Interspecies selective motoneuron projection patterns in chick-quail chimeras.

During normal development chick motoneurons have been shown to project selectively to appropriate muscles by responding to a series of cues, both specific and nonspecific, within the limb. We tested the ability of motoneurons from another avian species, the Japanese quail, to respond to these cues by transplanting chick limb buds onto quail embryos and quail limb buds onto chick embryos between stages 17 1/2 and 19. Feulgen staining, which distinguishes chick from quail cells on the basis of nuclear chromatin, revealed that all limb tissue, including muscle, was of donor origin, indicating that the migration of somite-derived muscle precursor cells had been completed by the time of transplantation. Normal quail motoneuron pools for most muscles were located in the same relative positions as homologous chick pools. In chick-quail chimeras we found that the motoneuron pools of one species selectively innervated the homologous muscles in the limb of opposite species with considerable precision. This was determined by defining the segmental innervation pattern of the muscles electrophysiologically and by retrogradely labeling motoneuron pools with HRP. Selective innervation was confirmed by using the functional activation patterns of the motoneuron pools as an additional means of identifying motoneurons. We conclude that any limb-derived cues required by motoneurons to project to their appropriate muscles must be similar in chick and quail and that the growth cones of both species must have similar detector systems for responding to these cues. Only 7 spinal segments were found to innervate the quail limb (versus 8 for the chick), resulting in an anterior shift in the spinal segments innervating several posterior quail muscles.(ABSTRACT TRUNCATED AT 250 WORDS)