ABSTRACT
Initial morphological characterization of the primary mesenchyme cell (PMC) homing sites in the embryos of the sea urchins, Hemicentrotus pulcherrimus, Mespilia globulus and Clypeaster japonicus, was conducted using time-lapse vedeomicroscopy, immunohistochemistry, and trasmission electron microscopy. In early gastrulae, the basal surface at the PMC homing sites undulated during termination of PMC migration, which was inhibited by the treatment with cytochalasin B. The sites were consisted of fan-shaped tall cells contained bundles of microtubules along the apico-basal axis, and fine fibrils in the cytoplasm on the basal side. The basal side extended cell processes to the blastocoel, but they never penetrated the basal lamina. The most characteristic feature of these fan-shaped cells was the presence of clusters of mitochondria which accumulated on the basal surface in early gastrulae, but were rather evenly distributed in swimming and mesenchyme blastulea.
