Many years have passed since the initial observations that led to the discovery of the origin of cortical interneurons in the subpallium of rodents (Porteus et al., 1994; De Carlos et al., 1996; Anderson et al., 1997; Tamamaki et al., 1997). Since then, it is becoming clear that understanding the development of cortical GABAergic interneurons may help to shed light on the problem of their diversity. The early description of mice lacking the transcription factor Nkx2-1, for example, made it evident that specific genes control the development of distinct classes of interneurons

(Sussel et al., 1999). More recently, analysis of the function of other transcription factors has revealed that each of the properties that contribute to the definition of specific

classes of interneurons is controlled by a defined high throughput screening assay set of genes. For example, acquisition of the fast-spiking characteristics and expression of Target Selective Inhibitor Library high throughput the calcium-binding protein parvalbumin (PV) seems to be defined by the concerted action of Nkx2-1, Dlx5, Dlx6, Lhx6 and Sox6, five genes expressed by specific cohorts of cortical interneurons (Liodis et al., 2007; Butt et al., 2008; Zhao et al., 2008; Azim et al., 2009; Batista-Brito et al., 2009; Wang et al., 2010). From this perspective, it is tempting to speculate that deciphering the origin of cortical interneurons may help us to generate a cladistic classification of these cells. Although it has been obvious for more than a century that many different classes of interneurons exists, for the purposes of this

review we have adopted a conservative grouping of GABAergic interneurons into four major classes: (1) fast-spiking, PV-containing basket and chandelier cells; (2) somatostatin (SST)-containing interneurons, which typically display intrinsic burst spiking or adapting non-fast-spiking electrophysiological profiles and many of which have long axons that extend into layer I; (3) rapidly adapting interneurons with bipolar or double-bouquet morphologies, which frequently express calretinin (CR) and/or vasointestinal peptide (VIP); and (4) rapidly adapting interneurons with multipolar morphologies and that express neuropeptide Y (NPY) and/or Demeclocycline reelin, but not SST (Fig. 1). Recent progress on the origin of interneurons suggests that these different classes of cells originate from three main sources in the developing subpallium: the medial ganglionic eminence (MGE), the caudal ganglionic eminence (CGE) and the preoptic area (POA), and reach the cortex following different migratory routes (Fig. 2). Here we review our current view on this process, which is largely based on studies in the mouse. The origin of some populations of GABAergic interneurons in the developing pallium of monkeys and human embryos will not be the addressed in this article, as this topic has recently been reviewed elsewhere (Jones, 2009).