulfate proteoglycan. Because amniotic fluid has a low protein content and amniotic epithelium (at least at term) lacks tight junctions, we postulate that the heparan sulfate proteoglycan associated with the amnion basal lamina may have an important function as a permeability barrier to anionic macromolecules.

Analysis of corneal development with monoclonal antibodies. I. Differentiation in isolated corneas

Monoclonal antibodies highly selective for developmentally regulated antigens present in the cornea (Zak and Linsenmayer, Dev. Biol. 99, 373-381, 1983) have been used to immunohistochemically evaluate differentiation in intact chick corneas cultured on the chorioallantoic membrane (CAM) of host embryos. One antibody is directed against the epithelial cell layer and the other is against the corneal stromal matrix. It has been established that both antigens recognized by the antibodies are expressed de novo in young explanted corneas and that the stromal matrix antigen is a product of the corneal fibroblasts. Thus expression of the antigens can be used as criteria for overt differentiation of the respective cell types. The antibodies have been employed to assess when the corneal epithelial and stromal cells become capable of autonomous differentiation within isolated corneas. To accomplish this, corneas of various ages were explanted with and without adjacent pericorneal tissues. The results indicate that, under the culture conditions employed, corneal stromal differentiation is dependent on the presence of the lens until stage 28 (51/2-6 days of development), which is the time when invasion of the stroma by pericorneal mesenchymal cells is initiated. After stage 28, the stromal matrix antigen was expressed by isolated corneas irrespective of the presence of the lens. Possibly the lens acts by maintaining the integrity of the corneal endothelial monolayer and thus promoting normal migration of pericorneal mesenchymal cells into the primary corneal stroma, where they undergo differentiation. Conversely, differentiation of the corneal epithelium was independent of any pericorneal structure from the earliest stage examined (41/2-5 days of development). It was even independent of overt stromal differentiation, thus suggesting an early and strong determination for this tissue.

Remodelling of the basement membrane: morphogenesis and maturation

We have analysed the reciprocal interactions between mouse embryo submandibular epithelium and mesenchyme which result in branching morphogenesis of the epithelium. The interactions modify the composition and metabolism of the basal and reticular laminae which comprise the basement membrane lying between these tissues. The mesenchyme remodels the basement membrane by depositing a type I collagen-rich matrix on the basal lamina and by producing a neutral hyaluronidase, which degrades hyaluronate and chondroitin sulphate, components of this basal lamina. By analogy with mouse mammary epithelial cells, the submandibular epithelial cells have a heparan sulphate-rich proteoglycan on their cell surfaces which is anchored to the cells. The extracellular domain of this integral membrane proteoglycan binds to interstitial collagen. Interfering with the collagen-proteoglycan interaction appears to reduce the morphological stability of the cells. Together with other processes, including epithelial cell proliferation, this remodelling leads to branching epithelial morphogenesis. Basement membrane remodelling may be a general process for regulating cell behaviour during development and is one of the mechanisms of morphogenetic tissue interaction. Remodelling may also cause maturation of basement membranes from a dynamic state of high turnover in the embryo to their persistence and stability in the adult organism.

Coated vesicles purified from chick tendon fibroblasts contain newly synthesized type I procollagen

Coated vesicles were purified from embryonic chick tendon fibroblasts pulsed with [3H]proline. They were morphologically and biochemically similar to coated vesicles purified from other sources. Furthermore, they contained newly synthesized Type I procollagen which was protected from bacterial collagenase digestion unless detergent was present. The procollagen remained associated with coated vesicles during immune precipitation and agarose gel electrophoresis. Data from pulse-chase experiments demonstrated that the specific activity of the coated vesicle preparations was approx. 5-fold higher at the 10 min chase point than at either the 0 or 40 min chase points. These data are consistent with the hypothesis that coated vesicles are intermediates in the intracellular transport of newly synthesized Type I procollagen in chick tendon fibroblasts.

