Figure 3.

Extrinsic and intrinsic mechanisms regulating the numbers of NSCs/progenitor cells in the SEZ. NSCs remain relatively quiescent; their random (oscillation-like) activation is indicative of the operation of intrinsic clock mechanisms (I). Their massive activation after the depletion of their progeny suggests the existence of progenitor-dependent feedback, extrinsic inhibition (1, 2). TaPs are mitotically active cells; their numbers are controlled via intrinsic mechanisms (II), as indicated by the observation that they start generating neuroblasts after certain numbers of divisions during regeneration of the niche, but they might be also receiving feedback inhibition similarly to NSC (3). Neuroblasts or OPC numbers in the niche are regulated by their controlled outward migration and cell death (4). NB = neuroblast.