Extracellular matrix components and somite chondrogenesis: A microscopic analysis

The stimulation of somite chondrogenesis by extracellular materials was studied using scanning and transmission electron microscopy and light microscopy. Analysis of control somite explants (no additives to the medium) cultured on Nuclepore filters for 24 h demonstrates cell processes extending to the undersurface of the filter. The cell processes secrete a matrix of fibers sparsely coated with granules which form amorphous sheets after 3 days in culture. Somite explants treated with proteoglycan complex, extracted from 13-day chick sterna, produce a dense matrix of fibers heavily coated with granules. Selective enzymatic digestions with chondroitinase ABC and purified collagenase demonstrate that the fibers are collagen and the granules are proteoglycans. Proteoglycan complex was separated into its components using cesium chloride density centrifugation. Each of these fractions was tested for its stimulating capacity in somite explants as analyzed using scanning electron microscopy. The importance of these components in relationship to the perinotochordal materials is discussed. When somite explants are cultured with the notochord, the matrix produced by somitic cells in the region of the notochord is similar to that of explants treated with proteoglycan complex. Away from the region of the notochord, the somitic cells produce a matrix similar to that of control explants. The evidence presented in this report suggests that it is the presence of the perinotochordal materials which creates the proper environment in vivo for the precise timing and phenotypic expression of somite chondrogenesis.