Connections between the sensory retina and the retinal pigment epithelium

The study reports on the presence of connections between the visual receptor outer segments and the apical villi on the retinal pigment epithelium in the human eye. The mechanisms underlying sensory retinal adhesion to the pigment epithelium, recognition and attachment of visual receptor outer segments to the pigment epithelium during phagocytosis, and communication and transport of substances between the two layers of retina have been difficult to define, and molecular connections between the two layers of retina could play an important role in these processes.

Elastin-proteoglycans association revealed by cytochemical methods

By using various cytochemical stains, proteoglycans are shown to be present inside elastic fibers in aortas of beta-aminopropionitrile-induced lathyritic chicks. Depending on the characteristics of the dyes, the shape, size and distribution of the proteoglycan-revealing precipitates are described. The monocationic dye toluidine blue O and the tetracationic dye Alcian blue in the presence of 0.3 M MgCl2 give the most detailed results. With these stains the proteoglycans inside lathyritic elastin appear to be lateral branches of matrix proteoglycans, lying on the external surface of the elastic fibers. A possible general biological significance of elastin-proteoglycan association is briefly discussed.

De novo cellular synthesis of sulfated proteoglycans of the developing renal glomerulus in vivo

The site of cellular synthesis of glomerular proteoglycans was investigated in developing glomeruli of 4- to 5-day-old rats. [35S]Sulfate was administered intravenously and animals were sacrificed 15 min to 12 hr later. The outermost layers of the kidney cortices were utilized for characterization of proteoglycans and electron microscopic autoradiography. Sepharose CL-6B chromatography and cellulose acetate electrophoresis revealed that most (approximately equal to 96%) of the radioactivity was associated with heparan sulfate-proteoglycan synthesized during maturation of glomerular capillaries. Tissue autoradiography revealed the following: (i) during the S-shaped body stage, there is rapid incorporation of [35S]sulfate by mesenchymal cells into the cleft region (site for development of future glomerular extracellular matrices); (ii) during the precapillary stage, mesenchyme-derived cells showed higher incorporation of radioisotope than did epithelial cells; and (iii) during the mature capillary stage, all glomerular cell types (mesangial, endothelial, and epithelial) incorporated [35S]sulfate, incorporation by mesangial cells being the greatest. Radiolabeling was also higher in the mesangial matrix than in the glomerular basement membrane of peripheral capillary loops. Synthesis of a single major species of sulfated glycosaminoglycan by cells of different embryologic origin may be unique to glomerular capillaries.

Leukocyte interleukins induce cultured endothelial cells to produce a highly organized, glycosaminoglycan-rich pericellular matrix

We report here that interleukins have a dramatic effect on extracellular matrix production by cultured endothelial cells. Human umbilical vein endothelial cells incubated with growth media conditioned by lectin-activated human peripheral blood mononuclear leukocytes undergo marked changes in cell shape and elaborate a highly organized extracellular material that is not detectable in untreated cultures. This material has the following characteristics: (a) it is not recognizable by electron microscopy unless the cationic dye, Alcian blue, is added to the fixative; (b) it is visualized as a network of branching and anastomosing fibrils of various thickness that can be resolved into bundles of fine filaments; (c) it is associated with the cell surface, extends between contiguous cells, and coats the culture substrate; (d) it is removed by digestion with glycosaminoglycan-degrading enzymes, such as crude heparinase and chondroitinase ABC. These results demonstrate that soluble factors released by activated peripheral blood mononuclear leukocytes (interleukins) stimulate cultured human umbilical vein endothelial cells to produce a highly structured pericellular matrix containing glycosaminoglycans (probably chondroitin sulfate and/or hyaluronic acid) as a major constituent. We speculate that this phenomenon corresponds to an early step of angiogenesis as observed in vivo as a consequence of interleukin release.

