Cortical distribution of GABAergic interneurons is determined by migration time and brain size

Cortical interneurons (CINs) originate in the ganglionic eminences (GEs) and migrate tangentially to the cortex guided by different attractive and repulsive cues. Once inside the cortex, the cellular and molecular mechanisms determining the migration of CINs along the rostro-caudal axis are less understood. Here, we investigated the cortical distribution of CINs originating in the medial and caudal GEs at different time-points. Using molecular and genetic labeling, we show that early and late-born CINs (E12 vs E15) are differentially distributed along the rostro-caudal axis. Specifically, late-born CINs are preferentially enriched in cortical areas closer to their respective sites of origin in the medial or caudal GE. Surprisingly, our in vitro experiments failed to show a preferential migration pattern along the rostro-caudal axis for medial or caudal born CINs. Moreover, in utero transplantation experiments suggested that the rostro-caudal dispersion of CINs depends on the developmental stage of the host brain and is limited by the migration time and the increasing size of the developing brain. These data suggest that the embryonic expansion of the cortex contributes to the rostro-caudal CINs distribution.