Induction of tenascin in healing wounds

The distribution of the extracellular matrix glycoprotein, tenascin, in normal skin and healing skin wounds in rats, has been investigated by immunohistochemistry. In normal skin, tenascin was sparsely distributed, predominantly in association with basement membranes. In wounds, there was a marked increase in the expression of tenascin at the wound edge in all levels of the skin. There was also particularly strong tenascin staining at the dermal-epidermal junction beneath migrating, proliferating epidermis. Tenascin was present throughout the matrix of the granulation tissue, which filled full-thickness wounds, but was not detectable in the scar after wound contraction was complete. The distribution of tenascin was spatially and temporally different from that of fibronectin, and tenascin appeared before laminin beneath migrating epidermis. Tenascin was not entirely codistributed with myofibroblasts, the contractile wound fibroblasts. In EM studies of wounds, tenascin was localized in the basal lamina at the dermal-epidermal junction, as well as in the extracellular matrix of the adjacent dermal stroma, where it was either distributed homogeneously or bound to the surface of collagen fibers. In cultured skin explants, in which epidermis migrated over the cut edge of the dermis, tenascin, but not fibronectin, appeared in the dermis underlying the migrating epithelium. This demonstrates that migrating, proliferating epidermis induces the production of tenascin. The results presented here suggest that tenascin is important in wound healing and is subject to quite different regulatory mechanisms than is fibronectin.     

Appearance of tenascin in healing skin of the mouse: possible involvement in seaming of wounded tissues

Distribution of the extracellular matrix glycoprotein tenascin during wound healing in mouse skin was studied immunohistochemically. Within 24 hours after wounding, and preceding the formation of granulation tissue, tenascin appeared in the basement membranes beneath epidermis and hair follicles adjacent to the wound edges and in the wounded edges of cutaneous muscle layer. Granulation tissue began to form in the wound space at about 1-2 days and was immediately covered by epidermis. Tenascin first appeared in the periphery of the granulation tissue beneath healing epidermis and around the wounded edges of cutaneous muscle layer. Then the tenascin-positive area extended into the inner region of granulation tissue. At about 5-7 days, all of the granulation tissue was intensely stained with anti-tenascin serum. Tenascin immunoreactivity decreased as granulation tissue was replaced with reconstructed dermal tissue at 7-14 days. In most cases, tenascin staining persisted longest in the dermis beneath the healing epidermis and at the juncture of healing edges of cutaneous muscle layer. It disappeared at about 10-14 days after wounding. These findings suggest that tenascin may play an important role in the seaming of wounded tissues.     

Tenascin gene expression in rat liver and in rat liver cells. In vivo and in vitro studies.

Tenascin is a major glycoprotein constituent of the extracellular matrix with a strong affinity to fibronectin; its distribution is believed to be temporarily and spatially limited. Tenascin gene expression is increased during wound healing processes. As repair mechanisms in chronic liver diseases resemble wound healing we studied tenascin gene expression in rat liver and in isolated rat liver cells. In normal rat liver a tenascin specific antiserum stains sinusoidal cells with fiber-like prolongations, which at the same time are desmin-positive (ITO-cells). In the CCl4-acutely-damaged liver a strong tenascin staining is detected in cells located among the mononuclear cells of the inflammatory infiltrates in the areas of necrosis and in cells of the sinusoids. In CCl4-chronically-damaged liver a strong tenascin staining is demonstrable in the connective tissue septa. In both cases, many of the tenascin-positive cells can be identified as desmin-positive by means of the double-staining fluorescence technique. The wall of larger vessels is always tensacin-negative. The staining pattern obtained with a fibronectin-specific antiserum is somewhat comparable with that of tenascin but the vessel wall was positive. hepatocytes, Kupffer cells, ITO-cells and endothelial cells were isolated from rat liver and studied for their capacity to express the tenascin gene. Biosynthetically labeled tenascin was immunoprecipated from supernatants and cell lysates obtained from cultured ITO-cells and to a much lesser extent from intracellular lysates obtained from endothelial cells; its synthesis in ITO-cells increased during the time in culture. Tenascin was also identified immuno-cytochemically in increasing amount in ITO-cells in culture.(ABSTRACT TRUNCATED AT 250 WORDS)     

