Role of basal lamina in tissue interactions

Basal lamina are extracellular structures found closely apposed to the plasma membrane on the basal surface of epithelial and endothelial cells and surround muscle and fat tissues. While the primary function of basal lamina in most adult tissues is probably supportive, significant evidence indicates that during embryonic development, basal lamina may be involved in regulating heterotypic tissue interactions. Changes in composition of the basal lamina of salivary and mammary gland epithelial tissues during development probably are important for branching morphogenesis which leads to the final form of the organ. During tooth organogenesis, selective basal lamina degradation and direct cell contact between developing epithelium and mesenchyme has been documented and suggested to be necessary for cytodifferentiation. Basal lamina turnover, as suggested by these morphological observations, no doubt involves both basal lamina synthesis and degradation. We have studied several factors evidently required for basal lamina reconstitution in vitro, and have discovered that fibronectin added to enamel organ epithelial cultures provided cues required for basal lamina formation in vitro. Both fetal calf serum and dental papilla mesenchyme-conditioned media also provided specific signals for basal lamina reconstitution. In addition, we have found that fibronectin is produced by the dental papilla mesenchyme, is released into the medium, and can be isolated from epithelial explants which have been cultured in mesenchyme-conditioned medium; the epithelial explants themselves do not make fibronectin in vitro.