Fibronectin in basal cell epithelioma: sources and significance

Cell-type related and spatial variation in the expression of integrins in cutaneous tumors

Integrins constitute a group of transmembrane proteins which mediate cell-cell and cell-matrix interactions. Previous studies have shown both increased and decreased expression of integrins in relation to malignancy and invasion. In the present study, we investigated integrin distribution in cutaneous tumors by using monoclonal antibodies on frozen tissue sections. Antibodies to integrin subunits alpha v, alpha 3, alpha 4, alpha 5, alpha 6, beta 1 and beta 3 were used. The study was designed to explore (i) the association between integrin expression and the tumor type, and (ii) the effect on the integrin expression of the location of the tumor, i.e. whether it grows intraepidermally or within various compartments of the dermis (papillary or reticular). Beta 1, beta 3 and alpha 3 were strongly or moderately expressed in the epithelial and stromal cells of basal cell carcinomas (BCC), seborrheic keratoses, solar keratoses, dermatofibromas (DF), and showed a variable expression in the nevic cells of benign and dysplastic nevocellular nevi. alpha v and in alpha 5 appeared strongly expressed in the stromal cells of BCC and DF, while only a focal, often weak staining was seen in nevic cells and in the epithelial cells of BCCs. In some nevocellular nevi, they were only expressed, together with alpha 4, in the deep-seated nevic cells in the reticular dermis. alpha 6 was expressed by tumor cells of BCCs and nevocellular nevi only within the dermo-epidermal junction. In seborrheic keratosis and solar keratosis a basement membrane-associated staining pattern for alpha 6 was seen in the basal cell layer, with focal discontinuities in solar keratosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunoelectron microscopic localization of fibronectin in cultured human keratinocytes

We investigated the ultrastructural localization of fibronectin (FN) in cultured human keratinocytes using an improved method of peroxidase-immunoelectron microscopy. This method enabled us to visualize the precise localization of FN within the cells while preserving the morphology. FN was localized in the protein synthetic and secretory organelles, including the rough-surfaced endoplasmic reticulum, Golgi complex, multivesicular bodies and perinuclear space. It was also detected in the extracellular space, on small regions of the villous projections of cell membranes at the site of secretion and at cell-substratum contact sites. These findings confirm that human keratinocytes synthesize, secrete and deposit FN in the pericellular matrix.

Differential expression of laminin isoforms and beta 4 integrin epitopes in the basement membrane zone of normal human skin and basal cell carcinomas

The basement membrane zone biology of normal human skin and basal cell carcinomas was explored by indirect immunofluorescence with monoclonal antibodies recognizing five subunit polypeptides of three different laminin isoforms as well as the beta 4 integrin epitopes. The laminin antibodies were specific for A, B1, and B2 chains of classic laminin, for the M chain of merosin, or for the S chain in S-laminin. Immunostaining of normal human skin revealed a strong signal with antibodies for A, B1, and B2 chain epitopes. A weak immunosignal was detected with an anti-M chain antibody, whereas the S-chain epitopes were undetectable, even following pretreatment of sections with hyaluronidase. Thus, the laminin at the epidermal-dermal junction of normal human skin is primarily of the classic type, with some merosin molecules being present. The staining of six nodular basal cell carcinomas revealed the presence of A, B1, and B2 chain epitopes in a linear pattern, but, in contrast to normal skin, the antibody recognizing M-chain epitopes yielded a strong immunosignal, and S-chain epitopes could also be readily detected. Staining for beta 4 integrins, potential receptors for laminin, revealed a strong staining reaction in normal skin as well as in the superficial portions of the basal cell carcinoma. However, the immunofluorescence pattern in the deeper portions of the lesions was scattered and interrupted. Thus, altered composition of the basement membrane of nodular basal cell carcinomas with respect to laminin isoforms and their interactions with putative cell-surface receptors, the beta 4 integrins, may change the containment of the tumor islands, contributing to the local aggressive behavior of basal cell carcinomas.

