Receptor tyrosine kinases (RTKs) are cell-surface molecules that transduce signals for cell growth and differentiation. The RTK encoded by the c-ret proto-oncogene is rearranged and constitutively activated in a large proportion of thyroid papillary carcinomas, and germ-line point mutations in c-ret seem to be responsible for the dominantly inherited cancer syndromes multiple endocrine neoplasia (MEN) types 2A and B. The gene is expressed in the developing central and peripheral nervous systems (sensory, autonomic and enteric ganglia) and the excretory system (Wolffian duct and ureteric bud epithelium) of mice, indicating that it may play a role in normal development. Here we show that mice homozygous for a targeted mutation in c-ret develop to term, but die soon after birth, showing renal agenesis or severe dysgenesis, and lacking enteric neurons throughout the digestive tract. Ret is thus an essential component of a signalling pathway required for renal organogenesis and enteric neurogenesis.

During development of the neocortex, the marginal zone (layer I) and the subplate (layer VII) are the first layers to form from a primordial plexiform neoropil. The cortical plate (layers II-VI) is subsequently established between these superficial and deep components of the primordial plexiform neuropil. Neurons in the early zones are thought to play important roles in the formation of the cortex: the Cajal-Retzius cells of the marginal zone are instrumental in neuronal migration and laminar formation, and cells of the subplate are involved in the formation of cortical connections. Using the fluorescent tracer 1,1'-dioctodecyl-3,3,3', 3'-tetramethylindocarbocyanine (DiI), we have shown here that a substantial proportion of neurons of the marginal zone, including cells with features of Cajal-Retzius cells, and of the subplate and lower intermediate zone are not born in the ventricular neuroepithelium but instead originate in the medial ganglionic eminence (MGE), the pallidal primordium. These neurons follow a tangential migratory route to their positions in the developing cortex. They express the neurotransmitter GABA but seem to lack the calcium binding protein calretinin; some migrating cells found in the marginal zone express reelin. In addition, migrating cells express the LIM-homeobox gene Lhx6, a characteristic marker of the MGE. It is suggested that this gene uniquely or in combination with other transcription factors may be involved in the decision of MGE cells to differentiate in situ or migrate to the neocortex.

Cortical pyramidal cells are generated from pallial neuroepithelial precursors, whereas GABAergic interneurons originate in subpallial germinal zones and migrate tangentially to reach the cortex. Using Cre-lox technology in transgenic mice and a series of molecular markers that subdivide the subpallial neuroepithelium into small domains, we fate-map precursor pools and identify interneurons generated from each domain. Cortical interneurons expressing calbindin, parvalbumin, and somatostatin are generated exclusively from Lhx6 (Lim homeobox 6)-expressing precursors in the medial ganglionic eminence (MGE). Martinotti cells that coexpress calretinin and somatostatin are generated from the dorsal region of the MGE neuroepithelium that expresses Nkx6.2 (NK2 transcription factor-related 6.2). Most neuropeptide Y-expressing cells and all bipolar calretinin-expressing interneurons are generated outside the MGE, from the germinal zones of the lateral/caudal ganglionic eminences that express Gsh2 (genomic screened homeobox 2). Our data demonstrate that subpallial neuroepithelial domains defined by expression of genetic determinants generate distinct interneuron subtypes, thereby contributing to the generation of cortical interneuron heterogeneity observed in the adult cortex.

The cerebral cortex contains two main neuronal cell populations, the excitatory glutamatergic (pyramidal) neurons and the inhibitory interneurons, which synthesize GABA and constitute 20 -30% of all cortical neurons. In contrast to the mostly homogeneous population of projection neurons, cortical interneurons are characterized by remarkable morphological, molecular, and functional diversity. Among the markers that have been used to classify cortical interneurons are the calcium-binding proteins parvalbumin and calretinin and the neuropeptide somatostatin, which in rodents identify mostly nonoverlapping interneuron subpopulations. Pyramidal neurons are born during embryogenesis in the ventricular zone of the dorsal telencephalon, whereas cortical interneurons are generated in the subpallium and reach the cortex by tangential migration. On completion of tangential migration, cortical interneurons switch to a radial mode of migration and enter the cortical plate. Although the mechanisms that control the generation of interneuron diversity are currently unknown, it has been proposed that their site of origin in the ventral forebrain determines their specification into defined neurochemical subgroups. Here, we show that Lhx6, a gene induced in the medial ganglionic eminence and maintained in parvalbumin-and somatostatin-positive interneurons, is required for the specification of these neuronal subtypes in the neocortex and the hippocampus. We also show that Lhx6 activity is required for the normal tangential and radial migration of GABAergic interneurons in the cortex.