Partial chemical characterization of the anionic sites in the basal lamina of fenestrated capillaries

The distribution of anionic sites in the basal laminae of the blood capillaries of the murine pancreas was studied in specimens fixed in ruthenium red (RR)-glutaraldehyde mixtures. The sites appeared as discrete, small (6 to 18 nm) particles distributed throughout the three laminae but concentrated primarily in the lamina rara externa, in which--spaced 80-100 nm apart--they formed a planar, partially ordered lattice comparable to that revealed by cationized ferritin in previous studies (M. Simionescu, N. Simionescu, and G. E. Palade, 1982, J. Cell Biol. 95, 425-434). The chemical nature of the anionic sites was explored by incubating fresh tissue specimens in solutions of selected enzymes before fixation in RR-glutaraldehyde mixtures. Pronase P and papain removed completely the anionic sites and left behind an extensively degraded and disorganized basal lamina. Trypsin caused the removal of anionic sites only, did not degrade the rest of the basal lamina, but detached it completely from the endothelium. Chondroitinase ABC reduced slightly the size and the surface density of RR-stainable particles, and detached focally the rest of the basal lamina from the endothelium and pericytes. Crude heparinase caused a nearly complete removal of anionic sites, and pure heparitinase gave comparable but less extensive results. Similar effects were recorded on the basal laminae of smooth muscle fibers and pancreatic acini and ducts. The results indicate that the anionic sites of all basal laminae examined are contributed primarily by heparin sulfate proteoglycans and trace amounts of chondroitin sulfate proteoglycans.

Similar growth pattern of mouse mammary epithelium cultivated in collagen matrix in vivo and in vitro

Mouse mammary ductal cells cultured in type I collagen gels give rise to three-dimensional multicellular outgrowths consisting of thin spikes which are often branched, and which may have pointed or blunt ends. The significance of these spikes to normal ductal morphogenesis has been unclear, since identical structures are not known to occur in vivo; conversely, it has not been possible to maintain in gel culture the highly structured end buds which are characteristic of ductal elongation in the animal. In order to evaluate whether the pattern of radiating spikes observed in collagen gel cultures results from chemical or physical peculiarities of the culture environment, a small volume of unpolymerized type I collagen solution was injected into mammary gland-free fat pads of young adult mice. After the bubble of collagen had polymerized, an implant of mammary ductal epithelium was introduced into the center of the gel. Histological examination of the implants after 3 to 6 days of growth revealed numerous small epithelial spikes, similar to those observed in gel culture, extending into the fibrous matrix. The early stages of regeneration of mammary implants placed in gland-free fat pads were then examined without the addition of exogenous collagen. In cases where the epithelium happened to contact a fibrous region of the fatty stroma, spikes were also seen to form in these natural collagenous substrates. Whether or not exogenous collagen was used, normal end buds were formed only when epithelial spikes contacted adipocytes. It was concluded that the three-dimensional pattern of radiating tubules in collagen gels in vitro is not merely an artifact of culture, but has a counterpart in vivo whereever regenerating mammary epithelium is surrounded by fibrous stroma. A model is presented in which the pattern of epithelial outgrowth is determined by the physical characteristics of the surrounding stroma; in collagen matrix a comparatively primitive and unspecialized type of morphogenesis occurs which may not require the participation of stromal cells. In contrast, epithelial-adipocyte interactions appear to be necessary for the formation of end buds and subsequent morphogenesis of fully structured mammary ducts.

Staining of proteoglycans in mouse lung alveoli. I. Ultrastructural localization of anionic sites

