Oligodendrocytes are cells that myelinate axons, providing saltatory conduction of action potentials and proper function of the central nervous system. Myelination begins prenatally in the human, and the sequence of oligodendrocyte development and the onset of myelination are not thoroughly investigated. This knowledge is important to better understand human diseases, such as periventricular leukomalacia, one of the leading causes of motor defi cit in premature babies, and demyelinating disorders such as multiple sclerosis (MS). In this review we discuss the spatial and temporal progression of oligodendrocyte lineage characterized by the expression of specifi c markers and transcription factors in the human fetal brain from the early embryonic period (5 gestational weeks, gw) until midgestation (24 gw). Our in vitro evidence indicated that a subpopulation of human oligodendrocytes may have dorsal origin, from cortical radial glia cells, in addition to their ventral telencephalic origin. Furthermore, we demonstrated that the regulation of myelination in the human fetal brain includes positive and negative regulators. Chemokines, such as CXCL1, abundant in proliferative zones during brain development and in regions of remyelination in adult, are discussed in the view of their potential roles in stimulating oligodendrocyte development. Other signals are inhibitory and may include, but are not limited to, polysialic acid modifi cation of the neural cell adhesion molecule on axons. Overall, important differences in temporal and spatial distribution and regulatory signals for oligodendrocyte differentiation exist between human and rodent brains. Those differences may underlie the unique susceptibility of humans to demyelinating diseases, such as MS.

The subplate lays the foundation of the developing cerebral cortex, and abnormalities have been suggested to contribute to various brain developmental disorders. The causal relationship between cellular pathologies and cognitive disorders remains unclear, and therefore, a better understanding of the role of subplate cells in cortical development is essential. Only by determining the molecular taxonomy of this diverse class of neurons can we identify the subpopulations that may contribute differentially to cortical development. We identified novel markers for murine subplate cells by comparing gene expression of subplate and layer 6 of primary visual and somatosensory cortical areas of postnatal day (P)8 old mice using a microarray-based approach. We examined the utility of these markers in well-characterized mutants (reeler, scrambler, and p35-KO) where the subplate is displaced in relation to the cortical plate. In situ hybridization or immunohistochemistry confirmed subplate-selective expression of complexin 3, connective tissue growth factor, nuclear receptor-related 1/Nr4a2, and monooxygenase Dbh-like 1 while transmembrane protein 163 also had additional expression in layer 5, and DOPA decarboxylase was also present in the white matter. Localization of marker-positive cells in the reeler and p35-KO cortices suggests different subpopulations of subplate cells. These new markers open up possibilities for further identification of subplate subpopulations in research and in neuropathological diagnosis.

Since the discovery that the vast majority of the GABA-containing interneurons of the cerebral cortex arise in the subpallium, considerable effort has been put into the description of the precise origin of these neurons in subdivisions of the ganglionic eminence and in the migratory routes they follow on their way to the developing cortex. More recently, studies have focused on the molecular and cellular mechanisms that guide their migration. Investigations of the molecular mechanisms involved have demonstrated important roles for numerous transcription factors, motogenic factors and guidance molecules. Here, we review results of very recent analyses of the underlying cellular mechanisms and specifically of the movement of the nucleus, cytoplasmic components and neuritic processes during interneuron migration.

Kallmann syndrome (KS) is a genetic disease characterized by hypogonadotropic hypogonadism and impaired sense of smell. The genetic causes underlying this syndrome are still largely unknown, but are thought to be due to a developmental defect in the migration of gonadotropin-releasing hormone (GnRH) neurons. Understanding the causes of the disease is hampered by lack of appropriate mouse models. GnRH neurons are hypothalamic cells that centrally control reproduction in mammals by secreting the GnRH decapeptide into the portal blood vessels of the pituitary to stimulate the production of gonadotropins. During development, these cells are born in the nasal placode outside the brain and migrate in association with olfactory/vomeronasal axons to reach the forebrain and position themselves in the hypothalamus. By combining the analysis of genetically altered mice with in vitro models, we demonstrate here that a secreted guidance cue of the class 3 semaphorin family, SEMA3A, is essential for the development of the GnRH neuron system: loss of SEMA3A signalling alters the targeting of vomeronasal nerves and the migration of GnRH neurons into the brain, resulting in reduced gonadal size. We found that SEMA3A signals redundantly through both its classical receptors neuropilin (NRP) 1 and, unconventionally, NRP2, while the usual NRP2 ligand SEMA3F is dispensable for this process. Strikingly, mice lacking SEMA3A or semaphorin signalling through both NRP1 and NRP2 recapitulate the anatomical features of a single case of KS analysed so far, and may therefore be used as genetic models to elucidate the pathogenesis of KS.

