Neurogenesis in the central olfactory pathway of adult decapod crustaceans: development of the neurogenic niche in the brains of procambarid crayfish

Between the end of embryogenesis and throughout the first post-embryonic stage (POI), a defined transverse band of mitotically active cells (which we will term 'the deutocerebral proliferative system' (DPS) appears. Just prior to hatching and in parallel with the formation of the DPS, the anlagen of the niche appears, closely associated with the vasculature. When the hatchling molts to the second post-embryonic stage (POII), the DPS differentiates into the lateral (LPZ) and medial (MPZ) proliferative zones. The LPZ and MPZ are characterized by a high number of mitotically active cells from the beginning of post-embryonic life; in contrast, the developing niche contains only very few dividing cells, a characteristic that persists in the adult organism.Our data suggest that the LPZ and MPZ are largely responsible for the production of new neurons in the early post-embryonic stages, and that the neurogenic niche in the beginning plays a subordinate role. However, as the neuroblasts in the proliferation zones disappear during early post-embryonic life, the neuronal precursors in the niche gradually become the dominant and only mechanism for the generation of new neurons in the adult brain.During embryonic development in the emerging ventral nerve cord of malacostracan Crustacea (for example, lobsters, crayfish and crabs), most neurons are generated by neuronal stem cells, the neuroblasts (NBs), by a division pattern called the stem cell mode [1-7]. NBs repeatedly divide asymmetrically, regenerating themselves and giving rise to one smaller ganglion mother cell that is pushed dorsally into the interior of the embryo; the ganglion mother cell divides once more to give rise to neurons or glial cells. In the crustacean brain, however, neurogenesis has not been as well documented as in the ventral nerve cord [8,9]. Evidence also suggests that the intermediate precursor cell type (ganglion mother cell) in the brain may divide more than once [10], unlike the stereotyped sequen