Growth of human basal cell carcinomas transplanted to C57/Balb/C bgJ/bgJ nu/nu (beige-nude) mice

The purpose of this study was to measure growth parameters of transplanted basal cell carcinoma (BCC) to beige-nude mice during a 4-month observation time. Forty male beige-nude mice were transplanted with human BCC with our reported subcutaneous implantation technique. Initial volume and wet weight were determined for each tumor. The tumors were measured every 2 weeks by calipers with a final volume determined at 120 days, at which time the tumors were harvested, weighed, and processed for routine histology. Thirty-two tumor sites were positive for persistent tumor at harvest. Tumor volumes declined by an average of 51% and average tumor weight by 33%. There were increased numbers of mast cells surrounding the BCC tumor lobules. These results indicate that BCC can survive for 120 days in the beige-nude mouse.

Biology of basal cell carcinoma (Part I)

Basal cell carcinoma is the most common malignancy in humans. Although rarely metastatic, it is capable of significant local destruction and disfigurement. This two-part article reviews the current understanding of basal cell carcinoma biology. Part I examines significant clinical, histologic, and ultrastructural features that relate to invasive potential. Genetic characteristics, including tumor growth rate, chromosomal abnormalities, and oncogene presence, are discussed, and expression of important cell and matrix proteins, including keratin, fibronectin, and HLA antigens, are reviewed. Further topics to be explored in Part II include host immunologic responses, theories of pathogenesis, and valuable second-line therapeutic regimens for treatment of multiple cancers.

Localization of integrin receptors for fibronectin, collagen, and laminin in human skin. Variable expression in basal and squamous cell carcinomas

VLA integrins in human skin were examined by indirect immunofluorescence utilizing antibodies recognizing the beta 1, alpha 2, alpha 3, or alpha 5 subunits. Staining of fetal, newborn, or adult skin with antibodies to beta 1, alpha 2, or alpha 3 subunits gave essentially similar staining patterns: intense staining was associated with the basal layer of the epidermis, hair follicles, and blood vessel walls. The alpha 5 subunit could be detected only in epidermis and the inner root sheath of hair follicles in fetal skin. In epidermis, the staining reaction for the beta 1 subunit was not only found in sites interfacing with the basement membrane zone, but also around the entire periphery of these cells. We speculate that these receptors might have previously unrecognized functions in cell-cell interactions or that these findings may suggest the presence of previously unrecognized ligands in the intercellular spaces of keratinocytes. Examination of nine nodular basal cell carcinomas revealed a prominent staining reaction with anti-beta 1 and anti-alpha 3 antibodies at the periphery of the tumor islands. In contrast, staining of five squamous cell carcinomas revealed either the absence of integrins or altered and variable expression. Thus, matrix components and their receptors may participate in modulation of growth, development, and organization of human skin.

Pathogenesis of tumor stroma generation: a critical role for leaky blood vessels and fibrin deposition

Tumor stroma formation results from the interaction of tumor cells and their products with the host and certain of its normal defense mechanisms, particularly the clotting and fibrinolytic systems. It is a process in which tumor cells render local venules and veins hyperpermeable with the result that fibrinogen and other proteins extravasate and clot, forming an extravascular crosslinked fibrin gel. Coagulation is mediated by an interaction between extravasated plasma clotting factors and tumor-associated and perhaps other tissue procoagulants. Parallel activation of the fibrinolytic system leads to substantial fibrin turnover, but fibrin nonetheless accumulates in amounts, variable from tumor to tumor, that are sufficient to provide a provisional stroma. This provisional stroma imposes on tumor cells a structure that persists even as tumor cells multiply and as the fibrin provisional stroma is replaced by mature connective tissue. The provisional fibrin stroma also serves to regulate the influx of macrophages, and perhaps other inflammatory cells, but at the same time, and in ways that are not fully understood, facilitates the inward migration of new blood vessels and fibroblasts, integral components of mature tumor stroma. Ascites tumors differ from solid tumors in that fibrin gel is not ordinarily deposited in body cavities and, as a result, there is no provisional stroma to impose an initial structure. Tumor stroma generation resembles the process of wound healing in many respects. However, it differs in the mechanism of its initiation, and in the apparent lack of a role for platelets. It also differs fundamentally in that invading tumor cells continually render new vessels hyperpermeable to plasma, thus perpetuating the cycle of extravascular fibrin deposition. In this sense, tumors behave as wounds that do not heal. Largely neglected in this review has been discussion of the numerous cytokines, mitogens, and growth factors that are widely believed to play important roles in tumor angiogenesis and wound healing; i.e., PDGF, FGF, EGF, TGF alpha, TGF beta, TNF, interferons, etc. This omission has been intentional, and for two reasons. First, these cytokines have already received considerable attention [100,123-128]. Second, it is not yet clear how closely the actions of these molecules, as described in vitro, relate to their functions in vivo. At present we are deluged with a surfeit of factors that have the capacity to induce new blood vessel formation in angiogenesis assays; these factors include not only peptides but lipids and even ions [126,129-131].(ABSTRACT TRUNCATED AT 400 WORDS)

