(A1) Serotonin (5-HT) transporter expression in a flat-mounted, tangential section of P6 brain delineates primary sensory areas. S1, primary somatosensory cortex; M1, primary motor cortex; A1, auditory cortex; V1, visual cortex; OB, olfactory bulb. (A2) Co-immunostaining against aCasp3 reveals strong area discrepancies in the density of apoptotic neurons: M1 (upper panel) displays a high density of aCasp3 positive cells compared with S1 (lower panel). (B) Representative DAPI staining of a coronal slice at P6 allows the identification of cortical layers and areas. Cortical regions were divided into sectors a to f containing M1 (sectors a and b) and S1 (sectors c to f) for further analysis. Based on DAPI staining, upper layers (layers I-IV), layer V and layer VI were identified. (C1) Representative aCasp3-positive neurons in M1 of a coronal slice from a P6 mouse brain. Cell bodies and basal dendrites are located in layers II-IV while apical dendrites project to layer I. Note that aCasp3 allows the detection of dying neurons at an early stage of apoptosis, when aCasp3 signal is not yet translocated into the nucleus and when the nucleus does not display any visible sign of degradation. (C2) Representative aCasp3-positive neuron in S1. The aCasp3 signal has translocated into the nucleus (green), the nucleus starts to condense (DAPI, blue) and neurites are not visible anymore.