Fig. 9. Model for Delta/Notch signaling in the sea urchin embryo. There appear to be at least three roles for Delta/Notch signaling during early development. (A) Blastula stage (eighth-tenth cleavage stage). Micromere derivatives (red) express LvDelta (arrows), and activate the Notch protein in neighboring cells to promote the specification of pigment cells and blastocoelar cells in the region that will become the non-skeletogenic mesoderm territory (pink). (B) Mesenchyme blastula stage to early gastrula stage. Prospective nonskeletogenic mesoderm (pink) in the vegetal plate expresses LvDelta (double-ended arrow) to promote the development of prospective muscle cells and blastocoelar cells. LvDelta expression by these cells may also activate the Notch protein in neighboring cells (single arrows from pink to yellow), promoting the development of prospective endoderm (yellow). Delta/Notch signaling also results in the expression of a secondary signal (arrows entirely within the yellow region) that promotes endoderm development in more animal cells (Sherwood and McClay, 2001).
