Ultrastructure of primary mesenchyme cell ingression in the sea urchin <i>Lytechinus pictus</i>

Abstract

Abstract The ultrastructural changes associated with primary mesenchyme cell formation in the sea urchin Lytechinus pictus were studied by transmission (TEM) and scanning electron microscopy (SEM). Early during ingression, one can recognize about 30 presumptive primary mesenchyme cells, which form a ring around eight central cells, located at the apex of the vegetal pole. The first steps involve the loss of the basal lamina in the vegetal plate region and the loss of cilia from the 30 presumptive primary mesenchyme cells. These cells then elongate into bottle‐shaped cells which have a bulbous, appendage‐free basal end that protrudes into the blastocoel, and a thin neck region that forms numerous apical cell processes. Longitudinally oriented microtubules are observed in the peripheral cytoplasm in the neck region of the bottle‐shaped cells and in the adjacent cytoplasm of closely adherent neighboring non‐mesenchymal blastomeres. After the disappearance of apical junctions, the neck regions of the bottle‐shaped cells shorten, resulting in the transposition of the cells into the blastocoel as rounded primary mesenchyme. Mesenchyme cell displacement results in the temporary formation of fenestrae in the blastular wall, which are filled in by apical cell processes that extend from neighboring blastomeres. These morphological changes have several features in common with other examples of cells which exit from epithelia and are consistent with a morphological sequence involving the relaxation of the basal cytoplasm of the presumptive primary mesenchyme cells, followed by their active elongation and lateral compression by adjacent cells.