Possible functions of mesenchyme cell-derived fibronectin during formation of basal lamina

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Abstract
Epithelial synthesis and secretion of basal lamina is considered a general feature of various vertebrate epithelium-mesenchyme interacting systems (e.g., salivary gland, mammary gland, feather, hair and tooth morphogenesis). Embryonic ectoderm and ectodermal derivatives, such as epithelial tissues associated with tooth morphogenesis, are assumed to synthesize and secrete basal lamina. Basal lamina of embryonic mouse tooth organs contain laminin, type IV collagen, glycosaminoglycans and possibly fibronectin. Ectodermally derived epithelia produce laminin, collagens and glycosaminoglycans, but they do not appear to produce fibronectin. Mesenchyme can effect basal lamina formation in vitro by releasing mesenchyme-derived fibronectin. Theiler stage 25 molar tooth mesenchymal and epithelial tissues were enzymatically separated and cultured in chemically defined media without serum, embryonic extracts or antibiotics for periods .ltoreq. 24 h. Isolated epithelia did not reconstitute a basal lamina. Mesenchyme-preconditioned media, fibronectin substrata or addition of 10% fetal calf serum induced reconstitution of epithelium-derived basal lamina. Dental mesenchyme-preconditioned medium contained, as a major component, a protein of .apprxeq. 2.3 .times. 105 identified as fibronectin by the criteria of gelatin binding and subunit MW. Fibronectin was not produced by isolated epithelia. Apparently, lamina ultrastructural organization results from supramolecular interactions between epithelium-derived macromolecules (e.g., type IV collagen, proteoglycans, glycosaminoglycans and laminin) with mesenchyme-derived cell surface fibronectin.