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FIGURE 3.9 The proliferation of neural progenitor cells can be studied in vitro. Progenitor cells from the
developing CNS can be studied in cell culture by dissociating them into single cells, diluting them to only a
few cells in each well of a tissue culture dish, and then examining them daily for increases in their numbers.
These micrographs, taken daily, document the proliferation of progenitors from the first division (A) to over
30 cells three days later B-D. Labeling the culture with antibodies against a neural specific protein shows
that several of the new cells have developed into neurons, while others express antigenic markers of either
oligodendrocytes or astrocytes. (From Qian et al., 1998, courtesy of S. Temple)
the large, highly convoluted structure at the back of
the brain that is critical for smooth movements. The
main output neurons from the cerebellar cortex are the
Purkinje neurons. The Purkinje neurons are generated
early in cerebellar development and form a layer one
cell thick throughout the cerebellum. One type of neuron
that connects with the Purkinje cell, the granule cell, is
made in huge numbers by a nearby progenitor zone,
the external granule layer. The Purkinje cells produce
the mitogen, Shh , while the granule cell progenitors
express the Shh receptors, patched and smoothened
(named for Drosophila mutants defective in the homol-
ogous genes). The Shh released from the Purkinje cells
stimulates the granule cell progenitors to make more
granule cells. If the Shh pathway is experimentally
blocked, fewer granule cells are produced. If the Shh
pathway is stimulated, granule cell production is
increased. In this way, Shh is utilized to mediate the
cell interactions between the differentiated Purkinje
neurons and the neural progenitors. This pathway also
provides another example of how a childhood tumor
can result from a misregulation of neurogenesis. Chil-
dren with mutations in the Shh receptor, patched , that
mediates Shh signaling will develop a tumor called
medulloblastoma, in which granule cell production is
fatally uncontrolled (Goodrich et al., 1997).
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