A dynamic population of stem cells and progenitors at the ventricular-subventricular zone drives the expansion of the cortex during human brain development. Studies in mice have demonstrated the sequential progression of cell types in cortical development: in one key transition, radial glia give rise to ependymal cells that eventually line the ventricle, displacing the remaining stem cell population but also contributing to the integrity of the stem cell niche. This developmental progression has, however, not been characterised in detail in the human brain, and now Joanne Conover and colleagues fill this gap. They first confirm that mouse ependymogenesis proceeds in a caudal to rostral direction, creating an ependymal monolayer interspersed with clusters of ventricle-contacting stem cell processes by postnatal day 30. Similarly, in humans, an occipital to frontal transition between radial glia and ependymal cell coverage occurs in the frontal horn of the lateral ventricle. Ventricle-contacting stem cells, initially arrayed in clusters along the ventricle wall, are greatly reduced in the perinatal period, accompanied by a reduction in neurogenesis; in adolescents and adults, no ventricle-contacting stem cells are observed. In contrast to the lateral wall, the medial wall completely loses all its stem cells soon after birth. Human ependymogenesis correlates with an increase of ventricle surface area and a change in anterior horn topography. This work thus characterises a vital developmental process in the human brain, with implications for our understanding of human pathologies characterised by ependymal dysfunction.