Tenascin gene expression in rat liver and in rat liver cells In vivo and in vitro studies

Tenascin is a major glycoprotein constituent of the extracellular matrix with a strong affinity to fibronectin; its distribution is believed to be temporarily and spatially limited. Tenascin gene expression is increased during wound healing processes. As repair mechanisms in chronic liver diseases resemble wound healing we studied tenascin gene expression in rat liver and in isolated rat liver cells. In normal rat liver a tenascin specific antiserum stains sinusoidal cells with fiber-like prolongations, which at the same time are desmin-positive (ITO-cells). In the CCl₄-acutely-damaged liver a strong tenascin staining is detected in cells located among the mononuclear cells of the inflammatory infiltrates in the areas of necrosis and in cells of the sinusoids. In CG⁴-chronically-damaged liver a strong tenascin staining is demonstrable in the connective tissue septa. In both cases, many of the tenascin-positive cells can be identified as desmin-positive by means of the double-staining fluorescence technique. The wall of larger vessels is always tensacin-negative. The staining pattern obtained with a fibronectin-specific antiserum is somewhat comparable with that of tenascin but the vessel wall was positive. Hepatocytes, Kupffer cells, ITO-cells and endothelial cells were isolated from rat liver and studied for their capacity to express the tenascin gene. Biosynthetically labeled tenascin was immunoprecipated from supernatants and cell lysates obtained from cultured ITO-cells and to a much lesser extent from intracellular lysates obtained from endothelial cells; its synthesis in ITO-cells increased during the time in culture. Tenascin was also identified immuno-cytochemically in increasing amount in ITO-cells in culture. We conclude that ITO-cells may play a major role in tenascin synthesis during liver fibrogenesis. Some of these results were presented at the Annual Meeting of the American Association Study of the Liver, Chicago, USA, 1990. G.R. holds a Hermann and Lilly Schilling professorship     

Regulation of Astrocytic Tenascin by Basic Fibroblast Growth Factor

Extracellular matrix (ECM) molecules have been implicated in the regulation of neuronal adhesion and neurite outgrowth both during development and after injury. It has been demonstrated in our laboratory that astrocytes are heterogenous in expression of the ECM molecule tenascin. High-tenascin astrocytes have a reduced ability to support neurite outgrowth. In addition, astrocytes treated with exogenous basic fibroblast growth factor (bFGF) supported reduced neuronal growth and adhesion. In the current study, the hypothesis was tested that bFGF could increase the expression of tenascin by these cells. Basic FGF was added to cultures of rat cerebral cortical astrocytes at concentrations of up to 30 ng/ml, concentrations shown to have a significant effect on neuronal adhesion. Tenascin levels were evaluated by Western blot analysis of both cell extracts and conditioned media and also by immunocytochemistry techniques. Tenascin levels began to increase after 24-48 hr and continued to increase throughout 8 days in culture. The increase in tenascin was concentration-dependent, with the largest increase seen at 5 ng/ml bFGF. Tenascin production was increased approximately 5.5-fold in serum-containing medium but only about 2-fold in serum-free medium. When heparin (10 μg/ml) was included along with bFGF in serum-free medium, tenascin production was further enhanced. The bFGF treatment was discontinued after 8 days, and the cells were maintained for an additional 8 days in culture. Tenascin levels returned to control values, demonstrating that the bFGF effect is transient. It is our hypothesis that the action of bFGF during injury may evoke the induction of tenascin on astrocytes, thereby reducing regeneration in the central nervous system.     

