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FIGURE 7.2 Light and electron micrographs of apoptosis in the developing cat retina. A. A low magnifi-
cation photomicrograph of the retina at embryonic day 57 contains two cells in the ventricular layer (arrows).
The retinal ganglion cell layer is at the top. B. A high-power photomicrograph shows two neurons with con-
densed chromatin in their nuclei (arrows). C. An electron micrograph shows a degenerating retinal ganglion
cell with a clearly pyknotic nucleus (pn). (Reprinted from Wong and Hughes, 1987).
count obtained in an adult. If the number is positive,
then neurons must have been born. If the number is
negative, then neurons must have died. However,
obtaining an accurate neuron count is trickier than one
might suppose. For example, if neurogenesis and cell
death overlap in time, then cell counts can remain rel-
atively stable and this will conceal the existence of cell
addition or elimination. A second difficulty revolves
around the counting strategy. Since it is often too labo-
rious to count each cell in a neuronal structure, esti-
mates are made from tissue sections, and changes in
the size or packing density of cell bodies can each
influence the final counts. Finally, neurons are not the
only type of cell to die during development. For
example, about 50% of oligodendrocytes in the rat
optic nerve die during development, and their sur-
vival depends upon the presence of retinal axons
(Barres et al., 1992). Therefore, those who tally up cell
bodies must be careful to discriminate neurons from
glia.
After it became clear that cell death was a general
feature of the developing nervous system, its magni-
tude became an issue, and a number of rigorous cell
counting studies appeared. One of the most convincing
ways to demonstrate that neurons are dying is to count
both the healthy cells and the pyknotic cells in the same
tissue section (Hughes, 1961). Counts of spinal motor
neuron (MN) cell bodies in the chick and frog demon-
strate that the decrease in number of healthy-looking
MNs is perfectly correlated with the appearance of
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