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2
2
4
6
4
8
6
10
8
12
10
14
Less cell death
in limb DRG
12
16
18
14
20
16
limb buds
22
18
20
22
24
26
28
34
24
34
30
26
32
32
28
30
Day 4.5
Day 7
FIGURE 7.7 The pattern of naturally occurring cell death in chick dorsal root ganglia. There is less cell
death among neurons in the DRGs that innervate limb buds (blue filled), compared to those that innervate
the axial musculature (unfilled). The numbers refer to the somitic segment of the spinal cord. Thus, the size
of DRGs at segments 14-16 decrease only slightly during development, whereas DRGs at segments 18-22
become much smaller. This result shows that neuron survival is correlated with the amount of target tissue.
(Adapted from Hamburger and Levi-Montalcini, 1949)
A
B
6000
NM
5000
Auditory
nerve
NM
4000
NL
3000
Cochlea
NL
2000
0
9
11
13
Age (days)
15
17
19
FIGURE 7.8 The period of cell death in pre- and postsynaptic nuclei. A. A transverse hemisection through
the chick auditory brainstem. The nucleus magnocellularis (NM) is a central auditory nucleus that is inner-
vated by auditory nerve terminals. It projects to a second-order nucleus, called nucleus laminaris (NL). B.
The graph shows the total number of cell bodies in NM and NL during the latter period of embryonic devel-
opment. In both nuclei, about 20% of the neurons are lost between embryonic day 9 and 17. (Adapted from
Rubel et al., 1976 and Solum et al., 1997)
the spinal cord becomes smaller (Figure 7.9). When an
extra limb bud is transplanted next to the original one,
providing a larger than normal target region, devel-
oping processes grow into the added target, and the
population of motor and DRG neurons is found to be
much larger than normal. The addition of an extra limb
bud saves up to 25% of the motor neurons that would
otherwise die. Experimental alteration in the size of the
periphery led to the hypothesis that the target pro-
vides a survival factor. When targets are eliminated,
there are fewer neurons to be found in the adult, and
when targets are enlarged neurons are found in greater
 
 
 
 
 
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