Fibronectin gene expression by epithelial tumor cells in basal cell carcinoma: an immunocytochemical and in situ hybridization study

Previous observations have demonstrated that fibronectin is deposited in high abundance in basal cell carcinoma stroma. In this study, the nature of fibronectin and the site of its synthesis were explored in 10 basal cell carcinomas of the nodulo-ulcerative type by immunocytochemistry and in situ hybridization. First, simultaneous localization of epithelial tumor cell islands and fibronectin epitopes was carried out by double immunofluorescence staining with monoclonal anti-cytokeratin antibodies and polyclonal fibronectin antibodies, the latter recognizing both the cellular and plasma types of the protein. Large amounts of fibronectin were deposited in the basal cell carcinoma stroma, with the highest concentration present in the immediate proximity of the epithelial cell islands. Immunofluorescence with a monoclonal anti-fibronectin antibody, which is directed against the ED-domain of cellular fibronectin and does not recognize the plasma type of fibronectin, revealed essentially the same staining pattern as that obtained with the polyclonal anti-fibronectin antibody. This observation suggested that fibronectin in BCC was predominantly of the cellular type. Second, in situ hybridizations, utilizing a human fibronectin specific cDNA, demonstrated that the highest concentration of fibronectin mRNA was found in the most peripheral cell layer of the epithelial tumor islands. The presence of fibronectin mRNAs in the tumor cells of the central regions of the islands, as well as within occasional stromal cells, was also noted. Thus, two lines of evidence suggest that the epithelial tumor cells are predominantly responsible for the synthesis and deposition of fibronectin in basal cell carcinoma. The presence of fibronectin may explain the characteristic biologic behavior of basal cell carcinomas, including low degree of metastatic potential and local destructive nature of the tumors.

Characterization and practical benefits of keratinocytes cultured in strontium-containing serum-free medium

Strontium (Sr2+) can substitute for Ca2+ and stimulate a variety of functions of numerous types of cells. The purpose of this study was to investigate the details of the biologic effects of Sr2+ on human keratinocyte growth, cell cycle, viability, and differentiation and to compare these effects with Sr2+ effects on cultured skin melanocytes. Cultured keratinocytes stimulated with 1.0-3.0 mM Sr2+ showed higher viability and almost a twofold increase in cell number compared with those grown in a standard calcium concentration. Time course studies revealed that 2.0 mM Sr2+ had no effects on growth of cultured melanocytes or fibroblasts. Sr2+ increased the percentage of cultured keratinocytes in G2/M phase, with a decrease in cells in G0/G1 phase. This effect of Sr2+ on the cell cycle was not seen in cultured melanocytes or fibroblasts. A 2 mM concentration of Sr2+ produced an increase in low-density keratinocytes separated by a Percoll gradient. In addition, increased expression of human fibronectin was observed in the cytoplasm and on cell membranes of keratinocytes cultured in Sr2+. Sr2+ can be of practical benefit in the culture of human keratinocytes in serum-free medium, increasing the viability and proliferative rate and producing a more uniform population of basaloid cells with increased expression of cell surface fibronectin.

Selectively enhanced procollagen gene expression in sclerosing (morphea-like) basal cell carcinoma as reflected by elevated pro alpha 1(I) and pro alpha 1(III) procollagen messenger RNA steady-state levels

