CELL-ADHESION AND CELL SIGNALING AT GASTRULATION IN THE SEA-URCHIN

The sea urchin embryo follows a relatively simple cell behavioral sequ ence in its gastrulation movements. The embryo reaches the gastrula st age as a spherical monolayer of cells. To form the mesoderm, primary m esenchyme cells ingress by delaminating from the vegetal plate, crossi ng the basal lamina and moving into the central blastocoelar cavity. T hese cells then migrate along the basal lamina lining the blastocoel a nd eventually manufacture the skeleton. The presumptive secondary mese nchyme and endoderm invaginate as a tubular sheet of cells from the ve getal pole of the embryo. The archenteron extends across the blastocoe l until its tip touches and attaches to the opposite side of the blast ocoel. Secondary mesenchyme cells, originally at the tip of the archen teron, differentiate to form a variety of structures including coelomi c pouches, esophageal muscles, pigment cells, and other cell types. Th e endoderm fuses with an invagination of the ventral ectoderm (the sto modaem), to form the mouth and complete the process of gastrulation. A number of experiments have established that these simple morphogeneti c 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 require ment for cell signaling has been demonstrated by manipulative experime nts 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 adhes ive changes. This review describes the adhesion changes and the signal ing that characterizes this early morphogenesis.