In order to contrast anionic sites in mouse lung alveoli, two staining procedures were applied: (a) staining with Ruthenium Red and Alcian Blue and (b) staining with Cuprolinic Blue in a critical electrolyte concentration method. The Ruthenium Red-Alcian Blue staining procedure revealed electron-dense granules in the alveolar basement membrane. The granules were closely associated with the epithelial cell membrane and continued to stain even when the procedure was carried out at a low pH, indicating the presence of sulphate groups in the granules. After staining with Cuprolinic Blue, electron-dense filaments, also closely associated with the cell membrane, became visible in the basement membrane of type I epithelial cells. Their length depended on the MgCl2 concentration used during staining. At 0.4 M MgCl2, the length was mostly within the range 100-180 nm. Using a modified Cuprolinic Blue method, the appearance of the filaments closely resembled that of spread proteoglycan monomers with their side-chains condensed. The basement membrane of type II epithelial cells also contained filaments positive towards Cuprolinic Blue; their length, however, was smaller in comparison with those of type I epithelial cells. The filaments lay in one plane and provided the whole alveolus with an almost continuous sheet of anionic sites. Cuprolinic Blue staining also revealed filaments in the basement membrane of the capillary endothelial cells. Furthermore, Cuprolinic Blue-positive filaments (average length about 40 nm) became apparent in close contact with collagen fibrils and separated from each other according to the main banding period of the collagen fibrils (about 60 nm), indicating a specific ultrastructural interaction between these two components. Filaments connecting collagen fibrils with each other were also detected.

Glycosaminoglycans of arterial basement membrane-like material from cultured rabbit aortic myomedial cells

Arterial basement membrane-like material was prepared by a sonication-differential centrifugation technique from cultures of rabbit aortic myomedial cells after metabolic labelling with [35S]sulphate and [3H]glucosamine. Labelled glycosaminoglycans were obtained from isolated basement membrane-like material by proteinase digestion and gel filtration. Glycosaminoglycans were identified by a combination of Sephadex G-50 chromatography and sequential degradation with nitrous acid, Streptomyces hyaluronidase, testicular hyaluronidase and chondroitinase ABC. The data showed that heparan sulphate and chondroitin sulphate were the predominant glycosaminoglycans of myomedial basement membrane-like material. Heparan sulphate accounted for about 55% of [3H]glucosamine-labelled glycosaminoglycans. In addition small amounts of hyaluronic acid was present. Only trace amounts of dermatan sulphate was found. The glycosaminoglycans were analysed by DEAE-cellulose chromatography. Two major peaks were found in the chromatogram consistent with the predominance of heparan sulphate and chondroitin sulphate.

Type, location and role of glycosaminoglycans in cloned differentiated chick retinal pigmented epithelium

In clonal culture, colonies of 3-4 week old chick retinal pigmented epithelial cells exhibit Alcian Blue positive extracellular matrix (ECM) material on the surface of the cells. Alcian blue positive ECM is located between undifferentiated cells at the edges of the disc-shaped colonies and beneath the differentiated cells in the colony center. The latter material is associated with the basement membrane. The staining properties suggest that glycosaminoglycans (GAG) are present in these regions. Extraction of GAG from homogenates of colonies, followed by electrophoresis on cellulose acetate strips, results in three bands with mobilities similar to those of hyaluronic acid, heparan sulfate, and chondroitin sulfate, respectively. All three bands label with [3H]glucosamine, and the last two also label with [35S]sulfate. The composition appeared to differ when colonies were grown in different media. Digestion of the GAG preparations with various enzymes suggests that bands II and III represent heparan sulfate and chondroitin sulfate, respectively, in colonies grown in Ham's F10g medium. The composition of band I is as yet undetermined. In minimal Eagle's medium (MEM), bands I and III consisted of hyaluronic acid and chondroitin sulfate, respectively, while band II had properties suggestive of a copolymer of heparan sulfate and an unidentified GAG. Cells release only one [3H]glucosamine-labelled GAG into the medium. This material has a mobility similar to hyaluronic acid and is digested by Streptomyces hyaluronidase, suggesting that it is hyaluronic acid. Staining with Alcian Blue at different pH suggests that it may represent the material associated with the upper surface of the cells. Some of the ECM located between the undifferentiated cells and associated with the basement membrane in the differentiated regions of the colonies stains with Alcian Blue at pH 1.0 and 0.2 suggesting that it may contain GAGs found in bands I and II. Colonies treated with medium containing 6-diazo-5-oxo-L-norleucine (DON), an inhibitor of GAG synthesis, for 48 hr showed a reduced Alcian Blue staining of the ECM in the undifferentiated regions. After 72 hr of treatment with DON, the undifferentiated cells had detached from the plate, whereas the differentiated cells remained intact. The results suggest that the GAG may be involved in cellular adhesion, particularly of the undifferentiated cells.