Sclerosing or morphea-like variant of basal cell carcinoma (BCC) is characterized by an extensive connective tissue stroma, and histopathology has suggested that the extracellular matrix is largely composed of collagen. In addition, fibronectin deposition has been proposed to modulate tumor growth in BCC. In this study, we examined the expression of genes coding for type I, III, and IV procollagens, as well as for fibronectin, in tissue from 10 patients with sclerosing BCC. For comparison, tissues from 5 patients with nodular BCC and 4 controls were examined. Total RNA was isolated by CsCl density gradient centrifugation, and messenger RNA (mRNA) steady-state levels were determined by slot-blot hybridizations with human sequence specific complementary DNAs (cDNAs). The abundance of type I procollagen mRNA in sclerosing BCC tissue was increased to 233.6 +/- 36.7% of the controls (mean +/- SEM). The corresponding value for type III procollagen mRNA in sclerosing BCC was 281.8 +/- 54.8% of the controls. Consequently, the steady-state ratio of type I/III procollagen mRNAs in sclerosing BCCs (5.0 +/- 1.2; mean +/- SD) was within the control range. Thus, there is a coordinate increase in type I and type III procollagen mRNA levels in sclerosing BCC. In contrast, the values for type I and type III procollagen mRNAs in nodular BCC were not different from the controls. In addition, type IV procollagen and fibronectin mRNA levels were not different from the controls either in sclerosing or nodular BCCs, attesting to the selectivity of the increase in type I and III procollagen mRNA levels in sclerosing BCC. These observations may relate to the excessive deposition of the extracellular matrix stroma surrounding the tumor cells in sclerosing BCC.

Growth and antigenic characteristics of basal cell carcinoma in culture

Basal cell carcinomas (BCCs) were cultured using an explant method and compared with normal cultured skin. Immunohistochemical staining revealed reduced beta 2 microglobulin uptake by BCCs in frozen section, but normal staining of the tumours in culture. In culture, fibronectin was detected on the cell surface of normal keratinocytes but not on BCCs. The above differences may explain some of the behaviour of BCCs in vivo.

Transplantation of human basal cell carcinoma to C57/BALB/C bgJ/bgJ-nu/nu (beige-nude) mouse

The purpose of this study was to evaluate the transplantation of BCC to the C57/BALB/C bgJ/bgJ-nu/nu (beige-nude) mouse. This animal has two gene defects which are expressed as impairment of natural killer cells (NK cells) as well as the absence of a thymus. The results demonstrate that human BCC can be successfully transplanted to the C57 beige-nude mouse without the need for supplemental immunosuppression. It may be that the impairment of NK cell activity in the beige-nude mouse model plays a significant role in tumor transplantation.

Peanut agglutinin immunohistochemistry of basal cell carcinoma

103 biopsies of basal cell carcinomas (BCCs) were studied using peanut agglutinin (PNA), PNA antibody and the peroxidase-antiperoxidase technique; 53 specimens of various skin tumors were examined as controls; 96% of the BCCs showed a band-like peritumorous reaction not seen in any other tumor except for the Pinkus' fibroepithelioma. The peritumorous PNA-binding was continuous in 51% of the BCCs studied and discontinuous in 45%; only 4% were completely negative. Both fibroepitheliomas revealed a discontinuous PNA-positive band. A narrow basement membrane-like positivity was seen around some small hair follicles situated within 2 neurofibromas and under one seborrhoeic keratosis. The PNA-binding band is apparently neither fibronectin, laminin, Type IV or Type V collagen and is not a constituent of normal epidermal, adnexal and vascular basement membranes.

Immunohistochemical studies of basal cell carcinomas transplanted into nude mice

Xenografting into nude mice forms a system for analysis of human tissues under experimental conditions. In this study, normal skin samples and basal cell carcinomas were investigated, prior to and after transplantation, using immunofluorescence methods with antibodies against keratins, laminin, and collagen type IV. Three groups of transplants were studied: intact tissue samples, human epithelium (either normal or neoplastic) recombined with normal human dermis and, human epithelium recombined with normal mouse dermis. Transplants recovered after 3 weeks showed the following characteristics. The xenograft system was satisfactory in terms of host survival and rate of successful tissue recovery except for recombinants between human epithelium and mouse dermis. Intact and recombined samples of normal skin retained their preexisting patterns of architecture, cytodifferentiation, and basement membrane staining. Solid nonfibrosing basal cell carcinomas showed altered architecture and differentiation of both the epithelium and the basement membrane zone after transplantation: the solid tumor pattern changed towards spreading of tumor cells, a more squamous differentiation pattern was apparent and was confirmed by reactivity with antibodies against large keratins. Discontinuities of the basement membrane zone were detected with antibodies against laminin and collagen type IV. These changes were seen in both intact and recombined tumor transplants.

The biology of basal cell carcinomas--a revisit and recent developments

This paper will review some of the present information on the biology of basal cell carcinoma. Recent developments include a better understanding of the basement membrane components surrounding the tumor nodules, as well as identifying the most actively dividing cells in solid basal cell carcinoma by autoradiographic techniques. The implications of a recently developed animal model will also be discussed.

