Initial analysis of immunochemical cell surface properties, location and formation of the serotonergic apical ganglion in sea urchin embryos

In the present study it was found that serotonergic epical ganglion (SAG)-f orming cells in plutei of the sea urchin, Hemicentrotus pulcherrimus, posse ssed a characteristic pear shape with broad apical sides and a pointed basa l side in the acron epithelium. The basal side extended axons through the s pace between the epithelium and the basal lamina toward the midline of the embryo that aligned parallel to the embryonic anteroposterior axis. Seroton ergic epical ganglion-forming cells had epithelial cell surface-specific pr oteins on their entire surface. The SAG in 4-arm plutei was composed of a 4 -cell trunk region that aligned at right angles to the embryonic anteropost erior axis, and forked into two branches of one to two cells at both ends. Two branches extended toward the oral and the other two toward the aboral r egion, respectively. Double-stained immunohistochemistry using antiserotoni n antibodies and oral ectoderm-specific anti-Ecto V monoclonal antibody or aboral ectoderm-specific anti-Ars antibodies indicated that SAG was in the aboral ectoderm region. Serotonergic apical ganglion cells were first detec ted in late gastrulae and increased in number rapidly between 36 and 48 h a fter fertilization, and then slowly afterwards. A 5-bromo-2-deoxyuridine in corporation study indicated that none of the increased SAG cells were in th e S phase during the aforementioned period, suggesting that SAG cells do no t proliferate by cell division, but acquire the property in particular cell s by transdifferentiation using a mechanism that has yet to be elucidated.