Aggregates of acetylcholine receptors are associated with plaques of a basal lamina heparan sulfate proteoglycan on the surface of skeletal muscle fibers

Hybridoma techniques have been used to generate monoclonal antibodies to an antigen concentrated in the basal lamina at the Xenopus laevis neuromuscular junction. The antibodies selectively precipitate a high molecular weight heparan sulfate proteoglycan from conditioned medium of muscle cultures grown in the presence of [35S]methionine or [35S]sulfate. Electron microscope autoradiography of adult X. laevis muscle fibers exposed to 125I-labeled antibody confirms that the antigen is localized within the basal lamina of skeletal muscle fibers and is concentrated at least fivefold within the specialized basal lamina at the neuromuscular junction. Fluorescence immunocytochemical experiments suggest that a similar proteoglycan is also present in other basement membranes, including those associated with blood vessels, myelinated axons, nerve sheath, and notochord. During development in culture, the surface of embryonic muscle cells displays a conspicuously non-uniform distribution of this basal lamina proteoglycan, consisting of large areas with a low antigen site-density and a variety of discrete plaques and fibrils. Clusters of acetylcholine receptors that form on muscle cells cultured without nerve are invariably associated with adjacent, congruent plaques containing basal lamina proteoglycan. This is also true for clusters of junctional receptors formed during synaptogenesis in vitro. This correlation indicates that the spatial organization of receptor and proteoglycan is coordinately regulated, and suggests that interactions between these two species may contribute to the localization of acetylcholine receptors at the neuromuscular junction.

Distribution of anionic sites on the basal lamina of Schwann cells

Basal laminae (BL) were separated from Schwann cells of rat sciatic nerves by means of weak sonication, and the anionic sites of the BL were demonstrated by using cationized ferritin (CF) or ruthenium red (RR). CF particles were deposited in clusters at intervals of 100-150 nm on the interstitial side of the BL facing the connective tissue, while the cellular side facing the Schwann cell plasmalemma showed only an occasional deposition of CF particles. RR-positive sites were found only on the interstitial side with a pattern of distribution comparable to that of CF-binding sites. These results indicate that the patterns of anionic site distribution are different between the inner and outer surfaces of the Schwann cell BL.

Culturing chick muscle cells on glycosaminoglycan substrates: attachment and differentiation

The effects of different glycosaminoglycans (GAGs) on myogenesis were tested by culturing embryonic chick myoblasts on tissue culture dishes to which either hyaluronic acid (HA) or chondroitin sulfate (ChS) was covalently bound. Both in cell number and in apparent cell type distribution, the population of cells bound to GAGs is similar to that on gelatin and significantly different from that observed with uncoated dishes. When plated on ChS, myoblasts proliferate, align, and fuse at a rate similar to cells plated on gelatin. The final fused cells appear as sheets rather than long, thin myotubes. On HA, the cells proliferate but are inhibited from differentiation. The extent of inhibition is dependent on the amount of HA present. The inhibition of myogenesis is maintained through four subcultures on HA, but can be reversed at any time by culturing cells on gelatin. These experiments indicate that different GAGs have different effects on myogenesis and that HA can actively inhibit the process.

