Journal of Neuroscience, Vol 1, 141-151, Copyright © 1981 by Society for Neuroscience
Cell death of motoneurons in the chick embryo spinal cord. V. Evidence on the role of cell death and neuromuscular function in the formation of specific peripheral connections
RW Oppenheim
Previous reports from this laboratory have shown that the chronic treatment
of chick embryos with neuromuscular blocking agents (e.g., curare,
alpha-bungarotoxin), during the period of naturally occurring cell death of
spinal motoneurons (days 5 to 10), greatly reduces the amount of cell death
in this system. The surviving motoneurons continue differentiation and
innervate the peripheral musculature. Since cell death has been prevented
in these preparations from the earliest stages of limb innervation on day 4
or 5, it was expected that any inappropriate synaptic connections present
at that time, or formed later, would be retained as long as the cell and
their axons were prevented from regressing. To test this possibility, small
injections of horseradish peroxidase were made into specific leg and wing
muscles on embryonic day 10 in order to label retrogradely motoneuron pools
in the spinal cord. The location of labeled motoneurons was found to be the
same in control and experimental embryos. The specific muscles examined
included the gastrocnemius, peroneus, adductor, and sartorius in the leg
and the biceps, triceps, extensor metacarpi radialis, and flexor carpi
ulnaris in the wing. In virtually all cases, there was a greater number of
labeled motoneurons in the experimental cases. Despite this difference, the
location of motoneuron pools in the rostral-caudal and transverse planes
were remarkably similar in control and experimental embryos. Thus, natural
cell death in this system is not primarily designed to remove errors in
synaptic connectivity. Since a normal pattern of neuromuscular connections
was formed in the virtual absence of functional synaptic interactions
between motoneurons and their targets, these data do not support the
contention that function is involved in the developmental specificity of
peripheral connectivity.