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.

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.