Epithelial induction of stromal tenascin in the mouse mammary gland: from embryogenesis to carcinogenesis

The distribution of the extracellular matrix glycoprotein tenascin was studied by immunofluorescence in the developmental history of the mouse mammary gland from embryogenesis to carcinogenesis. Tenascin appeared only in the mesenchyme immediately surrounding the epithelia just starting morphogenesis, that is, in embryonic mammary glands from 13th to 16th day of gestation, in mammary endbuds which are a characteristic structure starting development during maturation of the mammary gland, and in the stroma of malignant mammary tumors. However, tenascin was absent in the elongating ducts of embryonic, adult, proliferating, and involuting mammary glands and preneoplastic hyperplastic alveolar nodules. The transplantation of embryonic submandibular mesenchyme into adult mammary glands induces the development of duct-alveolus nodules, which morphologically resemble developing endbuds. Tenascin reappeared around those nodules during the initial stages of their development. Tenascin expression could be induced experimentally in several ways. First, tenascin was detected at the site where the first mammary tumor cells GMT-L metastasized. Second, tenascin was detected in the connective tissue in the tumors derived from the injected C3H mammary tumor cell line CMT315 into Balb/c nude mouse. Cross-strain marker anti-CSA antiserum clearly showed that the tenascin-positive fibroblasts were of Balb/c origin. Third, when embryonic mammary epithelium was explanted on to embryonic mammary fat pad cultures, the mesenchymal cells condensed immediately surrounding the epithelium. Tenascin was detected in these condensed cells. From these three observations we conclude that both embryonic and neoplastic epithelium induced tenascin synthesis in their surrounding mesenchyme.     

Mechanism of skin morphogenesis. I. Analyses with antibodies to adhesion molecules tenascin, N-CAM, and integrin.

To understand cell interactions during induction of skin appendages, we studied the roles of adhesion molecules N-CAM, tenascin, integrin, and fibronectin during feather development. Tenascin appeared in a periodic pattern on epithelia and was so far the earliest molecule detected in placodes. Three placode domains were identified: the anterior was positive for tenascin, the distal positive for N-CAM, and the posterior lacking both. Integrin appeared in dermal-epidermal junctions of placodes. In feather buds, sagittal sections revealed a transient anterior-posterior asymmetry with tenascin and N-CAM enriched in the anterior mesoderm. Tangential sections revealed a lateral-medial asymmetry with tenascin distributed in a ring shape and N-CAM in an "X" shape. Integrin was diffusely distributed within buds. Later tenascin and N-CAM were enriched in dermal papilla, the inducer of skin appendages. Perturbation of embryonic skin explant cultures with antibodies showed that anti-integrin beta 1 and anti-fibronectin blocked epithelial-mesenchymal interaction, anti-N-CAM caused uneven segregation of mesenchymal condensation, and anti-tenascin inhibited feather bud elongation. Dose-response curves showed gradual effects by these antibodies. The results indicated that these adhesion molecules are independently regulated and each contributes in different phases during morphogenesis of skin appendages.     

Tenascin induction in tenascin nonproducing carcinoma cell lines in vivo and by TGF-β1 in vitro

Tenascin, a novel six-armed extracellular-matrix glycoprotein, is expressed in a temporally and spatially restricted pattern during carcinogenesis in association with stromal-epithelial interactions. In this study, we have tested the hypothesis that tenascin expression depends upon the change of the cellular environment from in vitro to in vivo. The distribution and alterations in the expression of tenascin were compared between in vitro and in vivo studies in a variety of human epithelial- and nonepithelial-derived cell lines. When cell lines were transplanted into nude mice, all xenografts induced host-mouse-stroma-derived tenascin. Four carcinoma-derived cell lines and all sarcoma-derived lines, which secreted tenascin in vitro, were found to produce human tenascin after transplantation. Furthermore, three carcinoma-derived cell lines, A431, HEp-2, and MCF7, which did not synthesize tenascin in vitro, did synthesize human tenascin after transplantation. These tenascin nonproducing carcinoma cell lines did not express tenascin mRNA in vitro. The addition of TGF-β1 to the culture medium induced the synthesis and secretion of tenascin, but TGF-β2 and bFGF were less effective. TGF-β1 also induced other extracellular-matrix components, fibronectin and laminin. TGF-β1 did not induce tenascin in tenascin nonproducing carcinoma cell lines, such as WiDr and A549, in which human tenascin was not induced after transplantation. We have established an in vitro system in which tenascin is induced by the diffusible factor TGF-β1. This system could shed light on the mechanism of induction of human tenascin observed in vivo in tenascin nonproducing carcinoma cell lines. © 1994 wiley-Liss, Inc.     

