Cortical neurons are composed of excitatory projection neurons and inhibitory interneurons. Despite their small proportion, inhibitory interneurons play important roles in cortical functions such as modulation of plasticity and output. The specificity of cortical functions relies on their intracortical distribution, morphological, chemical, physiological properties and synaptic connectivities, all of which are outcome of precisely regulated development of these neurons. One notable feature of cortical interneuron development is their long-distance and long-term migration. Most of these neurons originate from the medial ganglionic eminence of the basal forebrain and migrate long distances to reach their final positions in the dorsal pallium. In this talk I will review how they perform their journey in the cortex, terminate migration and elongate axons. We focused on interneurons originating from the medial ganglionic eminence of the mouse at embryonic day (E) 12.5 and labeled these neurons by in utero electroporation of genes encoding for fluorescent proteins.

The earliest neurons labeled in this way have already arrived at the pallium at E14.5 but only in the intermediate/subventricular zones. At E15.5 many labeled neurons can be observed in the marginal zone (MZ), suggesting translocation of interneurons from the intermediate/subventricu1ar zone to the MZ. Curiously, these neurons do not migrate in specific directions in the MZ, but instead in all directions. They stay in the MZ for a long (>one day) period of time, being attracted by a chemokine SDF-l secreted from pial meninges. Thereafter, they migrate towards the cortical plate, where they complete migration; a process regulated both by intrinsic and extrinsic mechanisms. Postnatally, these neurons begin to initiate axons. Interestingly, the polarity of cortical interneurons as migrating neurons is lost and they begin to show morphologically non-polarized shape, alternately extending and retracting several processes. Then, one of these processes begins to elongate rapidly and becomes an axon. Although axon-dendrite polarity appears to derive from apico-basal po1arity of neuroepithelial cells in others systems, this does not appear to be the case for cortical interneurons. We will discuss how intracortical migration contributes to the establishment of their matured morphology and final distribution.