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telencephalon
Pio
EG
midbrain
S 2
Granule cell
precursor migration
Developing
cerebellum
rhombic lip
S 1
PCD
IVth ventricle
M
rhombencephalon
S 1
FIGURE 3.24 The precursors of the cerebellar granule cells come
from a region of the rhombencephalon known as the rhombic lip. The
rhombic lip is a region of the hindbrain that lies adjacent to the fourth
ventricle. Cells from this region migrate over the surface of the cere-
bellum to accumulate in a multicellular layer—the external granule
cell layer. This dorsal view of the developing brain shows the migra-
tory path of the granule cell precursors from the rhombic lip of the
rhombencephalon to the surface of the cerebellum (red arrows).
PC
P
GEC
G
External
granule
cell layer
CF
FIGURE 3.23 The neurons of the cerebellar cortex are arranged
in a highly ordered fashion. In the mature cerebellum, the very large
Purkinje cells (PC) lie in a single layer (P) and have an extensive
dendritic elaboration that lie in a single plane. The granule cells (red)
lie below the Purkinje cells in the granule cell layer (G) and have a
T-shaped axon that runs orthogonal to the plane of the Purkinje cell
dendrites, like phone wires strung on the Purkinje cell dendritic
“poles” in the molecular layer (M). In addition to these distinctive
cell types, the cerebellar cortex also contains other cell classes, the
stellate cells (S) and the Golgi epithelial cells (GECs). (Modified from
Rakic, 1971)
Purkinje
cell layer
Granule
cell layer
FIGURE 3.25 Granule cell production in the external granule cell
layer is followed by the migration of these cells to ultimately lie deep
to the Purkinje cell layer. Arrows show the migratory path a single
neuron would take from its birth to the granule cell layer. The
Bergmann glial cells are shown in red and function as guides for the
migrating neurons. The migration of a granule cell is thought to take
place along a single gila fiber, but in the diagram the migrating
neuron is shown to be associated with several glial cells for clarity.
(Modified from Ramon y Cajal, 1952)
cells. EM studies, similar to those described for the
cerebral cortex, first demonstrated the relationship
between the migrating granule cells and the Bergmann
glia (Rakic, 1971). Throughout the migration of the
granule cells, they are closely apposed to the
Bergmann glial processes. Hatten and her colleagues
have been able to demonstrate directly the migration
of granule cells on Bergmann glia using a dissociated
culture system. When the external granule cell layer is
removed from the cerebellum and the cells are cul-
tured along with cerebellar glia, the granule glial cells
migrate along the extended glial fibers in vitro. Time-
lapse video recordings have captured the granule cell
migration in action (Figure 3.26). The in vitro systems
have also provided a way to explore the molecular
basis of neuronal migration on glial cells in the CNS.
MOLECULAR MECHANISMS OF
NEURONAL MIGRATION
Several questions about the molecular mechanisms
of neuronal migration have been explored in recent
years using the in vitro cerebellar microculture system
developed by Hatten and her colleagues. A particular
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