Basal cell carcinomas grown in nude mice produce and deposit fibronectin in the extracellular matrix

Epidermal cells in vitro produce and deposit fibronectin (FN) in the pericellular matrix. Such FN production by epidermal cells may be involved in vivo in wound reepithelialization, tissue morphogenesis, and growth of epithelial tumors. The purpose of this study was to examine whether the FN, previously shown to be within and surrounding human basal cell carcinoma (BCC) lobules, was in part the product of epidermal-derived tumor cells. To examine this question we took advantage of our ability to grow human BCC in nude mice. Since we could demonstrate that all stromal cells surrounding the BCC were of mouse origin, antibodies specific for human FN would distinguish epithelial-derived FN from mesenchymal-derived FN. Five solid BCCs were implanted subcutaneously in nude mice. Growing tumors were removed after 60 days, snap-frozen, sectioned on a cryostat, and verified microscopically as BCC. The Hoescht DNA stain, which can distinguish mouse and human nuclei, demonstrated that mouse, not human, fibroblasts occupied the stroma surrounding each tumor lobule. Sections of all 5 BCCs were stained by immunofluorescence and immunoperoxidase techniques with antibodies to bullous pemphigoid (BP) antigen, laminin (LM), and FN. BP antigen and LM were present at the basement membrane zone (BMZ) of all tumor lobules as previously described for in situ BCC. FN staining was present along the BMZ, within the tumor lobules, and in the surrounding stroma. Antibodies to human FN were passed over a mouse FN affinity column to absorb antibodies which cross-reacted with mouse FN. The resultant antibody preparation, which was specific for human FN in this system, continued to demonstrate FN along the BMZ and within the tumors, but failed to stain FN in more distant stroma. Epidermal-derived cells, therefore, can synthesize and deposit FN in vivo in adjacent extracellular matrix. We speculate that this FN matrix may facilitate growth of BCC in this model.

Expression of fibronectin, laminin, and factor VIII-related antigen during development of the human cutaneous microvasculature

The regulation of angiogenesis during human skin development is poorly understood. Since fibronectin is involved in cell movement and organization during embryogenesis and morphogenesis in a variety of species, we investigated the expression of fibronectin and factor VIII-related antigen, a marker for endothelial cells, at various stages in the development of the human cutaneous microvasculature. Skin specimens were obtained from 4 human fetuses during the second trimester (14-18 weeks), from newborn foreskins, and from consenting normal adults. Cryostat sections were stained with both fluorescein-conjugated rabbit antihuman fibronectin and rhodamine-conjugated goat antihuman factor VIII-related antigen. Expression of fibronectin in the microvasculature was striking in fetal skin but became progressively less prominent with maturation. Fibronectin appeared in fetal blood vessels as a bright continuous linear array, in neonatal blood vessels as a bright interrupted linear and speckled array, and in adult blood vessels as a sparse interrupted linear and speckled array. In contrast, expression of factor VIII-related antigen by the endothelium became more prominent with the degree of maturation of the microvasculature. Granular factor VIII-related antigen staining was scant in the newly forming blood vessels of fetal skin, bright but focal in the microvasculature of newborn skin, and intense and almost confluent in the blood vessels of adult skin. Although expression of fibronectin and factor VIII-related antigen changed, expression of laminin was consistent throughout development. Staining of the same skin specimens with fluorescein-conjugated sheep antihuman laminin produced a bright continuous linear pattern in all blood vessels. The reciprocal relationship manifested by intense fibronectin staining during human blood vessel development and prominent factor VIII-related antigen staining in mature blood vessels supports the hypotheses that fibronectin plays a role in human blood vessel modulation and morphogenesis, and that factor VIII-related antigen is a marker for endothelial cell differentiation.

Transplantation of human basal cell carcinomas to athymic mice

Basal cell carcinomas (BCCs) obtained from 22 subjects undergoing microscopically controlled surgery were transplanted to 40 athymic (nude) mice. With no further immunosuppression of the mice, no tumor growth was noted in the first 14 attempts. When mice were further immunosuppressed with anti-lymphocyte serum (ALS) injections and by splenectomy, successful tumor growth was achieved in 15 of 22 mice by a subcutaneous implantation technique and in 1 of 4 by a superficial grafting technique. Transplanted BCC retained the morphology and basement membrane proteins typical of human BCC. As determined by autoradiography, 3H-thymidine was incorporated primarily in the peripheral palisaded cells of the transplanted tumor. Successful use of the athymic mouse model for study of human BCC requires use of mice further immunosuppressed by splenectomy and ALS, and the use of a subcutaneous implantation technique. With the use of this model, studies of the biology of human BCC may be possible.

