Programmed cell death in the developing human telencephalon

Programmed cell death (PCD) in the form of apoptosis is recognized as one of the central events in the development of the central nervous system. To study the time of onset, extent and distribution of PCD in the human telencephalon, embryos and fetuses from 4.5 to 27 gestational weeks (g.w.) were examined using the TUNEL (TdT-mediated dUTP-biotin nick-end labelling) in situ method. At 4.5 g.w. sparse TUNEL(+) nuclei were observed in the ventricular zone of the neural tube. With the formation of the cortical plate at 7-8 g.w., TUNEL(+) nuclei were seen in all developmental layers of the cortical anlage, as well as in the subcortical regions such as the ganglionic eminence and the internal capsule. The proliferative zones (the ventricular zone, the subventricular zone and the ganglionic eminence) contained the majority of all apoptotic nuclei observed in each specimen. However, the apoptotic index was highest in the subplate zone and in layer I. Double-labelling experiments suggested that neuronal precursors were the main population of cells undergoing PCD in the first trimester of gestation, whereas glial cells probably start dying around midgestation. The onset of labelling of microglial cells and apoptotic nuclei were synchronous, indicating the involvement of microglia in PCD. In conclusion, two distinct types of PCD were observed during human telencephalic development: embryonic apoptosis, which was synchronous with proliferation and migration of neuronal cells and probably not related to establishment of neuronal circuitry, and fetal apoptosis, which coincided with differentiation and synaptogenesis, and therefore may be related to the development of axonal-target connectivity.