Surface charges associated with fenestrated brain capillaries. I. In vitro labeling of anionic sites

Ferritin derivatives with different pI values and the basic dye ruthenium red have been used as cationic probes to localize anionic sites associated with fenestrated brain capillaries. Cationic ferritin was found in the endothelial basement membrane and the basement membrane of the perivascular cellular linings in amounts far exceeding those observed with anionic derivatives, the degree being greater for the more cationized ferritin molecules. Labeling of the luminal endothelial front with cationic ferritin was only achieved when a serum- or albumin-free medium was applied. Furthermore, the striated collagen fibers were coated with cationic ferritin molecules in a highly ordered fashion. Ruthenium red localized to the same sites. The findings suggest the existence of a perivascular charge filter around fenestrated capillaries of the brain. Some physiological roles of this filter are discussed, as related to its possible function in regulating homeostasis of cerebrospinal fluid.

Biosynthesis of proteoglycans in organ cultures of developing kidney mesenchyme

The biosynthesis of proteoglycans was studied in organ cultures of differentiating metanephric mesenchymes. When triggered by a contact-mediated inductive interaction, this tissue undergoes transition from a mesenchyme to an epithelium. In the present study, proteoglycans were extracted by guanidinium hydrochloride in the presence of protease inhibitors. We found that, as a response to induction, the differentiating mesenchyme begins to synthesize large size proteoglycans with an apparent molecular weight (MW) of 1 X 10(6) D. The major glycosaminoglycans detected were chondroitin sulfates. Heparan sulfate proteoglycans were also detected, constituting 20% of the proteoglycans. An inhibitor of glucosamine synthesis, 6-diazo-5-oxo-norleucine (DON) was found to inhibit glycosaminoglycan synthesis by approx. 60%, and the size of the proteoglycans was also diminished. Our studies suggest that the transition of the mesenchyme to epithelium is associated with initiation of synthesis of large size proteoglycans.

Keratinocytes synthesize basal-lamina proteins in culture

On histologic vertical sections of skin, the epidermis is separated from the dermis by an amorphous thin membrane, the basal lamina. Ultrastructurally, the basal lamina is composed of four areas, including the basal-cell plasma membrane and hemidesmosomes, the lamina lucida, the lamina densa, and the sub-lamina densa fibrillar region. In culture, epidermal keratinocytes are able to produce hemidesmosomes, lamina lucida, and lamina densa. There is no evidence that cultured keratinocytes can produce sub-lamina densa fibrils. Biochemically, the lamina lucida contains two major glycoproteins. One, the bullous pemphigoid antigen, is synthesized by epidermal keratinocytes in vitro. These cells also synthesize laminin, the other glycoprotein of lamina lucida. At the interface between lamina lucida and lamina densa there is probably a heparan sulfate proteoglycan. Whether this proteoglycan is produced by keratinocytes in culture is not known, but the possibility can be considered. Lamina densa contains collagen IV, and this collagen is synthesized by keratinocytes in culture. However, cultured keratinocytes may also synthesize collagen types I, III, and V. Type V is associated with the basal lamina, but its exact location is unknown. Types I and III (if they are produced in vivo) would be situated in the sub-basal lamina region. The problem of fibronectin remains unsolved. There is "some" fibronectin in the lamina lucida, but its origin is not clear.

Mammary ductal elongation: differentiation of myoepithelium and basal lamina during branching morphogenesis