Tenascin: cDNA cloning and induction by TGF-beta.

cDNA clones coding for tenascin, an extracellular matrix glycoprotein with a restricted tissue distribution, were isolated from a chicken fibroblast cDNA expression library using a specific tenascin antiserum. Antibodies eluted from the cDNA-encoded fusion proteins reacted exclusively with tenascin. Limited trypsin treatment of purified tenascin resulted in a peptide which confirmed the deduced protein sequences. The largest clone encoding 632 amino acids showed a cysteine-rich region containing 13 consecutive epidermal growth factor-like repeats of unusual uniformity. Northern blot analysis revealed 8- to 9-kb messages. Tenascin is shown to be induced in vitro by fetal calf serum as well as by transforming growth factor beta (TGF-beta). A 4-fold increase in tenascin secretion by chick embryo fibroblasts was seen after TGF-beta treatment. The induction of tenascin protein synthesis was preceded by an increase of tenascin mRNA as determined by Northern blot analysis. The induction of tenascin was compared with fibronectin. The accumulation of the two extracellular matrix proteins in the medium was differentially affected by fetal calf serum and TGF-beta and the increase was in both cases higher for tenascin.     

Tenascin Expression in Vitro and in Vivo: Comparison between Epithelial and Nonepithelial Rat Cell Lines

Tenascin is a novel six-armed extracellular-matrix glycoprotein expressed in association with mesenchymal-epithelial interactions, and its expression is temporally and spatially restricted during organogenesis and carcinogenesis. The distribution and alterations in the expression of fibronectin, laminin, and especially of tenascin, were compared between in vitro and in vivo studies with rat epithelial (hepatocyte-derived) and nonepithelial (sarcoma-derived) cell lines. Immunoprecipitation studies revealed that the production of extracellular-matrix glycoproteins varied among the cell lines. Two ascites-hepatoma-derived cell lines and one sarcoma-derived line were found to synthesize tenascin in vitro. Their major tenascin isoform yielded a molecular weight of 220 kDa under reducing conditions. The other cell lines examined, including all of those derived from normal hepatocytes, were negative for the expression of tenascin. Coculture studies were performed between epithelial and nonepithelial cell lines. No drastic change in tenascin expression was found after coculturing the cells. As an in vivo study, cell lines were transplanted into nude mice. All xenografts of the epithelial lines were associated with a strong positive reaction for extracellular-matrix glycoproteins, and especially for tenasein, in the mouse fibrous stroma adjacent to them. This represents the epithelial induction of stromal tenascin. Whether or not they produced tenascin in vitro, after transplantation none of the epithelial cell lines themselves produced tenascin, whereas both of the nonepithelial cell lines prominently produced tenascin. These findings suggest that, in the process of interactions between epithelial and nonepithelial cells, the expression of tenascin depends on the switch from in vitro to in vivo.     

The human placenta: a model for tenascin expression

Summary Tenascin is a large glycoprotein of the extracellular matrix. Previous reports have demonstrated that it is associated with epithelial-mesenchymal interfaces and is expressed during embryonic and tumour development, wound healing, cell proliferation and it may be involved in immunomodulation. The human placenta shows numerous features related to these aspects. We have investigated the presence of tenascin in the human placenta throughout pregnancy by immunohistochemistry. We used monoclonal (mAb) and polyclonal (pAb) antibodies to tenascin, a mAb to fibrin, a pAb to fibrinogen, and the mAb Ki-67 as proliferation marker. Tenascin was highly expressed in the mesenchymal villi which are considered the basis of growth and differentiation of the villous trees. Moreover, fibrinoid deposits at the surfaces of the villous trees were always separated from the fetal stroma by tenascin. The stroma of villi encased in fibrinoid was also positive for tenascin. This glycoprotein was also expressed in the villous stroma directly apposed to cell islands and cell columns. In the proximal portions of both epithelial structures, cytotrophoblast was Ki-67 positive. These data show that tenascin is expressed during the development of the placenta, particularly in the mesenchymal villi, cell islands and cell columns. These structures are considered to be the proliferating units of the villous trees. Tenascin underlying fibrinoid deposits suggests that it also participates in repair mechanisms. Thus, in the human placenta tenascin expression can be correlated with villous growth, cell proliferation, and fibrinoid deposition. Its role in immunoprotection of fetal tissues in areas where syncytiotrophoblast as barrier is missing or damaged is discussed.     