In this study we examine possible origins and migratory routes of human cortical neurons, with special emphasis on the preplate and layer I. In embryonic stages, two main cell types, Cajal-Retzius cells, and cells labeled with interneuron markers (calretinin, calbindin and GABA), were present in the preplate layer. In addition, a number of preplate GABAergic cells co-expressed either Nkx2.1 or Dlx transcription factors, findings consistent with their origin in the ganglionic eminence and subsequent tangential migration to the layer I. The orientation of the leading process indicates that some of these cells descend to the cortical plate. However, the finding of radially oriented GABAergic, NKX2.1 + and DLX + cells in the cortical ventricular zone, argues that, unlike in rodents, a significant subpopulation of these cells originates in the cortical ventricular zone. In embryonic stages, expression of Reelin in Cajal-Retzius cells as well as Reelin/DLX2 + cells in the embryonic ganglionic eminence and the olfactory region, suggest that these cells in human may have diverse origins. In later fetal stages in human (17-22 gestational weeks) layer I and the newly formed subpial granular layer, contained a population of small interneurons that originated mainly in the lateral ganglionic eminence, since the majority of these cells were double-labeled with DLX/GABA, and rarely with NKX2.1/GABA. Therefore, neurons in the human cortical layer I are heterogeneous, with more complex origin and migratory routes than in rodents. In addition to the ganglionic eminence, both the expended subventricular zone and subpial granular layer, contribute to the neuronal population of the developing layer I and underlining cortical plate.

Cortical interneurons in rodents are generated in the ventral telencephalon and migrate tangentially into the cortex. This process requires the coordinated action of many intrinsic and extrinsic factors. Here we show that Robo1 and Robo2 receptor proteins are dynamically expressed throughout the period of corticogenesis and colocalize with interneuronal markers, suggesting that they play a role in the migration of these cells. Analysis of Robo mutants showed a marked increase in the number of interneurons in the cortices of Robo1(-/-), but not Robo2(-/-), animals throughout the period of corticogenesis and in adulthood; this excess number of interneurons was observed in all layers of the developing cortex. Using BrdU incorporation in dissociated cell cultures and phosphohistone-3 labeling in vivo, we demonstrated that the increased number of interneurons in Robo1(-/-) mice is, at least in part, due to increased proliferation. Interestingly, a similar increase in proliferation was observed in Slit1(-/-)/Slit2(-/-) mutant mice, suggesting that cell division is influenced by Slit-Robo signaling mechanisms. Morphometric analysis of migrating interneurons in Robo1(-/-), Robo2(-/-) and Slit1(-/-)/Slit2(-/-), but not in Slit1(-/-) mice, showed a differential increase in neuronal process length and branching suggesting that Slit-Robo signaling also plays an important role in the morphological differentiation of these neurons.

Programmed cell death (PCD) in the form of apoptosis is recognized as one of the central events in the development of the central nervous system. To study the time of onset, extent and distribution of PCD in the human telencephalon, embryos and fetuses from 4.5 to 27 gestational weeks (g.w.) were examined using the TUNEL (TdT-mediated dUTP-biotin nick-end labelling) in situ method. At 4.5 g.w. sparse TUNEL(+) nuclei were observed in the ventricular zone of the neural tube. With the formation of the cortical plate at 7-8 g.w. , TUNEL(+) nuclei were seen in all developmental layers of the cortical anlage, as well as in the subcortical regions such as the ganglionic eminence and the internal capsule. The proliferative zones (the ventricular zone, the subventricular zone and the ganglionic eminence) contained the majority of all apoptotic nuclei observed in each specimen. However, the apoptotic index was highest in the subplate zone and in layer I. Double-labelling experiments suggested that neuronal precursors were the main population of cells undergoing PCD in the first trimester of gestation, whereas glial cells probably start dying around midgestation. The onset of labelling of microglial cells and apoptotic nuclei were synchronous, indicating the involvement of microglia in PCD. In conclusion, two distinct types of PCD were observed during human telencephalic development: embryonic apoptosis, which was synchronous with proliferation and migration of neuronal cells and probably not related to establishment of neuronal circuitry, and fetal apoptosis, which coincided with differentiation and synaptogenesis, and therefore may be related to the development of axonal-target connectivity.

Type I lissencephaly, a genetic disease characterized by disorganized cortical layers and gyral abnormalities, is associated with severe cognitive impairment and epilepsy. Two genes, LIS1 and doublecortin (DCX), have been shown to be responsible for a large proportion of cases of type I lissencephaly. Both genes encode microtubule-associated proteins that have been shown to be important for radial migration of cortical pyramidal neurons. To investigate whether DCX also plays a role in cortical interneuron migration, we inactivated DCX in the ganglionic eminence of rat embryonic day 17 brain slices using short hairpin RNA. We found that, when DCX expression was blocked, the migration of interneurons from the ganglionic eminence to the cerebral cortex was slowed but not absent, similar to what had previously been reported for radial neuronal migration. In addition, the processes of DCX-deficient migrating interneurons were more branched than their counterparts in control experiments. These effects were rescued by DCX overexpression, confirming the specificity to DCX inactivation. A similar delay in interneuron migration was observed when Doublecortin-like kinase (DCLK), a microtubuleassociated protein related to DCX, was inactivated, although the morphology of the cells was not affected. The importance of these genes in interneuron migration was confirmed by our finding that the cortices of Dcx, Dclk, and Dcx/Dclk mutant mice contained a reduced number of such cells in the cortex and their distribution was different compared with wild-type controls. However, the defect was different for each group of mutant animals, suggesting that DCX and DCLK have distinct roles in cortical interneuron migration.