The Questions of Developmental Biology

Fig 12.16
Cerebellar organization
In the brain, cell migration, differential neuronal proliferation, and selective cell death
produce modifications of the three-zone pattern (Figure 12.16). In the cerebellum, some neuronal
precursors enter the marginal zone to form clusters of neurons called nuclei. Each nucleus works
as a functional unit, serving as a relay station between the outer layers of the cerebellum and other
parts of the brain. In the cerebellum, some neuronal precursors can also migrate away from the
germinal epithelium. These precursor cells, called neuroblasts, migrate to the outer surface of the
developing cerebellum and form a new germinal zone, the external granule layer, near the outer
boundary of the neural tube. At the outer boundary of the external granule layer (which is one to
two cells thick), neuroblasts proliferate. The inner compartment of the external granule layer
contains postmitotic neuroblasts that are the precursors of the major neurons of the cerebellar
cortex, the granule neurons. These granule neurons migrate back into the developing cerebellar
white matter to produce a region called the internal granule layer. Meanwhile, the original
ependymal layer of the cerebellum generates a wide variety of neurons and glial cells, including
the distinctive and large Purkinje neurons. Purkinje neurons are not only critical in the electrical
pathway of the cerebellum, they also support the granule neurons. The Purkinje cell secretes
Sonic hedgehog, which sustains the division of granule neuron precursors in the external granule