Isolation of chick tenascin variants and fragments. A C-terminal heparin-binding fragment produced by cleavage of the extra domain from the largest subunit splicing variant.

The extracellular-matrix glycoprotein, tenascin, consists of disulfide-linked subunits of 190, 200 and 230 kDa (the three splicing variants reported in chicken) and usually exists as a six-armed structure under the electron microscope. We used monoclonal antibodies to isolate and characterize different splicing variants and proteolytic fragments obtained from the native protein. Purified monomeric tenascin has a native molecular mass of 216 kDa and is structured as single arms. Tenascin fragments obtained by pepsin digestion bind to monoclonal antibody (mAb) TnM1 which is directed against epidermal-growth-factor-like repeats in the N-terminal half of all subunits. These fragments represent the thin proximal part of the tenascin arms and they are still partially linked to dimers and trimers via disulfide bridges. Using mAb Tn68, that reacts with a fibronectin-type-III repeat towards the C-terminus, a tenascin fragment, generated by treatment with pronase, can be isolated. Ultrastructurally, this fragment looks like the thicker distal part of the tenascin arms. Only the 230-kDa variant of tenascin gives rise to this distal fragment after cleavage within the alternatively spliced fibronectin-type-III repeats. Native tenascin and all fragments containing the distal part of its arms bind to heparin-agarose, whereas the proximal fragments do not. Oligomeric and monomeric tenascin inhibit fibronectin-mediated fibroblast spreading with comparable efficiency when added to the culture medium, while the proximal fragment has no effect. The distal fragment as well as reduced and alkylated tenascin are active in this assay, but only at higher molar concentrations when compared to the native protein.     

The distribution of immuno-reactive tenascin in the epithelial-mesenchymal junctional areas of benign and malignant squamous epithelia

Tenascin is an extra cellular matrix glycoprotein which is distributed in the mesenchyme surrounding various organs during embryogenesis. It has also been demonstrated in some normal adult tissues and in the matrix of human tumours. The present study has been carried out to analyse the distribution of tenascin in non malignant and malignant skin disorders, in squamous cell carcinomas of the head and neck, in squamous cell carcinoma xenografts and in a squamous cell carcinoma cell line grown on collagen gel. Immunohistochemical localisation of tenascin was performed, using a monoclonal antibody specific for tenascin, by the indirect immunoperoxidase method with silver enhancement. Tenascin was heterogeneously distributed in the extra cellular matrix of squamous cell carcinomas and in squamous cell carcinoma xenografts. It was absent in basal cell carcinoma and in the squamous cell carcinoma cell line grown on collagen gel. The distribution of tenascin in squamous cell carcinoma and basal cell carcinoma is discussed in relation to tumour invasion and differentiation.     

The human placenta: a model for tenascin expression.

Tenascin is a large glycoprotein of the extracellular matrix. Previous reports have demonstrated that it is associated with epithelial-mesenchymal interfaces and is expressed during embryonic and tumour development, wound healing, cell proliferation and it may be involved in immunomodulation. The human placenta shows numerous features related to these aspects. We have investigated the presence of tenascin in the human placenta throughout pregnancy by immunohistochemistry. We used monoclonal (mAb) and polyclonal (pAb) antibodies to tenascin, a mAb to fibrin, a pAb to fibrinogen, and the mAb Ki-67 as proliferation marker. Tenascin was highly expressed in the mesenchymal villi which are considered the basis of growth and differentiation of the villous trees. Moreover, fibrinoid deposits at the surfaces of the villous trees were always separated from the fetal stroma by tenascin. The stroma of villi encased in fibrinoid was also positive for tenascin. This glycoprotein was also expressed in the villous stroma directly apposed to cell islands and cell columns. In the proximal portions of both epithelial structures, cytotrophoblast was Ki-67 positive. These data show that tenascin is expressed during the development of the placenta, particularly in the mesenchymal villi, cell islands and cell columns. These structures are considered to be the proliferating units of the villous trees. Tenascin underlying fibrinoid deposits suggests that it also participates in repair mechanisms. Thus, in the human placenta tenascin expression can be correlated with villous growth, cell proliferation, and fibrinoid deposition. Its role in immunoprotection of fetal tissues in areas where syncytiotrophoblast as barrier is missing or damaged is discussed.     

