Cell adhesion and cell signaling at gastrulation in the sea urchin

Abstract

The sea urchin embryo follows a relatively simple cell behavioral sequence in its gastrulation movements. The embryo reaches the gastrula stage as a spherical monolayer of cells. To form the mesoderm, primary mesenchyme cells ingress by delaminating from the vegetal plate, crossing the basal lamina and moving into the central blastocoelar cavity. These cells then migrate along the basal lamina lining the blastocoel and eventually manufacture the skeleton. The presumptive secondary mesenchyme and endoderm invaginate as a tubular sheet of cells from the vegetal pole of the embryo. The archenteron extends across the blastocoel until its tip touches and attaches to the opposite side of the blastocoel. Secondary mesenchyme cells, originally at the tip of the archenteron, differentiate to form a variety of structures including coelomic pouches, esophageal muscles, pigment cells, and other cell types. The endoderm fuses with an invagination of the ventral ectoderm (the stomodaem), to form the mouth and complete the process of gastrulation. A number of experiments have established that these simple morphogenetic movements are accompanied by a number of cell adhesion changes plus a series of cell-cell interactions that provide spatial, temporal, and scalar information to cells of the mesoderm and endoderm. The requirement for cell signaling has been demonstrated by manipulative experiments where it has been shown that axial, temporal, spatial, and scalar information is obtained by mesoderm and endoderm from other embryonic cells. That information governs pattern formation and subsequent adhesive changes. This review describes the adhesion changes and the signaling that characterizes this early morphogenesis.