Figure 11.

A: Schematic representation of the cytoarchitecture of the clump of cells in the lateral soma cluster of P. argus. The clump of cells is composed of bipolar clump-forming cells (CFC) whose somata form a continuous cortex around a nucleus-free center. One process of the clump-forming cells is short and faces inward; the other one is long and faces outward. Together, the outward-facing processes form the duct connecting with the adjacent proliferation zone. The clump-forming cells completely enclose one adult neuroblast (aNB), which consists of two large domains connected by a thin bridge and thus has an hourglass-like shape. The peripheral domain of the adult neuroblast contains the nucleus and is located at the apical pole of the clump of cells. The other domain forms a bulbous foot (asterisk) within the nucleus-free center of the clump of cells and is surrounded by the inner processes of CFCs. The clump of cells and the duct are surrounded by a continuous layer of processes of multipolar cell body glia (G). The clump of cells is associated with an arteriole (A) whose wall is constituted by processes of perivascular cells (PVC). The duct contains elongated cells that most likely represent neuronal progenitor cells (NPC) that are produced by asymmetric divisions of the aNB and migrate through the duct to the adjacent proliferation zone. B: Schematic representation of the current model of cell divisions and lineages maintaining adult neurogenesis in the olfactory deutocerebrum of P. argus. In each neuronal soma cluster of the olfactory deutocerebrum, production of new neurons is based on one neural stem cell, which is an adult neuroblast (aNB) located close to the proliferation zone and embedded in a clump of cells. Through serial asymmetric divisions, the aNB renews itself and produces differentiating daughter cells at its apical pole. Within a duct formed by processes of the clump-forming cells (CFC), the daughter cells migrate to the proliferation zone, where they replenish the pool of neuronal progenitor cells (NPCs). Ultimately, neuronal progenitor cells divide once symmetrically and produce two daughter cells that represent immature neurons (iN). Migrating out of the proliferation zone, most of the immature neurons become differentiated neurons (dN) within months, but some die by apoptosis. We propose that, upon their arrival in the proliferation zone, daughter cells generated by the aNB may undergo one or more fast rounds of cell divisions and thus represent transit amplifying intermediate progenitor cells, accommodating the continuous replenishment of 30 (MC) or 100 (LC) cells in S-phase in the proliferation zone by only one aNB. For adult P. argus, there is no evidence for proliferation of the CFCs or the cell body glia (G) surrounding the clump of cells, making it unlikely that these two cell types contribute to maintaining adult neurogenesis.