Development of the Basal Lamina and Its Role in Migration and Pattern Formation of Primary Mesenchyme Cells in Sea Urchin Embryos: (sea urchin/primary mesenchyme cell/basal lamina/TEM/SEM).

The development and substructure of the basal lamina and its role in migration and pattern formation of primary mesenchyme cells (PMCs) in normal as well as Li - and Zn -treated embryos of sea urchins were investigated by electron microscopy. Major findings were as follows. 1) Network fibrils appear along the basal surface of the blastular wall by the hatching blastula stage. The area covered with fibrils is restricted to the vegetal hemisphere at earlier stages, but extends to the animal hemisphere as development proceeds. 2) Nonfibrous fuzzy material embeds the fibrils to form a basal lamina, but in places the fibrils project from the basal lamina into the blastocoel. The major components of the fuzzy material were digested by glycosidase, which failed to digest the fibrous components. 3) The fibrils can be classified into two types, one Ca -independent and the other Ca -dependent. PMCs apparently utilize the Ca -indepndent fibrils as a substratum for locomotion. 4) After migration, PMCs accumulate in a specific region to form the PMC pattern. This is formed in the area of greatest concentration of Ca -independent fibrils. 5) PMCs in embryos treated with LiCl, in contrast to normal embryos, accumulate in the animal pole region where the Ca -independent fibrils are markedly concentrated.