Tenascin‐c renders a proangiogenic phenotype in macrophage via annexin II

Tenascin‐c is an extracellular matrix glycoprotein, the expression of which relates to the progression of atherosclerosis, myocardial infarction and heart failure. Annexin II acts as a cell surface receptor of tenascin‐c. This study aimed to delineate the role of tenascin‐c and annexin II in macrophages presented in atherosclerotic plaque. Animal models with atherosclerotic lesions were established using ApoE‐KO mice fed with high‐cholesterol diet. The expression of tenascin‐c and annexin II in atherosclerotic lesions was determined by qRT‐PCR, Western blot and immunohistochemistry analysis. Raw 264.7 macrophages and human primary macrophages were exposed to 5, 10 and 15 μg/ml tenascin‐c for 12 hrs. Cell migration as well as the proangiogenic ability of macrophages was examined. Additionally, annexin II expression was delineated in raw 264.7 macrophages under normal condition (20% O₂) for 12 hrs or hypoxic condition (1% O₂) for 6–12 hrs. The expression of tenascin‐c and annexin II was markedly augmented in lesion aorta. Tenascin‐c positively regulated macrophage migration, which was dependent on the expression of annexin II in macrophages. VEGF release from macrophages and endothelial tube induction by macrophage were boosted by tenascin‐c and attenuated by annexin II blocking. Furthermore, tenascin‐c activated Akt/NF‐κB and ERK signalling through annexin II. Lastly, hypoxia conditioning remarkably facilitates annexin II expression in macrophages through hypoxia‐inducible factor (HIF)‐1α but not HIF‐2α. In conclusion, tenascin‐c promoted macrophage migration and VEGF expression through annexin II, the expression of which was modulated by HIF‐1α.     

Possible involvement of matrix metalloproteinase-9 in Langerhans cell migration and maturation

Epidermal Langerhans cells (LC) are potent dendritic cells in the induction of primary T cell-mediated immune responses in the skin. They capture foreign Ags and migrate to regional lymph nodes to carry and present these Ags to naive T cells. We investigated the role of matrix metalloproteinase-9 (MMP-9) in LC migration using an anti-MMP-9 mAb. Intradermal injection of anti-MMP-9 mAb before rhodamine B or oxazolone painting markedly inhibited these hapten-induced decreases in LC number in the epidermis and the accumulation of dendritic cells in the regional lymph nodes, indicating that MMP-9 plays some important roles in LC migration in the induction phase of contact sensitization. Treatment with anti-MMP-9 mAb also blocked the increase in cell size, dendrite development, and the enhanced expression of MHC class II Ags in LC induced by hapten painting. In addition, intradermal injection of purified MMP-9 induced marked increases in cell size, dendrite extension, and enhanced expression of MHC class II Ags in LC. These results strongly suggested that MMP-9 is involved not only in LC migration, but also in their morphological and phenotypic maturation in the skin.     

