Behavior and Ultrastructure of Primary Mesenchyme Cells at Sessile Site during Termination of Cell Migration in Early Gastrulae: (sessile site/cell migration/primary mesenchyme cell/sea urchin/gastrula).

The behavior and ultrastructure of primary mesenchyme cells at two ventrolateral sessile sites in early gastrulae were examined by time-lapse videomicroscopy, scanning electron microscopy, and immunotrans-mission electron microscopy using the sea urchin, Hemicentrotus pulcherrimus and the sand dollar. Clypeaster japonicus. At sessile sites in early gastrulae, PMCs terminated their migration after "touch-and-go" behavior, and even after the termination they retained a pulsatile movement. These behaviors indicate that the termination of PMC migration is not due to deprivation of cell motility nor the establishment of firm adhesion between PMCs and the site. PMCs used short cell processes during migration, and extended longer ones during the early period of migration termination. During the final period of migration at the sessile sites, PMCs extended characteristically thin and long cell processes to the basal lamina. These cell processes, as far as present results indicate, never attach to the blastocoel wall cells through the basal lamina. Thus it is indicated that the primary interaction site for PMCs to terminate their migration is the basal lamina.