Effects of acid on the basal lamina of the rat stomach and duodenum

Rapid restitution of the gastric and intestinal epithelium after acute injury involves emigration of cells from the gastric glands and basal half of the intestinal villi. An intact basal lamina is prerequisite to the restitution process. The present study was performed to determine the effects of acid on the rat gastric and duodenal basal lamina. The basal lamina was denuded in vitro by ultrasonic vibration. The tissue was then immersed in 0.2 M mannitol (control) or in HCl (5-50 mM) for 10 min. Samples of the tissues were examined by transmission and scanning electron microscopy. Some samples were stained with ruthenium red to demonstrate glycosaminoglycans. The lower concentrations of acid (5 and 10 mM) had little or no effect on the structure of the basal lamina. However, exposure to 20 and 50 mM HCl caused extensive damage to the basal lamina and exposed the underlying connective tissue matrix of the lamina propria. Ruthenium red staining demonstrated differences in size and location of glycosaminoglycans within the basal laminae of stomach and intestine. Exposure to acid at concentrations of 20 or 50 mM caused total loss of ruthenium red staining in both intestinal and gastric basal laminae. Exposure to 10 mM acid resulted in loss of the outermost (luminal) layer of anionic sites from the gastric basal lamina. These studies demonstrate that brief exposure to acid, in concentrations which are necessary for the formation of hemorrhagic erosions in the stomach, caused damage to the basal lamina. This damage may impair epithelial restitution and thus account, in part, for the role of acid in ulcerogenesis.     

The role of Plasmodium berghei ookinete proteins in binding to basal lamina components and transformation into oocysts.

The ookinete is a motile form of the malaria parasite that travels from the midgut lumen of the mosquito, invades the epithelial cells and settles beneath the basal lamina. The events surrounding cessation of ookinete motility and its transformation into an oocyst are poorly understood, but interaction between components of the basal lamina and the parasite surface has been implicated. Here we report that interactions occur between basal lamina constituents and ookinete proteins and that these interactions inhibit motility and are likely to be involved in transformation to an oocyst. Plasmodium berghei ookinetes bound weakly to microtitre plate wells coated with fibronectin and much more strongly to wells coated with laminin and collagen IV. A 1:1 mixture of collagen and laminin significantly enhanced binding. Binding increased with time of incubation up to 10 h and different components showed different binding profiles with time. Two parasite molecules were shown to act as ligands for basal lamina components. Western blots demonstrated that the surface molecule Pbs21 bound strongly to laminin but not to collagen IV whereas a 215 kDa molecule (possibly PbCTRP) bound to both laminin and collagen IV. Furthermore up to 90% inhibition of binding of ookinetes to collagen IV/laminin combination occurred if parasites were pre-incubated with anti-Pbs21 monoclonal antibody 13.1. Some transformation of ookinetes to oocysts occurred in wells coated with laminin or laminin/collagen IV combinations but collagen IV alone did not trigger transformation. No binding or transformation occurred in uncoated wells. Our data support the suggestion that ookinete proteins Pbs21 and a 215 kDa protein may have multiple roles including interactions with midgut basal lamina components that cause binding, inhibit motility and trigger transformation.     

Thinning of the basement Membrane and Localized Cell Proliferation during Gland Formation of the Chick Proventriculus

The early processes of proventricular gland formation in the chick embryo were investigated. The glands appeared as intra-epithelial invaginations of the proventricular endoderm on day 6 of incubation. By day 6.5 they began to protrude into the mesenchyme and elongated without branching until day 9. Before elongation of the glands, the immunofluorescence of laminin and the ultrastructure of the basal lamina were consistently observed in the intra-epithelial invaginations as well as in other regions, and the mitotic activity in the gland rudiments was not different from that in other regions. However, at the tips of the elongating glands, little laminin was detected and the basal lamina were thin and discontinuous. The mitotic activity at the tip of the glands was higher than that in non-glandular epithelium or in the stalk of the glands. These results suggest causal relationships between thinning of the basement membrane and localized epithelial cell proliferation at the tip of the elongating glands.     