Downregulation of tenascin expression by glucocorticoids in bone marrow stromal cells and in fibroblasts

Tenascin, a predominantly mesenchymal extracellular matrix (ECM) glycoprotein has a rather restricted tissue distribution, but until now factors that inhibit its expression have not been identified. Glucocorticoids are known to be beneficial for establishment of myelopoiesis in long-term bone marrow cultures. Tenascin was found to be expressed in the bone marrow, and glucocorticoids were found to affect bone marrow tenascin expression. Both tenascin mRNAs and the mRNA of another ECM protein, laminin B1 chain, were drastically downregulated by glucocorticoids during initiation of bone marrow cultures. However, in already established long-term cultures glucocorticoids did not affect laminin B1 chain mRNA levels although tenascin mRNAs continued to be downregulated. Studies with a stromal cell line (MC3T3-G2/PA6) and fibroblasts (3T3) suggested that glucocorticoids act directly on the stromal cells that produce tenascin. In 3T3 cells this downregulation occurred within 12 h of glucocorticoid-treatment, suggesting that glucocorticoids acted through cis regulatory elements of the tenascin gene. We suggest that glucocorticoids in part regulate hematopoiesis by modifying the ECM. Furthermore, downregulation of tenascin expression by glucocorticoids may in part explain the restricted tissue distribution of tenascin in other tissues.     

Tenascin: a potential modulator of cell-extracellular matrix interactions during vertebrate embryogenesis.

Tenascin is a large oligomeric extracellular matrix (ECM) glycoprotein whose expression is highly restricted during vertebrate development. It has a characteristic hexameric quaternary structure with six arms linked to a central globular domain. Each arm contains a single polypeptide with the central globular domain formed by the covalent association of the N-terminal ends of the six polypeptides. Tenascin first appears during development, associated with the neural crest cell migration pathways of mammalian, avian and amphibian embryos. During later development, it is observed at sites of cartilage, bone and tendon formation. Tenascin expression also occurs in defined areas in the developing nervous system and in condensing mesenchyme, in response to epithelio-mesenchymal interactions. The function of tenascin in these different morphogenetic processes is not yet clearly understood. Tenascin can promote neurite outgrowth in vitro and can inhibit cell interactions with fibronectin. Results based on antibody mapping and molecular cloning indicate that these properties involve two distinct cell binding sites. Together with its highly regulated expression in the embryo, these properties suggest that tenascin plays a key role in the control of cell migration and differentiation during development.     

Tenascin is associated with chondrogenic and osteogenic differentiation in vivo and promotes chondrogenesis in vitro

The tissue distribution of the extracellular matrix glycoprotein, tenascin, during cartilage and bone development in rodents has been investigated by immunohistochemistry. Tenascin was present in condensing mesenchyme of cartilage anlagen, but not in the surrounding mesenchyme. In fully differentiated cartilages, tenascin was only present in the perichondrium. In bones that form by endochondral ossification, tenascin reappeared around the osteogenic cells invading the cartilage model. Tenascin was also present in the condensing mesenchyme of developing bones that form by intramembranous ossification and later was present around the spicules of forming bone. Tenascin was absent from mature bone matrix but persisted on periosteal and endosteal surfaces. Immunofluorescent staining of wing bud cultures from chick embryos showed large amounts of tenascin in the forming cartilage nodules. Cultures grown on a substrate of tenascin produced more cartilage nodules than cultures grown on tissue culture plastic. Tenascin in the culture medium inhibited the attachment of wing bud cells to fibronectin-coated substrates. We propose that tenascin plays an important role in chondrogenesis by modulating fibronectin-cell interactions and causing cell rounding and condensation.     

Tenascin: a modulator of cell growth.

The large, multidomain extracellular matrix protein tenascin displays a markedly restricted tissue distribution during embryogenesis and remains present only in a few adult tissues. The protein is reexpressed, however, during wound healing and in the stroma of malignant tumours. While a variety of studies have dealt with the important role of tenascin in the development of neural and non-neural tissues, there is growing evidence that tenascin expression may be associated with proliferation of cells lining these tissues. The presence of repeating domains in tenascin similar to those in epidermal growth factor prompted us to investigate the ability of tenascin to modulate the growth of different cell types. Tenascin was actually found to be mitogenic for several cell types. This mitogenic activity, however, appears to be associated with a region in the fibronectin type III domains. The mitogenic mechanism is clearly distinct from pathways used by peptide growth factors such as epidermal growth factor and platelet-derived growth factor, which activate the intrinsic tyrosine kinase activity of their cell-surface receptors. However, we show that this large extracellular matrix molecule is efficiently internalised and may be processed by responding cells.