The patterns of innervation and movements of ectopic hindlimb supplied by brachial spinal cord segments in the chick

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Abstract
Pools of motoneurons in the ventral horn of the brachial spinal cord segments 13–16, innervating muscles of an ectopic hindlimb were mapped in chicks with the use of retrograde axoplasmic transport of horseradish peroxidase (HRP). On the third day of incubation the right wing bud of the acceptor embryo was removed and replaced by either a right or left hindlimb bud of the donor embryo. HRP was injected tnto selected muscles of the ectopic and intact hindlimbs as well as into muscles of the intact left wing between 17–21 days of incubation and after hatching. The position of labelled motoneuron pools representing individual muscles was determined. In 19–21 day old embryos and in hatched chicks the movement of the ectopic limbs was also studied. HRP histochemistry showed that the ectopic limb muscles of dorsal and ventral embryonic muscle mass origin were innervated by motoneurons located in the lateral and medial parts of the ventral horn, respectively. This is similar to the innervation pattern of normal wing and hindlimb muscles. Depite the ordered motor nerve supply to the ectopic limbs they failed to display any co-ordinated movements characteristic of a normal hindlimb. These observations indicate specific target affinities between pools of motoneurons in the spinal cord segments and appropriate limb muscles, even when the motoneuron pools and the limb are inappropriately matched. Nevertheless, it is evident that the ‘wing-like’ motor activation pattern of the brachial spinal cord segments is unable to initiate coordinated movements of the ectopic hindlimb.