Rapid onset of (R)-α-methylhistamine protection in response to ethanol-induced histologic damage in rat gastric mucosa

The influence of pretreatment with (R)-α-methylhistamine, selective agonist of histamine H₃ receptors, has been investigated on gastric mucosal lesions at different time intervals, from 5 to 60 minutes, after administration of absolute ethanol in the rat. The amount and depth of lesions were quantitatively evaluated by light microscopy. In rats pretreated with (R)-α-methylhistamine, the integrity of the mucosa was preserved in approximately 80% of the total mucosal length measured despite ethanol challenge. Prevention of lesion formation was as great at 5 min after ethanol administration as at 60 min. When present, damage involved mainly superficial mucosa and lesions extending deeply into the gland region were evident in 1–2.5% of the total mucosa. Present findings indicate that mechanisms by which (R)-α-methylhistamine operates enable the mucosa to counteract damage just from the moment of exposure to ethanol and that protection is exerted both on surface and pit cells and on gastric glands.     

Basal lamina formation by porcine thyroid cells grown in collagen- and laminin-deficient medium

Summary Porcine thyroid cells isolated by dispase treatment were cultured in either (a) Matrigel, (b) agarose with the addition of different combinations of basic fibroblast growth factor and laminin, or (c) on agarose-coated dishes. The formation of follicles and the presence of a basal lamina was investigated by routine electron microscopy of Araldite-embedded material and by light and electron microscopical immunocytochemical detection of the basal lamina components, laminin and collagen type IV. After 10 days of culture in Matrigel or agarose, a basal lamina-like structure surrounded most follicles. Follicles of cells growing in agarose and overlaid with a medium containing thyrotropin and fibroblast growth factor showed a fluorescent band at the basal side of the follicles after immunocytochemical staining with anti-laminin and anti-collagen antibodies. Routine electron microscopy showed that a basal lamina-like structure lined the outside of the follicle. This structure could be subdivided into a lamina lucida and a lamina densa. Electron microscopical immunogold labelling revealed that immunologically detectable laminin was confined to the lamina densa. These findings suggest that even in the absence of basal lamina components in the culture medium, thyroid cells are able to form follicles with a regular basal lamina when they are cultured in a three-dimensional environment.     

Basal lamina formation on thyroid epithelia in separated follicles in suspension culture

When thyroid follicles are isolated by collagenase treatment of minced thyroid lobes, the basal lamina around each follicle is removed. The basal lamina does not reform when follicles are cultured in suspension in Coon's modified Ham's F-12 medium containing, in addition, 0.5% calf serum, insulin, transferrin, and thyrotropin. We have added acid soluble collagen and/or laminin to see if they would result in the formation of a basal lamina. An extended basal lamina did not form when follicles were embedded in a gel formed from acid-soluble rat tendon collagen or from calf skin collagen when added at a concentration of 100 micrograms collagen/ml. However, laminin at a concentration of 5.1 micrograms/ml gave rise to short segments of a basal lamina within 30 min. At longer time intervals, the segments lengthened and covered the base of many cells, and were continuous across the gap between cells and across the mouth of a coated pit. Not all basal surfaces were covered, and no exposed apical surfaces with microvilli had a basal lamina. There was no obvious difference in the appearance of the basal lamina if collagen was added in addition to laminin, but collagen, in contact with the plasma membrane when added alone, was lifted off the membrane in the presence of the basal lamina. The basal lamina appeared denser if formed in the presence of 5% serum instead of 0.5%.     

The roles of ethanol and of acid in the production of gastric mucosal erosions in rats

This study was undertaken to differentiate between the morphological changes produced in chambered rat gastric mucosae by 40% ethanol and by 50 mM HCl. 40% ethanol produced both focal mucosal hyperemia and widespread exfoliation of the surface epithelium. Massive release of mucus accompanied both events. In the absence of acid the released mucus was stabilized by a network of fibrin, and epithelial continuity was re-established over non-hyperemic regions by migration of epithelial (and parietal) cells from the gastric pits. Hemorrhagic erosions occurred only in the presence of acid, but were limited to the hyperemic regions. Acid had the following effects: (1) platelet thrombi were destroyed, thus promoting hemorrhage; (2) destruction of the fibrin network by acid caused dissipation of the adherent mucous coat; (3) vulnerable cells which had previously shown only ischemic damage were irreversibly damaged by acid; (4) exposed basal lamina was destroyed, thus removing the substratum necessary for orderly epithelial re-establishment.     

