Newly formed undifferentiated
cells of the pseudostratified neuroepithelium migrate outward forming a
3-layered tube consisting of 1) an internal, columnar
ependymal layer
- becomes the ependymal lining and epithelium of choroid plexus, 2) a middle,
densely packed layer of
mantle cells
- becomes the gray matter of the CNS, and 3) an external
marginal layer
composed mainly of the processes of cells of the mantle layers - becomes the
white matter of the CNS.
Neurons develop from neuroblasts of the neuroepithelium and migrate into the
mantle layer. The neuroblast develops into a bipolar cell having a primitive
axon and dendrite. The single dendrite degenerates and is replaced by multiple
dendrites forming a multipolar neuroblast. Axons have few branches and their
axonal growth cones are directed to their targets by tropic factors. Primitive
supporting cells are glioblasts and migrate into the mantle and marginal layers
to become astrocytes and oligodendrocytes. Microglia are derived from invading
blood monocytes.
