Corridors of Migrating Neurons in Human Brain and Their Decline during Infancy

The subventricular zone (SVZ) of many adult non-human mammals generates large numbers of new neurons destined for the olfactory bulb (OB)(1–6). Along the walls of the lateral ventricles, immature neuronal progeny migrate in tangentially-oriented chains that coalesce into a rostral migratory stream (RMS) connecting the SVZ to the OB. The adult human SVZ, in contrast, contains a hypocellular gap layer separating the ependymal lining from a periventricular ribbon of astrocytes(7). Some of these SVZ astrocytes can function as neural stem cells in vitro, but their function in vivo remains controversial. An initial report finds few SVZ proliferating cells and rare migrating immature neurons in the RMS of adult humans(7). In contrast, a subsequent study indicates robust proliferation and migration in the human SVZ and RMS(8,9). Here, we find that the infant human SVZ and RMS contain an extensive corridor of migrating immature neurons before 18 months of age, but, contrary to previous reports(8), this germinal activity subsides in older children and is nearly extinct by adulthood. Surprisingly, during this limited window of neurogenesis, not all new neurons in the human SVZ are destined for the OB – 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 SVZ and cortex. These pathways represent potential targets of neurological injuries affecting neonates.