[Quantitative study of autoradiographic labelling in the nervous system of the rat. I. Early stage characteristics in the neural tube]

Regulation of neuroblast cell-cycle kinetics plays a crucial role in the generation of unique features of neocortical areas

Cortical neurons are generated in the germinal zones lining the ventricles before migrating predominantly radially. To investigate regional differences in the cell-cycle kinetics of neuroblasts, pulse [3H]-thymidine injections were made throughout corticogenesis, and labeled neuron counts were compared in areas 3, 6, 17, and 18a in the adult mouse. The relationship between height in the cortex and intensity of autoradiographic signal distinguishes first generation and subsequent generations of neurons. This provides the mitotic history of defined sets of neurons and is a powerful tool for analyzing areal differences in cell-cycle kinetics. The infragranular laminar labeling indices of different generations show significant differences in areas 3 and 6. The labeling index of first generation neurons shows that the rate of neuron production is higher in area 3 than in area 6. This increased generation rate in area 3 was accompanied by two major changes. First, computation of the labeling index of the subsequent generation neurons (which reflects percentages of precursors in S-phase at the moment of the pulse) indicates a shorter cell cycle in area 3. Second, the total population of labeled neurons contains a higher proportion of first generation neurons in area 3, implying a higher leaving fraction in this area. Computer simulations of these areal differences of cell-cycle kinetics generate neuron numbers that are in close agreement with published data. Altogether these findings reveal an early regionalization of the ventricular zone that serves to generate unique features of future cortical areas.

Prenatal and postnatal ontogenesis of neurotransmitter-synthetizing enzymes and [125I]tetanus toxin binding capacity in the mouse hypothalamus

In order to estimate the early neuronal maturation in the hypothalamus, we followed the development of 3 neurotransmitter-synthetizing enzymes (TH, GAD, ChAT) and a neuronal cell surface marker (tetanus toxin) in the hypothalamus as compared to cerebral hemispheres. This showed that TH, ChAT and GAD activities were present in both structures on the fourteenth fetal day. Yet, before birth, whereas GAD and ChAT activities remained low and followed a similar development in the 2 structures, TH activity was important and higher in hypothalamus than in brain hemispheres. After birth the activities of the 3 enzymes increased rapidly between day 5 and day 20, but their evolution in the hypothalamus always preceded that in the cerebral hemispheres. Tetanus toxin binding capacity was present on the thirteenth fetal day in the 2 structures, but during fetal life in the hypothalamus, the level of toxin binding was always higher than in the cerebral hemispheres. After birth, the toxin binding increased between day 2 and day 10 and reached adult level at the same time in both structures, around day 15. We conclude that neuronal maturation proceeds earlier in the hypothalamus than in brain hemispheres, and that the differentiation of a neuronal surface marker appears concomitantly with that of specific intraneuronal enzymes.