Lectin-binding abnormalities in the stromal and epithelial components of basal cell carcinoma

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Alteration of the extracellular matrix of cultured human keratinocytes by transformation and during differentiation

We have investigated the production of 3 extracellular matrix proteins (fibronectin, laminin and entactin) and glycosaminoglycans (GAGs) by keratinocytes derived from human squamous-cell carcinomas (SCCs). All the SCC lines retained the ability to synthesize fibronectin, laminin and entactin, and to incorporate them into an extracellular matrix. In some of the SCC lines fibronectin production was higher than in normal keratinocytes, and in most lines laminin production was equal to or higher than that seen in normal keratinocyte strains. All the tumorigenic SCC lines produced less entactin than normal keratinocytes. Five out of 7 SCC lines showed a shift in GAG production compared with normal keratinocytes, so that in these lines heparan sulphate was the major GAG as opposed to hyaluronic acid in the normal keratinocytes. As these changes could have been a consequence of the reduced differentiation found in the SCC lines, we fractionated normal keratinocytes into 4 size classes corresponding to different stages of differentiation, using Percoll gradient centrifugation. It was found that the ability to produce these extracellular matrix glycoproteins and GAGs was lost as keratinocytes terminally differentiated. However, this did not explain the matrix changes seen in the SCC lines, since the undifferentiated normal keratinocytes produced a normal pattern of extracellular matrix components. Nonetheless, the loss of extracellular matrix production may well explain the reduction in substratum adhesiveness which occurs as keratinocytes terminally differentiate.

Cellular differentiation of basal cell carcinoma studied with fluorescent lectins and cytokeratin antibodies

The expression of cellular glycoconjugates and cytokeratin polypeptides in 8 basal cell carcinomas (BCC) was studied using fluorochrome-coupled lectins and different keratin-antibodies. Peanut agglutinin and Wistaria floribunda agglutinin, binding to all layers of normal human epidermis, also stained all cells in the basal cell carcinomas. Dolichos biflorus agglutinin, which in normal epidermis binds only to the basal cells, gave a mottled staining pattern in most of the tumors. Instead, Ulex europaeus I agglutinin and soybean agglutinin, which in normal epidermis only bind to the spinous and granular cell layers, did not stain tumor cells in basal cell carcinomas. Rabbit antibodies to human 43-50kD epidermal keratin polypeptides and 2 monoclonal cytokeratin antibodies, PKK1 reacting only with follicular epithelium, and PKK2 reacting also with the basal epidermal cells, brightly stained all cells of the basal cell carcinomas studied, whereas antibodies to human 60-67kD epidermal keratin polypeptides did not bind to the carcinoma cells. The results suggest that the cells in basal cell carcinomas resemble epidermal basal cells both by their glycoconjugate pattern and keratin expression. However, the tumor cells also express cytokeratins, which can be found only in the follicular epithelium, but not in normal interfollicular epidermis.

Human keratinocytes synthesize, secrete, and deposit fibronectin in the pericellular matrix

Fibronectin production by human keratinocytes cultured in serum-free, low-calcium medium without a fibroblast feeder layer was examined using several techniques. Immunohistochemical examination confirmed that the cultures were not contaminated with fibroblasts or Langerhans cells. By indirect immunofluorescence, fibronectin appeared as intracellular granules within all cells and as short radial fibrils between the cells and the substratum, and in the pericellular matrix. Conditioned media taken from 4-, 6-, 8-, 10-, and 12-day cultures contained fibronectin as measured by enzyme-linked immunosorbent assay in proportion to the cell number in cultures. Synthesis of fibronectin by 7-day keratinocyte cultures was determined by 18-h [35S]methionine metabolic labeling followed by sodium dodecyl sulfate polyacrylamide gel electrophoresis and autoradiography. Fibronectin accounted for 12% of the secreted protein under these culture conditions. Our results indicate that human keratinocytes under conditions that prevent terminal differentiation in vitro can synthesize, secrete, and deposit fibronectin in the extracellular matrix.