Localization of motor neuron pools supplying identified muscles in normal and supernumerary legs of chick embryo.

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Abstract
Neuromuscular specificity was investigated in chick embryos with a grafted supernumerary leg. The nerves of the lumbar plexus were divided between the 2 legs so that rostral nerves innervated the grafted leg and the caudal nerves supplied the host''s original leg. The basic topographic organization of the histologically definable motor neuron clusters of the lateral motor columns remained unchanged by the addition of a supernumerary leg. Intramuscular injections of identified leg muscles were used to map the intraspinal location of specific motor pools in stage 38 (12-day) embryos. In the normal embryo, the gastrocnemius muscle was innervated by neurons in a central dorsal cluster of motor neurons in segments 26-29. In 6 experimental cases, the motor neurons supplying the gastrocnemius muscle of a rostrally placed grafted leg were consistently located in a specific medial cluster of neurons in segments 23-25. Motor neurons in this location never normally innervate a gastrocnemius muscle, even in the very young embryos during the period of naturally occurring cell death. This observation of a systematic mismatch between a particular motor cluster and an abnormally innervated muscle indicated the operation of a selective developmental process. A hierarchy of selective chemoaffinities may best explain the experimental results.