Elongation of mammary ducts in the immature mouse takes place as a result of rapid growth in end buds. These structures proliferate at the apex of elongating ducts and are responsible for penetration of the surrounding adipose stroma; by turning and branching, end buds give rise to the characteristic open pattern of the mammary ductal tree. We have used a variety of techniques to determine the cellular and structural basis for certain of these end bud activities, and now report the following. (1) The end bud tip is covered with a monolayer of epithelium, the "cap cells," which are characterized by a relative lack of intercellular junctions and other specialized features. (2) The cap cell layer extends along the end bud flank and neck regions where it is continuous with the myoepithelium which surrounds the subtending mature duct. A linear sequence of differentiative changes occur in the cap cells in this region as they progressively alter in shape and accumulate the cytological features of mature myoepithelium. Cap cells may therefore be defined as a stem cell population providing new myoepithelial cells for ductal morphogenesis and elongation. (3) Differentiation of cap cells into myoepithelium is associated with conspicuous changes in the basal lamina. At the tip, cap cells form a 104-nm lamina similar to that described in expanding mammary alveoli and in embryonic tissues. Along the end bud flanks the basal lamina is raised from the cell surface and extensively folded, resulting in a greatly thickened lamina, measuring as much as 1.4 microns. At the surface of the subtending ducts the lamina becomes structurally simplified and resembles that at the tip, but has a significantly greater thickness, averaging 130 nm. (4) The codifferentiation of myoepithelium and its basement membrane is associated with changes in the surrounding stroma. Undifferentiated mesenchymal-like cells attach to the surface of the basal lamina in the midportion of the end buds and become increasingly numerous in the neck region, forming a monolayer over the myoepithelial basal lamina. These stromal cells progressively differentiated into fibrocytes which participate in collagen fibrillogenesis and give rise to the fibrous components of the stroma surrounding the mature duct.

Topographical arrangement of basement membrane proteins in lactating rat mammary gland: comparison of the distribution of type IV collagen, laminin, fibronectin, and Thy-1 at the ultrastructural level

The topographical distribution of type IV collagen, laminin, fibronectin, and the thymocyte differentiation antigen Thy-1 in the basement membrane of the lactating rat mammary gland was investigated. Small cubes of tissue, which had not been subjected to prior fixation or freezing, were incubated with monospecific or monoclonal antibodies to these proteins, and the antibodies were located by an indirect immunoperoxidase staining technique and observed in the electron microscope. The lamina densa stained uniformly with antibodies to type IV collagen and laminin. In addition, both laminin and type IV collagen were present in semiperiodic clusters that traversed the lamina lucida from the cell surface to the lamina densa. Fibronectin was present only in the semiperiodic clusters and not elsewhere in the basement membrane. These clusters were irregularly spaced along the cell surface and heterogeneous in size. It remains to be determined if these three proteins are present in the same clusters. Thy-1 was largely present on the lamina densa and not on the lamina lucida. The Thy-1 staining of the lamina densa occurred in discrete maxima and minima. These maxima occurred in regions adjacent to Thy-1-bearing stromal cells. Thus, the topographical distribution of proteins within a basement membrane varies in a nonrandom manner, and local factors can modify this distribution. We suggest that this topographical variability may play a role in cell recognition and signalling processes that occur across the basement membrane.

Distribution of anionic binding sites in extravascular space of skeletal muscle demonstrated with polycationized ferritin

Polycationized ferritin (PCF) and native ferritin (NF) were microinjected into rat muscle interstitium. The injection site was fixed in situ and processed for electron microscopy. PCF decorated the external aspect of the lamina densa of pericapillary and muscular basal lamina as quasi-regular aggregates, up to approximately 200 nm wide, separated with spaces greater than 20 nm wide. The internal aspect of the lamina densa possessed negative binding sites with a similar distribution, whereas PCF was never retained within the lamina densa. PCF bound very closely to the sarcolemma but did not approximate intimately the endothelial plasmalemma including vesicles. PCF also interacted with strict regularity with collagen fibrils (at an axial repeat of greater than 60 nm) forming transverse bands of PCF across collagen bundles. No ordered interaction between NF and extracellular structures was observed. The ordered distribution of negative binding sites in interstitium, as revealed with PCF, should affect the blood-lymph movement of macromolecules.

Distribution of polyanionic sites in the developing gonads and the dorsal mesentery of the chick embryo

The distribution of glycoconjugates was investigated in the embryonic trunk mesoderm used as a substrate by migrating primordial germ cells (PGCs) by means of ultrastructural cytochemistry. In both mesentery and developing gonads polyanionic sites were abundant in epithelial and mesenchymal cell coats, basal laminae, and extracellular matrices (ECM). In the latter, polyanions distributed on microfibrils and granules were associated with collagen fibers, forming an entangled network. No preferential association of this fibrillo-granular material with PGCs was observed, suggesting that polyanions present in ECM likely act by promoting inflation of the extracellular spaces rather than by providing mechanical guides for the moving cells.

