What is known of the production of basement membrane components.

Immunohistochemistry was used to identify basement membrane components and examine their production by associated cells. Four substances were identified in a series of basement membranes in rats aged 20 days to 34 months, namely, type IV collagen, laminin, heparan sulfate proteoglycan, and fibronectin. They were then all localized to the basal lamina part of basement membranes and, presumably, are integrated within this layer. The production of type IV collagen was first examined in the embryonic endodermal cells associated with Reichert's membrane in the rat parietal yolk sac. The rough endoplasmic reticulum (rER), Golgi apparatus, and putative secretory granules of endodermal cells were immunostained, suggesting that these organelles participated in the biogenesis of type IV collagen. However, in rats aged 20 days or more, the cells associated with basement membranes were usually unstained. An exception was noted in the continually growing incisor tooth where the endothelial cells at the proliferating end usually showed immunostaining of rER and Golgi apparatus. It is, therefore, proposed that the formation of type IV collagen for basement membrane occurs at an early stage of development in the life of producer cells. Little is known of the formation of other basement membrane components during development, but there is immunohistochemical evidence that laminin and fibronectin are produced along the same secretory pathway as type IV collagen.