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

In the decapod crustacean brain, neurogenesis persists throughout the animal's life. After embryogenesis, the central olfactory pathway integrates newborn olfactory local and projection interneurons that replace old neurons or expand the existing population. In crayfish, these neurons are the descendants of precursor cells residing in a neurogenic niche. In this paper, the development of the niche was documented by monitoring proliferating cells with S-phase-specific markers combined with immunohistochemical, dye-injection and pulse-chase experiments. 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.