Corridors of migrating neurons in the human brain and their decline during infancy

The subventricular zone of many adult non-human mammals generates large numbers of new neurons destined for the olfactory bulb. Along the walls of the lateral ventricles, immature neuronal progeny migrate in tangentially oriented chains that coalesce into a rostralmigratory stream (RMS) connecting the subventricular zone to the olfactory bulb. The adult human subventricular zone, in contrast, contains a hypocellular gap layer separating the ependymal lining from a periventricular ribbon of astrocytes. Some of these subventricular zone astrocytes can function as neural stem cells in vitro, but their function in vivo remains controversial. An initial report found few subventricular zone proliferating cells and rare migrating immature neurons in the RMS of adult humans. In contrast, a subsequent study indicated robust proliferation and migration in the human subventricular zone and RMS. Here we find that the infant human subventricular zone andRMScontain an extensive corridor of migrating immature neurons before 18 months of age but, contrary to previous reports, this germinal activity subsides in older children and is nearly extinct by adulthood. Surprisingly, during this limitedwindowofneurogenesis, not all new neurons in the human subventricular zone are destined for the olfactory bulb—we describe a major migratory pathway that targets the prefrontal cortex in humans. Together, these findings reveal robust streams of tangentially migrating immature neurons in human early postnatal subventricular zone and cortex. These pathways represent potential targets of neurological injuries affecting neonates. We collected human brain specimens from 10 neurosurgical resections and 50 autopsied brains, ranging in age from birth to 84 years (Supplementary Fig. 1 and Supplementary Table 1), using a protocol that allows subsequent analysis by fluorescent immunohistochemistry and in situ hybridization (seeMethods). As shown (Fig. 1), staining of horizontal sections (30mm) through the anterior horn of the lateral ventricle demonstrates that, in the first 6months of life, the structure of the human subventricular zone in infants differs considerably from that observed in adults. The astrocytic ribbon and gap layer are not evident (Fig. 1) and, as seen in the fetal human brain, cells with elongated radial glial processes line the lateral ventricular wall and express vimentin and glial fibrillary acidic protein (GFAP). Adjacent to these radial glia, we observed a dense network of elongated unipolar and bipolar cells oriented tangentially to the ventricular lining.Many of these cells expressed the immature neuronal markers doublecortin (DCX) (Fig. 1) and b-III tubulin (TUBB3). Some putative immature neurons also expressed polysialylated neural cell adhesion molecule (PSA-NCAM), which is present in migratory cells. They also had ultrastructural features of immature migrating neurons (Supplementary Fig. 2) similar to neuroblasts described in the rodent subventricular zone. Progressively, between 6 to 18months of age, the subventricular zone is depleted of this dense network of putative migratory neurons and adopts the characteristic adult structure with an astrocyte ribbon and hypocellular gap layer. The emergence of the gap layer coincides with the decline in DCX immature neurons (25-fold during the first 6 months; n5 16, ages 0–17 years; Fig. 1) and proliferation, suggesting that human subventricular zone neurogenesis decreases drastically during the first 6 months of life. Only a small number of proliferating cells were present in adolescents and adults. Expression of the proliferation marker Ki67 was not associated with pyknotic nuclei, although we cannot exclude that some Ki67 nuclei correspond to apoptotic cells induced by ischaemia. We also identified a subpopulationof epidermal growth factor receptor (EGFR)-positive cells, amarker associated with early progenitors including neural stem cells and transit-amplifying cells in mice, which similarly diminished with time (Supplementary Fig. 3). A subset of EGFR cells expressed Ki67, while others co-localized with DCX or PSA-NCAM, potentially representing transitional stages from transit-amplifying cells to immature neurons. Whole-mount en face preparations of the subventricular zone also confirmed massive numbers of tangentially oriented DCX chains within the gap layer at 1 week and 2months of life. In stark contrast, cells with themorphologyandmarker expressionof immaturemigratory neurons were extremely rare in adults (Supplementary Fig. 4). Our results indicate that robust streams of tangentially migrating immature neurons initially populate the postnatal human gap layer; these pathways become depleted between 6–18months and this region transitions into a hypocellular gap. Previous work has also demonstrated a similarly sharp decline in EGFR and PSA-NCAM immunoreactivity in the human subventricular zone during the first year of life. Within the subventricular zone, the decline of migratory immature neurons appears first along the posterior third of the lateral ventricle and then progresses in a posterior-to-anterior trajectory towards the ventral tip (data not shown). After 18 months, Ki67 proliferative activity and the number of DCX immature neurons assume trace levels seen in adults, leaving behind the gap layer characteristic of the adult human subventricular zone. We next investigatedwhether proliferation and the presence of putatively migrating, immature neurons in the subventricular zone were associated with an active RMS. Using autopsied material (n5 6) (ages 1 day; 1 week; 1, 3 and 6 months), serial sagittal and coronal reconstruction of the ventral forebrain revealed an uninterrupted column of cells that connected the ventral tip of the subventricular zone to the olfactory peduncle (Fig. 2). The descending/proximal limb of the paediatric RMS contained cells organized as chains—large collections of elongated immature neurons expressing DCX and surrounded by glial cells and processes (Fig. 2)—or as broad streams of individual cells. A subpopulation of these DCX cells expressed PSA-NCAM. Conversely, analysis of tissue fromolder children (n5 7; 2, 3, 7, 16, 17 years) failed to reveal chains of migrating cells or evidence of an active RMS. However, individual or pairs of elongated DCXPSA-NCAM