The number and distribution of neurons in the murine neocortex are altered by loss of function of p27Kip1, a cyclin-dependent kinase inhibitor that regulates cell cycle progression at the G1 phase. We show that a temporary decline in the production of non-GABAergic projection neurons occurs in the dorsomedial neopallium of the p27Kip1 knockout mouse during the mid-term of neuronogenesis. It is followed by an augmentation of neuron production later in neuronogenesis leading to an increased production of projection neurons for the upper layers of the neocortex (layers II-IV). p27Kip1 is likely to play a critical role in cell internal regulatory mechanisms of proliferation/differentiation behavior of neural progenitor cells and may be directly involved in the control of neuron number during the period when non-GABAergic projection neurons for layers II-IV are being produced.
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