Chain formation and glial tube assembly in the shift from neonatal to adult subventricular zone of rodent forebrain

The subventricular zone (SVZ) is regarded as an embryonic germinal layer persisting at the end of cerebral cortex neurogenesis and capable of generating neuronal precursors throughout life. The two distinct compartments of the adult rodent forebrain SVZ, astrocytic glial tubes and chains of migrating cells, are not distinguishable in the embryonic and early postnatal counterpart. In this study we analyzed the SVZ of mice and rats around birth and throughout different postnatal stages, describing molecular and morphological changes which lead to the typical structural arrangement of adult SVZ. In both species studied, most changes occurred during the first month of life, the transition being slightly delayed in mice, in spite of their earlier development. Important modifications affected the glial cells, eventually leading to glial tube assembly. These changes involved an overall reorganization of glial processes and their mutual relationships, as well as gliogenesis occurring within the SVZ which gives rise to glial cell subpopulations. The neuroblast cell population remained qualitatively quite homogeneous throughout all the stages investigated, changes being restricted to the relationships among cells and consequent formation of chains at about the third postnatal week. Electron microscopy showed that chain formation is not directly linked to glial tube assembly, generally preceding the occurrence of complete glial ensheathment. Moreover, chain and glial tube formation is asymmetric in the medial/lateral aspect of the SVZ, being inversely related. The attainment of an adult SVZ compartmentalization, on the other hand, seems linked to the pattern of expression of adhesion and extracellular matrix molecules.