Muscle fibre types and innervation of the chick embryo limb following cervical spinal cord removal.

Several segments of spinal cord were removed from the cervical regions of stage-13 or -14 (day-2) chick embryos. After further incubation to day 17 or 18, the patterns of end-plate distribution and ATPase typing of muscle fibres in the anterior and posterior latissimus dorsi and the ulnimetacarpalis dorsalis, and the ATPase typing of the forearm muscles were examined. No differences from control embryos were found. The embryos had normal numbers of lateral motor column motoneurons in both the brachial and lumbar enlargements and the positions of motoneurons supplying the biceps as identified with retrograde horseradish peroxidase labelling were consistent with the normal patterns of motor projection into the limb. These results show that the fibre typing of limb muscles and their patterns of innervation are independent of descending inputs until just before hatching in the chick.