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.     

Impairment of H+-K+-ATPase-dependent proton transport and inhibition of gastric acid secretion by ethanol

Ethanol (1-20% vol/vol) caused a dose-dependent reduction in the basal rate of acid formation in isolated rabbit gastric glands with a calculated EC(50) value of 4.5 +/- 0.2%. Ethanol also reduced ATP levels in isolated gastric glands and in cultured parietal cells (EC(50): 8.8 +/- 0.4% and 8.5 +/- 0.2%, respectively) and decreased both basal and forskolin-stimulated cAMP levels. In studies carried out in gastric gland microsomes, ethanol inhibited the hydrolytic activity of H+-K+-ATPase(EC(50): 8.5 +/- 0.6%), increased passive proton permeability (EC(50): 7.9%), and reduced H+-K+-ATPase-dependent proton transport (EC(50): 3%). Our results show that the inhibition of gastric acid secretion observed at low concentrations of ethanol (< or =5%) is mainly caused by the specific impairment of H+-K+-ATPase-dependent proton transport across cell membranes rather than inhibition of the hydrolytic activity of H+-K+-ATPase, reduction in the cellular content of ATP, or increase in the passive permeability of membranes to protons, although these changes, in combination, must be relevant at concentrations of ethanol > or =7%.     

Location and possible function of fibronectin and laminin in clones of chick retinal pigmented epithelial cells

Summary Distribution and organization of the extracellular glycoproteins, fibronectin and laminin, in clonal cultures of chick retinal pigmented epithelial cells have been investigated using indirect immunofluorescence microscopy. Fibronectin is located on the apical and basal cell surfaces and between the cells in the undifferentiated regions of the colony (outer edge and stratified region). It seems to run parallel to intracellular microfilament bundles and to be associated with them across the cell membrane. In the differentiated region of thecolony (center), it is located exclusively on the basal cell surface and seems to be primarily associated with the collagen bundles of the basement membrane. The locations suggest that it may be necessary to stabilizing the sheet of differentiated cells in the colony center. In all regions except the outer edge of the colony, laminin is associated with the basal cell surfaces where it forms a meshwork of short, fine strands. The laminin has a totally different staining pattern from the fibronectin and does not seem to be associated with collagen bundles. The location suggests that laminin may be present in the basal lamina and may be involved in adhesion of the cells to the substratum. This work was supported by Medical Research Council of Canada (MA-6337).     

Matrix metalloproteinase‐3 removes agrin from synaptic basal lamina

Agrin, a heparin sulfate proteoglycan, is an integral member of the synaptic basal lamina and plays a critical role in the formation and maintenance of the neuromuscular junction. The N‐terminal region of agrin binds tightly to basal lamina, while the C‐terminal region interacts with a muscle‐specific tyrosine kinase (MuSK) to induce the formation of the postsynaptic apparatus. Although the binding of agrin to basal lamina is tight, the binding of agrin to MuSK has yet to be shown; therefore, basal lamina binding is critical for maintaining the presentation of agrin to MuSK. Here we report evidence that supports our hypothesis that matrix metalloproteinase‐3 (MMP‐3) is responsible for the removal of agrin from synaptic basal lamina. Antibodies to the hinge region of human MMP‐3 recognize molecules concentrated at the frog neuromuscular junction in both cross sections and whole mounts. Electron microscopy of neuromuscular junctions stained with antibodies to MMP‐3 reveals that staining is found in the extracellular matrix surrounding the Schwann cell. Treatment of sections from frog anterior tibialis muscle with MMP‐3 results in a clear and reproducible removal of agrin immunoreactivity from synaptic basal lamina. The same MMP‐3 treatment does not alter anti‐laminin staining. These results support our hypothesis that synaptic activity results in the activation of MMP‐3 at the neuromuscular junction and that MMP‐3 specifically removes agrin from synaptic basal lamina. © 2000 John Wiley & Sons, Inc. J Neurobiol 43: 140–149, 2000     

