Involvement of Delta and Nodal signals in the specification process of five types of secondary mesenchyme cells in embryo of the sea urchin, Hemicentrotus pulcherrimus

Secondary mesenchyme cells (SMCs) of the sea urchin embryo are composed of pigment cells, blastocoelar cells, spicule tip cells, coelomic pouch cells and muscle cells. To learn how and when these five types of SMCs are specified in the veg₂ descendants, Notch or Nodal signaling was blocked with γ‐secretase inhibitor or Nodal receptor inhibitor, respectively. All types of SMCs were decreased with DAPT, while sensitivity to this inhibitor varied among them. Pulse‐treatment revealed that five types of SMCs are divided into “early” (pigment cells and blastocoelar cells) and “late” (spicule tip cells, coelomic pouch cells and muscle cells) groups; the “early” group was sensitive to DAPT up to the hatching, and the “late” group was sensitive until the mesenchyme blastula stage. Judging from timing of the shift of Delta‐expressing regions, it was suggested that the “early” group and “late” groups are derived from the lower and the middle tier of veg₂ descendants, respectively. Interestingly, numbers of SMCs were also altered with SB431542; blastocoelar cells, coelomic pouch cells and circum‐esophageal muscles decreased, whereas pigment cells and spicule tip cells increased in number. Pulse‐treatment showed that the “early” group was sensitive up to the mesenchyme blastula stage, while the “late” group up to the onset of gastrulation. Thus, it became clear that precursor cells of the “early” and “late” groups, which are located in different regions in the vegetal plate, receive Delta and Nodal signals at different timings, resulting in the diversification of SMCs. Based on the obtained results, the specification processes of five types of SMCs are diagrammatically presented.