Distribution and fine structural analysis of undifferentiated cells in the primate subependymal layer.

The subependymal layer (SEL) of the postnatal marmoset, a simian primate, has been investigated by histological and electron microscopic techniques. Although well documented in rodents, little is known about this layer in primates. The distribution of the SEL in marmosets is most extensive at birth around the anterior lateral ventricles, where the layer is generally 5-10 cells deep; however, there is considerable regional variation. With age the size of the SEL decreases dramatically, becoming very poorly demarcated in adult animals. Ultrastructurally, many subependymal cells in neonatal and young brains display the features of undifferentiated cells, although neurons and glia are also present. Cells displaying features intermediate between astrocytes and undifferentiated SEL cells are occasionally encountered. In adults undifferentiated cells are seen rarely and the former SEL is composed primarily of glial and neuronal processes. Thus the layer in primates probably represents a site of continued cellular differentiation in the postnatal brain and as such must play an important role in the final stages of cortical development.