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Furthermore, these errant projections remain into
adulthood when one nare is closed during develop-
ment, suggesting that sensory experience is required
for the formation of adult maps (Chapter 6).
Elimination of commissural axons, projecting from
one side of the cerebral cortex to the other, occurs in
the absence of cell death (Innocenti et al., 1977). This
was demonstrated in rats by labeling commissural
neurons twice, once early in development and then
once again after certain axons retract (O'Leary et al.,
1981). Neurons that projected to the other hemisphere
were retrogradely labeled at birth by injecting a dye on
one side of the brain and allowing commissural axons
to transport it back to the cell body (Figure 9.4). Two
weeks later, a second dye was injected in the same
spot, and the remaining commissural axons retro-
gradely transported it to their cell bodies. When the
tissue was examined, many cells were stained with
only the first dye (Figure 9.4, green), but only a frac-
tion of these labeled cells also contained the second
dye (Figure 9.4, red). The green cells must have sent
axons through the commissure at birth, but some of the
cells had apparently retracted their axons before the
red dye was injected. Therefore, many cortical neurons
generate transient projections through the cerebral
commissure (corpus callosum), and some of these
axons are eliminated during development.
The wholesale withdrawal of axons provides a
wonderful example of developmental refinement, but
unfortunately for the anatomist this is not the norm.
Changes in terminal arbor morphology are usually
quite subtle. An axon tends to innervate the correct
target region (Chapter 6), extend a bit beyond the
correct topographic position, and then pull back to the
adult boundary. Perhaps the best characterized exam-
ples of axon terminal elimination come from the devel-
oping visual pathway. In cats and primates, retinal
ganglion cells from each eye project to separate layers
in the lateral geniculate nucleus (LGN). The LGN
neurons then project to Layer IV of the visual cortex,
forming segregated eye-specific termination zones,
called ocular dominance columns or “stripes” (Figure
It is possible to visualize the projection pattern of
an entire eye by injecting 3 H-proline into the eye cup.
This label is taken up by retinal ganglion cells and
transported down their axons to the LGN where it
crosses the synapse, enters the postsynaptic LGN
neuron, and is carried by the axons to their terminals
in the cortex. Autoradiographic images were obtained
from the cortex, which showed the termination
pattern of LGN axons originating in one eye-specific
layer (Figure 9.5B). The LGN afferents from one eye
were widespread in cortical Layer IV at 7 days post-
Inject first dye at P2
Labelled cells at P21
(retrograde transport from the injection)
Inject first dye at P2
Inject second dye at P20
Double-labelled cells at P21
Cells containing first dye only
(withdrew their axons
between P2 and P20)
FIGURE 9.4 Elimination of commissural projections during
development. At postnatal day 2 (P2), a dye (green) was injected into
the cortex, and it was retrogradely transported by commissural
neurons. At P21, the animal was sacrificed, and the tissue was
processed to reveal the labeled neurons. In a second experiment, the
first (green) dye was injected at P2, and a second (red) dye was
injected at P20. When the tissue was processed, some commissural
neurons were double labeled (green/red), whereas others contained
only the green dye. Thus, the green-labeled cells projected to the con-
tralateral cortex at P2, but retracted their axons during postnatal
development and maintained local connections. (Adapted from
O'Leary et al., 1981)
natal. Over the next several weeks, this diffuse label
breaks up into discrete patches that represent eye-
specific termination zones (LeVay et al., 1978; Crair
et al., 2001). The light-evoked responses of Layer IV
visual cortex neurons are consistent with the anatomy.
By monitoring an intrinsic optical signal that is pro-
duced by electrically active brain tissue (see BOX:
Watching Neurons Think), it is possible to show that
visual cortex responds uniformly to stimulation of one
eye at postnatal day 8. However, the same stimulus to
one eye begins to activate patches of cortex by post-
natal day 14 (Crair et al., 2001). Therefore, individual
LGN arbors from one eye must retract a portion of
their terminals during development or there is a selec-
tive elimination of entire geniculate axons. The
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