Comparative expression of laminin and smooth muscle actin in the testis and epididymis of poultry and rabbit

The present investigation was conducted to demonstrate laminin and α smooth muscle actin (αSMA) in the testis and epididymis of adult chickens, Sudani ducks, pigeons, and rabbits. This study may represent the first indication for the presence of laminin in the male reproductive organs of birds and rabbits and might therefore serve as a milestone for further reports. In the testis of chicken, Sudani duck, pigeon, and rabbit, the laminin was localized in the basal lamina of the seminiferous tubules and of the peritubular myoid cells, in the testicular capsule and to a small extent in the vicinity of Leydig cells. The testicular vasculature also exhibited intense laminin immunostaining. Weak laminin staining was additionally seen in the cytoplasm of the duck Sertoli cells. In the epididymis, the basal lamina of the epididymal epithelium showed a distinctly positive reaction in all birds and rabbit. The basal lamina of the periductal myoid cells also showed a positive reaction. In the interductal tissue, laminin immunostaining was particularly observed in chicken, duck and pigeon. Laminin positive reaction was also seen in the epididymal vasculatures of all birds and rabbit. Interestingly, weak to moderate laminin staining was observed in the apical surface of the ciliated cells of the proximal and distal efferent ductules in chicken, duck and pigeon. αSMA positive reaction was seen in the testicular capsule and in the peritubular myoid cells of all birds and rabbit. In the testicular capsule, αSMA staining was either observed in the inner portion (chicken) or throughout the tunica albuginea (Sudani duck and pigeon), or in the outer aspect (rabbit). Distinct αSMA reaction was additionally observed in the testicular vasculature. In the epididymis of all birds and rabbit, the αSMA was particularly seen in the periductal and interductal myoid cells as well as in the epididymal vasculatures. No αSMA specific staining was however detected in the epididymal epithelium, fibrous lamina propria, and luminal spermatozoa of all birds and rabbits. In conclusion, the distribution of laminin and αSMA in the testis and epididymis might point out to their roles in the male reproduction.     

Embedding in a collagen gel stabilizes the polarity of epithelial cells in thyroid follicles in suspension culture

Separated thyroid follicles are stable in suspension culture in Coon's modified Ham's F12 medium containing 0.5% calf serum. They resemble follicles in vivo except for the absence of a basal lamina. However, the epithelial cells reverse polarity and the follicles invert when the serum concentration is raised to 5%. A number of substances, especially components of extracellular matrix, were added to the medium to ascertain if they could stabilize the follicles against inversion in 5% serum. Cellular and plasma fibronectin, gelatin, heat-denatured collagen, methylcellulose and laminin did not stabilize. The addition to the medium of as little as 50 micrograms/ml of acid-soluble collagen prepared from calf skin or rat tail tendons resulted in the formation of small clouds of gel. Follicles embedded within the gel were stabilized. Follicles in the same dish but not embedded in the gel inverted. Stabilization was not specific for collagen, since follicles embedded in a plasma clot were also stabilized. A gel was not sufficient for stabilization, since embedding in an agarose gel did not stabilize. Ultrastructural studies indicate that adherence to a limited number of gelled fibers of collagen covering only a small fraction of the basal plasma membrane may be sufficient to stabilize and that a basal lamina formed in the presence of laminin but without added collagen does not stabilize.     

Ultrastructural orientation of laminin 5 in the epidermal basement membrane: an updated model for basement membrane organization.

