The development of motor projection patterns in the chick hind limb.

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Abstract
Retrograde transport of horseradish peroxidase was used to map the initial projection patterns of lumbosacral motoneurons to the embryonic chick hind limb. The stage 28 segmental projection pattern to each of the 4 primary muscle masses was characteristic and indistinguishable from the stage 36 projection pattern to the sum of the muscles derived from that mass. The adductor motoneuron pool was similar in position (both rostro-caudal and medio-lateral) at stages 29, 30, 32, 33 1/2 and 36. Axons from lumbosacral motoneurons project for the most part only to appropriate regions from early times shortly after they grow into the limb bud. The attainment of the segmental projection pattern occurs prior to the normal time without the aid of cell death, as indicated by electrophysiological recordings made from muscle nerves. A regionalization of the projection patterns within a single muscle mass was shown both anatomically and physiologically prior to the cleavage of the mass into individual muscles and the projections were in a general way appropriate for the muscles derived from those regions. The process of muscle cleavage does not in itself create the specific projection patterns observed. Motoneuron axons appear to grow, ramify and make synapses only within regions which correspond to their adult muscles. The termination site of each motoneuron axon in the early limb was tightly correlated in a somatotopic fashion with the position occupied by its soma in the cord. Some feature of the motoneuron related to its position may be of importance in achieving the specific projection patterns.