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The process of neurogenesis ceases in most regions
of the nervous system in most animals. Neurons them-
selves are terminally differentiated cells, and there are
no well-documented examples of functional neurons
reentering the mitotic cycle. However, it has long been
appreciated that in most species some new neurons are
generated throughout life. There is considerable re-
modeling of the nervous system of insects during meta-
morphosis. Much of this remodeling occurs through
cell death, but new neurons are also produced.
Many amphibians also go through a larval stage.
Frogs and toads have tadpole stages where a consid-
erable amount of body growth takes place prior to
metamorphosis into the adult form. During larval
stages, many regions of the frog nervous system
continue to undergo neurogenesis similar to that in
embryonic stages. One of the most well-studied exam-
ples of larval frog neurogenesis is in the retinotectal
system. The eye of the tadpole, like that of the fish,
increases dramatically in size after embryonic devel-
opment is complete. During this period, the animal
uses its visual system to catch prey and avoid preda-
tors. The growth of the retina, however, does not occur
throughout its full extent, but rather is confined to the
periphery (Figure 3.30). Similar to the way in which a
tree grows, the retina adds new rings of cells at the pre-
existing edge of the retina. This provides a way for
new cell addition to go on at the same time the central
retina functions normally. As the new retinal cells are
added, they are integrated into the circuitry of the
previously differentiated retina, into a seamless struc-
ture. At the same time that new cells are added to
the peripheral retina, the optic tectum also adds new
neurons. The coordination between neurogenesis in
these two regions likely involves their interaction via
the retinal ganglion cell projection of the tectum. The
growth of the optic tectum, the brain center to which
the retina sends its axons, occurs at its caudal margin,
so the axons of the ganglion cell must shift caudally
during this time. Fish retina also has an additional
means of growth. As the retina grows, the sensitivity
to light declines as retinal stretch causes a reduction in
the number of rod photoreceptors. The fish maintains
a constant sensitivity by adding new rods throughout
the retina, not just at the peripheral edge. The new rods
are generated by a specialized cell, the rod progenitor,
which under normal circumstances generates new rod
photoreceptors (Raymond and Rivlin, 1987). This spe-
cialized progenitor may not be entirely restricted in
its potential, however, since, following damage to the
“stem” cells
FIGURE 3.30 The eyes of frogs grow by the addition of new cells
to the margin. The neural retina of the frog tadpole is derived from
the neural tube, as described in a previous chapter. The initial retinal
neurons are generated during embryogenesis. However, as the eye
grows, the neural retina grows by means of a specialized ring of
retinal stem cells at the peripheral margin of the eye (red). The retinal
stem cells generate all the different types of retinal neurons to
produce new retina that is indistinguishable from the retina gener-
ated in the embryo, and thoroughly integrated with it. In the newly
post-metamorphic Rana pipiens frog, nearly 90% of the retina has
been generated during the larval stages; all this time the retina has
been fully functional. This process continues even after metamor-
phosis but much more slowly.
retina, these cells are stimulated to generate other
retinal cell types as well.
One of the most well-studied examples of neuroge-
nesis in mature animals comes from studies of song
birds. In 1980, Fernando Nottebom reported that there
was a seasonal change in the size of one of the brain
nuclei important for song production in adult male
canaries. In song birds, specific nuclei in the telen-
cephalon of the brain are critical for the production of
the song. The HVC nucleus is of particular importance
for both song learning and song production (see
Chapter 10). The HVC is almost twice as large in the
spring, when male canaries are generating normal
adult song, than in the fall, when they no longer sing.
Nottebom initially proposed that this change in size
might be due to seasonal changes in the numbers of
synapses. In further studies of the HVC in male and
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