Prenatal development of GABA-immunoreactive neurons in the human striate cortex.

The prenatal development of neurons immunoreactive to gamma-aminobutyric acid (GABA) in the striate cortex (area 17) of human foetuses aged from 14 weeks to term was studied immunocytochemically. In the 14 week foetus GABA-immunoreactive cells occurred in all layers of area 17 with the highest density in the marginal zone (MZ), subplate (SP), deep intermediate zone (IZ) and ventricular zone (VZ). The cortical plate (CP), which gives rise to most of the definitive adult cortical layers, had relatively low concentrations of GABAergic cells. By 17 weeks the density in the proliferative VZ had declined. At 20 weeks some of the adult layers were recognisable; the density of GABA-positive neurons was now highest in the definitive cortex, especially the deep layers (layers VI and V), was lower in the superficial cortical plate, and was lowest in IZ, where the white matter would form. The peak of GABA-immunoreactive neuronal density continued to move superficially during development, and was in layer IVc by 30 weeks. The laminar distribution stabilised from 30 weeks with three dense bands: in layer IVc and superficial V, layer IVa, and layers II and superficial III. The tangential distribution of GABAergic neurons was determined in two older brains (32 and 39 weeks) and no unequivocal spatial periodicity was observed in this plane. The mean cross-sectional area of GABAergic neurons in area 17 increased with foetal age, and also increased from superficial to deep layers at each age. Most GABA-immunoreactive neurons in younger brains contained immunonegative or weakly positive nuclei and had few visible processes, while in the older brains most neurons contained positive nuclei and had more visible processes. The proportion of GABA-immunoreactive bipolar cells declined during development while that of multipolar cells increased. GABAergic neurons thus differentiate early in human foetal striate cortex. They are initially most numerous in the proliferative layers deep to the developing definitive cortex; from 20 weeks of gestation, their peak moves superficially into the maturing deep layers (VI and V) and a stable laminar distribution is attained by 30 weeks, with peaks in layers II/IIIm, IVa and IVc/V. There is no obvious horizontal periodic distribution before term.