The Structure and Development of the Apical Ganglion in the Sea Urchin Pluteus Larvae of Strongylocentrotus droebachiensis and Mespilia globulus

The structure of the apical ganglion is described using transmission electron microscopy and ultrastructural immunohistochemical localization with anti-serotonin, and the development of these nerve cells in animalized and vegetalized embryos, and embryos treated with Ca²⁺-deficient sea water are demonstrated with immunofluorescence microscopy. The axons within the neuropile contain clear and dense-core vesicles, but all the vesicles appear anti-serotonin immunoreactive. The principal type of neuron within the apical ganglions is anti-serotonin immunoreactive. In the animalized embryos, serotonergic neuroblasts and neurons appear in the second quarter of animal-vegetal axis. Serotonergic cells were not detected in vegetalized exogestrulae. Mespilia globulus embryos treated with Ca²⁺-deficient sea water ruptured to form a cellular sheet on the substratum. In ruptured embryos serotonergic neurons formed a ring around the area corresponding to the apical plate of normal embryos. These findings indicated that the serotonergic preoral nerve cells of echinopluteus derive from cells on the periphery of the apical plate.