These studies in fetal monkeys and ferrets show that the two fundamental types of thalamic GABA neuron populate the thalamus during widely separated developmental epochs. Those of the ventral thalamus (VT) and epithalamus (ET) appear very early in conjunction with the GABA cells of the substantia nigra, globus pallidus, and pretectum (PT). GABA neurons intrinsic to the dorsal thalamus (DT) appear later, long after proliferative activity has ceased in the wall of the third ventricle, after dorsal thalamic nuclei have differentiated and at the same time as principal neurons are acquiring a glutamatergic phenotype. The exact origins of the two waves of GABA cells are uncertain but some appear to arrive from extrinsic sources that include the ganglionic eminence (GE) of the basal forebrain.

Neuronal migration is guided by long radially oriented glial fibres. During late stages of development radial glial cells are transformed into astrocytes. A predominant intermediate filament protein within radial glial cells and immature astrocytes is vimentin. In this study fetal brain sections were used to demonstrate the transient features of vimentin-positive radial glia. In the lower half of the cerebral wall of the 6th gestational month bundles, curvature, and crossing of vimentin-positive fibres are regularly seen. Moreover, fibres terminating on vessels are observed. In the upper half fibres are radially oriented; when ascending towards the pial surface the number and diameter of fibres appears conspicuously decreased. Radially aligned fibres display numerous varicosities. In the 8th month the bulk of vimentin-positive fibres is encountered next to the ganglionic eminence and below isocortical cerebral fissures. The dentate gyrus is conspicuous due to its high amount of immunolabelled fibres. Furthermore, densely packed fibres are visible within the internal and external capsule and in the vicinity of the anterior commissure. Radial glial somata are found in the proliferative areas as well as in the adjacent white matter. In the latter location bipolar, monopolar and stellate vimentin-positive cells are present. The results demonstrate an area-specific distribution pattern of vimentin-positive structures which can be correlated with migrational events. Areas maturing late in development for instance, reveal dense immunolabelling in the 8th month. The orientation and position of radial fibres point to an additional developmental role of these fibres, i.e. their involvement in the guidance of growing axons. Moreover, the arrangement and morphology of vimentin-positive fibres, such as retraction of fibres or occurrence of varicosities, are indicative of degenerative events. Accordingly, a transformation of radial glial somata, their displacement towards the white matter and finally the growth of stellate processes can clearly be demonstrated.