Developmental Expression of Cell-Surface (Glyco)proteins Involved in Gastrulation and Spicule Formation in Sea Urchin Embryos

Gastrulation in the sea urchin embryo begins shortly after the embryo emerges from the fertilization envelope (Gustafson and Wolpert 1967). This process, which eventually produces the germ layers, involves a series of cell migration and differentiation events. First, the descendants of the micromeres enter the blastocoel where they become the primary mesenchyme cells. Second, the blastoderm invaginates, beginning to form the archenteron. Secondary mesenchyme cells at the tip of the archenteron extend filopodia to the inner surface of the blastocoel, helping to elongate the archenteron into a tube. During invagination of the archenteron, the filopodia of the primary mesenchyme cells fuse to form a syncytial cable-like structure that later will be the site for deposition of the skeletal matrix of the spicule. This cable system, which is initially triradiate, seems to predetermine the shape of the spicule (Okazaki 1975). During the late stages of gastrulation, the primary mesenchyme cells initiate...