Laminin 5 is a trimeric glycoprotein involved in cell adhesion in the epidermal basement membrane. To determine the precise orientation of laminin 5 in adult human skin, we used plural epitope-specific monoclonal antibodies, a polyclonal antiserum, and postembedding immunogold electron microscopy (IEM). Immunogold labeling distances from the basal keratinocyte plasma membrane (PM) were measured for each gold particle (>200 particles) and the mean distance (nm) calculated. Antibodies included BM165 (recognizing the alpha 3-chain first globular domain) that was measured at 35.40 +/- 2.20 nm from the keratinocyte PM, K140 (recognizing a region adjacent to the beta 3-chain globular domain IV) that measured 45.20 +/- 3.60 nm from the PM, and an anti-laminin 5 polyclonal antiserum that was 43.43 +/- 6.28 nm from the PM. The laminin 5 gamma 2-chain short arm hinge domain was previously localized to the lower lamina densa (LD) at approximately 56.30 +/- 1.65 nm from the keratinocyte PM. Taken together with previous gamma 2-chain data and the distribution of the polyclonal antisera, we estimate that the long axis of laminin 5 is oriented at an angle of approximately 27 degrees from the horizontal lamina lucida (LL)/LD border and propose that the gamma 2-chain lies farthest from the PM. This novel orientation, with the majority of the laminin 5 molecule lying obliquely along the LL/LD border and not perpendicularly, as was first thought, sheds new light on the organization of the basement membrane and likely molecular interactions.     

Immunolocalization of laminin during estrogen-induced differentiation of quail oviduct epithelial cells

During estrogen-induced development of the quail oviduct, tubular glands are formed by evagination of epithelial cells into the stroma. The distribution of laminin was studied during the early stages by means of immunofluorescence and immunoperoxidase techniques. Ultrastructural changes in the basal lamina were studied by electron microscopy. Basement membranes at all stages of development were delineated with 3 polyclonal antilaminin antisera. However, in ovariectomized birds, laminin could not be detected by one of the polyclonal antilaminin antisera. Subsequently, this antibody detected laminin as epithelial cell evaginations were induced by estradiol benzoate. The heavy and light chains of Engelbreth Holm sarcoma (EHS) laminin were revealed in immunoblotting by all antibodies. By electron microscopy after the immunoperoxidase technique with antilaminin antisera laminin appears to be accumulated mainly in the lamina densa. Furthermore, the thickness of the basal lamina increases during oviduct development. These data indicate that basal lamina organization is modified during oviduct cell differentiation and that immunoreactivity of epithelial basement membrane laminin changes during development.     

The matrix form of collagen and basal microporosity influence basal lamina deposition and laminin synthesis/secretion by stratified human keratinocytes in vitro

Summary The ability of the collagen matrix form to support the formation of a basal lamina by cultured normal human epidermal keratinocytes (NHEK) was determined using transmission electron microscopy. The collagen matrix forms tested in this study were a) a dry type I collagen film and b) a type I collagen gel. NHEK were grown for 14 days on the following five different substrates: plain plastic culture dishes without the addition of collagen (PP); plain plastic culture dishes overlaid with a dry, aldehyde-crosslinked type I collagen film (DCF-P); plain plastic culture dishes overlaid with an aldehyde-crosslinked type I collagen gel (GEL-P); Millipore Millicell CM microporous membranes overlaid with a dry, aldehyde-crosslinked type I collagen film (DCF-CM); and Millipore Millicell CM microporous membranes overlaid with an aldehyde-crosslinked type I collagen gel (GEL-CM). NHEK maintained for 2 wk on PP and DCF-P were unable to secrete a basal lamina. NHEK grown for 2 wk on the GEL-P and GEL-CM substrates, however, secreted a contiguous basal lamina at the GEL-NHEK interface. To determine if the appearance of this basal lamina correlated with laminin synthesis, laminin was immunoprecipitated from cellular extracts, as well as media from the apical and basal chambers. NHEK grown on the GEL-P substrate synthesized more laminin than did NHEK grown on the other four alternative substrates. In addition, NHEK grown on GEL-CM were able to direct more laminin to the basal compartment than NHEK grown on DCF-CM substrates. Taken together, the data indicate that the matrix form of collagen can influence basal lamina deposition, laminin synthesis, and laminin trafficking in NHEK.     

