Early regionalisation of the neocortex and the medial ganglionic eminence.

The two major functional classes of neurons that build the cerebral cortex are generated in two distinct parts of the telencephalon. Excitatory long distance projecting neurons are produced dorsally in the pallium, whereas local inhibitory interneurons are mainly produced in the medial ridge of the ventral telencephalon. These two parts of the telencephalon are molecularly regionalized from early embryonic stages, but cellular indices of regionalisation are observed only at later stages of development. We have looked for cellular indices of regionalisation in the cortical anlage at early embryonic stages, when the first efferent cortical neurons are generated. Similarly, we have looked for functional regionalisation of the medial ganglionic eminence at the same stages, when the future cortical interneurones are generated. Here, we summarize data showing that two regions in the mouse cortex embryo, the lateral and dorsal cortex, differ strongly in their early neurogenesis. Moreover, the two domains differ in their capacity to produce GABAergic neurons in vitro; this capacity is only observed in the dorsal cortex. The differentiation of the two domains appears to be independent of the laterorostral to mediocaudal gradient of maturation of the cortex. In the basal telencephalon too, the capacity to differentiate GABAergic neurons is not uniformly distributed across the medial ganglionic eminence. The neurogenesis of future cortical interneurons is seen to be highly active in a small area located in the rostral MGE, at mid dorso-ventral level.