Localization of type IV collagen, laminin, heparan sulfate proteoglycan, and fibronectin to the basal lamina of basement membranes

Electron microscopic immunostaining of rat duodenum and incisor tooth was used to examine the location of four known components of the basement-membrane region: type IV collagen, laminin, heparan sulfate proteoglycan, and fibronectin. Antibodies or antisera against these substances were localized by direct or indirect peroxidase methods on 60-microns thick slices of formaldehyde-fixed tissues. In the basement-membrane region of the duodenal epithelium, enamel-organ epithelium, and blood-vessel endothelium, immunostaining for all four components was observed in the basal lamina (also called lamina densa). The bulk of the lamina lucida (rara) was unstained, but it was traversed by narrow projections of the basal lamina that were immunostained for all four components. In the subbasement-membrane fibrous elements or reticular lamina, immunostaining was confined to occasional "bridges" extending from the epithelial basal-lamina to that of adjacent capillaries. The joint presence of type IV collagen, laminin, heparan sulfate proteoglycan, and fibronectin in the basal lamina indicates that these substances do not occur in separate layers but are integrated into a common structure.

Collagen fibrillogenesis in tissues, in a solution and from modeling: a synthesis

Collagen fibril formation has been studied in tissues by light and electron microscopy; in solution by light scattering and microscopy; and from modeling based on the amino acid sequence of type I collagen. Taken together these studies indicate that collagen fibril assembly involves a stepwise formation of intermediate aggregates in which each intermediate is formed from earlier aggregates. In this sequence, monomeric collagen contributes only to the formation of early aggregates; and fibrils grow in length by the addition of intermediate aggregates to the end of a subfibril and in width by lateral wrapping of subfibrils. Modeling based on amino acid sequence data of possible intermolecular charge-charge interactions indicate 2 different kinds, one which promotes linear aggregation and the other which promotes linear aggregation. The effects of different collagens and coprecipitants such as glycoproteins and proteoglycans can begin to be explained by their influence on the character of intermediate subassemblies. Ultrastructural data from 2 tissues, embryonic cornea and tendon, indicate that the site of fibril growth and assembly is at the cell surface.

Structure and function of basement membrane

Progress has been made in identifying and characterizing basement membrane macromolecules, including type IV collagen, laminin, a heparan sulfate proteoglycan and bullous pemphigoid antigen. Basement membrane contains a unique collagen, type IV collagen, which is formed of pro alpha 1(IV) (Mr = 185,000) and pro alpha 2(IV) (Mr = 170,000) chains. As opposed to the fibrillar pattern seen with other collagens, the type IV collagen molecules are thought to be arranged in a honey-comb or reticular pattern which provides the major structural element of the basement membrane. Consistent with this model, type IV collagen has been localized to the basement membrane lamina densa, a nonfibrillar structure. Laminin is a large (Mr = 1,000,000) noncollagenous glycoprotein with chains of 200,000 and 400,000 daltons. It has been localized to the basement membrane lamina lucida and functions to bind epithelial cells to the basement membrane. A heparan sulfate proteoglycan has also been identified in the basement membrane. Its biological function may be to restrict the penetration of anionic macromolecules through the basement membrane. In contrast to the above-mentioned components which are found in all tissue basement membranes, bullous pemphigoid antigen is only found in certain basement membranes, mostly those of stratified squamous epithelia. Bullous pemphigoid antigen is a protein, synthesized by keratinocytes in culture, with disulfide-linked chains (Mr = 220,000). By immunoelectron microscopy, it is localized in the lamina lucida of epidermal basement membrane and is closely associated with the basal cell surface. Its biological function is not known, but could involve epidermal basal cell-substrate interactions which occur when basal cells re-epithelialize wounds.