Fig. 1.

Cell cycle regulation in the adult hippocampal neurogenic niche. Some niche-specific cell cycle regulators in the adult hippocampus have been identified. Cdk6-cyclin D2 and Cdk4-cyclin D1 complexes promote the expansion of the neural progenitor pool. P21 and p27 Cdk inhibitors have a role in proliferation arrest, both at the G1 and G2 phase. E2F1 has an important role in the neurogenic process by inducing the expression of genes involved in cell proliferation and differentiation. Cdk5 activity is associated with cell cycle reentry inhibition in postmitotic neurons. Signaling pathways, such as Notch, BMP, Shh and Wnt, are also involved in proliferation regulation, and in the balance between proliferation induction and stem cell quiescence maintenance. Nevertheless, several key molecules remain to be identified in this process in the context of adult hippocampal neurogenic niche (represented by a question mark). Rb retinoblastoma protein