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The area of contact between adenohypophysis and diencephalon rudiments of human embryos (5-7 weeks of development) was studied using immunohistochemistry and electron microscopy. Basement membranes of Rathke's pouch ectoderm and of diencephalon bottom neuroectoderm are connected by means of a complex consisting of thin fibrous material and collagen-like fibrils. After Ca+2 and Mg+2 ions removal, this complex with basement membranes was separated from epithelial layers. The material in the contact area differed in its composition from the basement membranes covering the adenohypophysis and diencephalon rudiments by the high content of tenascin and the absence of EDB-fibronectin. Other basement membrane components (collagen IV, heparan-sulphate proteoglycans, entactin and laminin) were also present in this area. Tenascin accumulation was also found in Rathke's pouch epithelium and cavity.

A protein with properties of tenascine has been discovered in extracellular matrix of human embryonic cartilage tissue. Its Mr, as determined by electrophoresis in PAAG with SDS, is 205 and 195 kDa. Immunoblotting with monoclonal antibodies raised against tenascine, revealed 2 precipitation bands, which permitted to identify the discovered protein as tenascine. This is the first report of detection of tenascine in rib cartilage matrix of human embryos. A protein identified as tenascine, has been also detected in a culture of human embryonic fibroblasts.

A series of reciprocal interactions between epithelial and mesenchymal tissues control the morphogenesis and cell differentiation in the developing tooth. The molecular mechanisms operating in these interactions are, however, unknown at present. Structural components of the extracellular matrix (ECM) affect cellular behavior in the embryo and appear to be involved also in these regulatory processes. The ECM molecules exert their effects on cells through binding to specific matrix receptors on the cell surface. This review article summarizes our findings on the distribution patterns during tooth development of the ECM glycoproteins, fibronectin and tenascin, and of the cell surface proteoglycan, syndecan, which functions as a receptor for interstitial matrix. Based on the observed changes in these distribution patterns and on experimental evidence, roles for these molecules in epithelial-mesenchymal interactions during tooth development are suggested. Fibronectin and tenascin are enriched in the dental basement membrane at the time of odontoblast differentiation. These matrix glycoproteins may be involved in the cell-matrix interaction which controls differentiation of the dental mesenchymal cells into odontoblasts. Tenascin and syndecan are accumulated in the dental mesenchyme during bud stage of development. We have shown in tissue recombination experiments that the presumptive dental epithelium induces the expression of tenascin and syndecan in mesenchyme. We suggest that these molecules are involved in cell-matrix interactions, which regulate mesenchymal cell condensation during the earliest stages of tooth morphogenesis.