Methodological dependence in the ultrastructural immunolocalization of laminin in tubular basement membranes of the mouse kidney

The ultrastructural localization of the basement membrane glycoprotein laminin was investigated in basement membranes of proximal tubules of the mouse kidney. The localization of laminin was determined using two different immunoperoxidase and one immunogold preembedding technique and one immunogold postembedding technique on unfixed and formaldehyde fixed tissue. Strong differences in the immunolocalization for laminin were found in the lamina densa of the tubular basement membrane using different techniques. After preembedding immunostaining for laminin using IgG--PO as secondary antibody, a positive reaction for the lamina densa was found in the formaldehyde fixed as well as in the unfixed kidney. After preembedding immunostaining for laminin using Protein-A--PO, staining of the 1. densa was seen in the unfixed, but not in the fixed kidney. It was striking that no clear immunoreaction in the 1. densa of the tubular basement membrane was seen in either the fixed or unfixed tissue after preembedding immunostaining for laminin using protein A-gold. With a direct postembedding immunogold technique laminin was localized only in the 1. fibroreticularis and the 1. rara but not in the 1. densa of basement membranes of proximal tubules of the unfixed and the fixed kidney.     

Simultaneous labelling of basal lamina components and acetylcholinesterase at the neuromuscular junction

Summary A double labelling technique has been developed which permits the concomitant localization of basal lamina constituents together with acetylcholinesterase in mouse skeletal muscles. First, using the protein A-gold technique, type IV collagen and laminin were revealed on basal laminae ensheathing skeletal muscle fibres. The immunolabelling for both proteins was higher in synaptic than extrasynaptic regions. At synaptic sites the anti-type IV collagen immunolabelling exhibited an asymmetry; it was more intense on the portion of basal lamina closest to the postsynaptic membrane, whereas the anti-laminin immunolabelling was more uniformly distributed. It was also observed that the laminin immunoreactivity associated with Schwann and perineural cells was higher than that of skeletal muscle fibres. Secondly, the two basal lamina antigens were revealed simultaneously with another synaptic protein, acetylcholinesterase, using a refined cytochemical technique prior to the immunolabelling. The cytochemical reaction, which facilitates the location of endplates, did not alter the immunolabelling pattern. This double labelling procedure permits ready comparison of the distributions of type IV collagen and laminin with that of acetylcholinesterase, and may prove to be a useful approach in studies on synaptic components in developing and diseased muscle.     

Application of frozen thin sectioning immunogold staining to the study of the developing neuroepithelial basal lamina

In order to examine the deposition of basal lamina components in the developing neuroepithelium, a technique for frozen thin sectioning and immunogold staining of early embryonic tissue was developed. Different fixatives and buffer systems were evaluated to determine which best retained immunoreactivity and satisfactory ultrastructure of day 9 and 10 mouse embryos. Fixation in sodium phosphate and sodium bicarbonate buffers did not retain antigenicity, and incubations in TBS (trishydroxymethyl-aminomethane buffered saline) in an effort to 'restore' immunoreactivity were similarly unsuccessful. Fixation in sodium cacodylate buffer, however, did retain the antigenicity of basal lamina components; the pattern of type IV collagen and laminin distribution was clearly determined. These results represent the first report of on-grid immunocytochemistry of early embryonic material.     

Basal lamina formation by epithelial cell lines correlates with laminin A chain synthesis and secretion.

Laminin is a major component of the basal lamina upon which all epithelial cells reside in vivo. The synthesis of basal lamina components and their subsequent assembly into a morphologically distinct basal lamina is a differentiated function of epithelial cells in vivo. Ultrastructural studies in our laboratory show that some epithelial cell lines (P-MDCK) form a basal lamina when cultured on membrane-permeable substrate (Millipore Millicells or type I collagen gels). Under the same conditions other epithelial cell lines (MDCK-AA7, M-mTAL-1P, and T84) do not form a basal lamina. When metabolically labeled with [35S]methionine, laminin A and B chains can be immunoprecipitated from the culture medium and culture lysates of P-MDCK cells. In contrast, labeled laminin chains cannot be immunoprecipitated from the culture medium of MDCK-AA7, M-mTAL-1P, and T84 cells. Immunoprecipitates of MDCK-AA7, M-mTAL-1P, and T84 culture lysates demonstrate the presence of one or both B chains but not A chains. These results suggest that laminin B chain synthesis is constitutive in MDCK-AA7, M-mTAL-1P, and T84 cells and that B chains, in the absence of A chains, are not secreted. Furthermore, laminin secretion and basal lamina formation are not required to maintain structural and functional polarity in these cell lines.