Neuronal death mechanisms in the mature and immature CNS are dissimilar in the timing of the death process. The process of neuronal apoptosis is accelerated in immature CNS compared to adult CNS. Thalamic neurons in adult and immature brain die similarly after cortical injury, but neuronal apoptosis occurs much faster in the immature brain (204). In newborn mice (10-day-old), nuclear p53 accumulation and internucleosomal fragmentation of DNA coincide at 24 h postlesion, and maximal neuronal apoptosis occurs at 36 h postlesion. This time course contrasts sharply with the time course of neuronal apoptosis in the adult brain. Thus, neuronal apoptosis occurs more rapidly in the immature brain compared to adult brain (which occurs over 6-7 days), although p53-modulated mechanisms may participate in both the adult and newborn brain.