The influence of presumptive limb connective tissue on motoneuron axon guidance.

During embryogenesis in the chick, the lumbosacral (LS) somatopleure gives rise to the connective tissue and the epidermis of the limb. We wished to determine if the LS somatopleure was a primary source of guidance cues for motoneuron pathway choices along the anteroposterior axis of the limb. At stage (st) 15, prior to its population by muscle cell precursors and the neural crest, the LS somatopleure was shifted anteriorly. This surgery resulted in the development of limbs that were shifted one to four segments into the thoracic region. Muscles within the anterior thigh of the shifted limb were normally patterned and of composite origin: connective tissues were of LS origin, while muscle cells were of LS and thoracic origin. Retrograde HRP labeling at st 35-37 indicated that motoneuron pools to these anterior thigh muscles were located within LS rather than thoracic cord segments. Pools to individual muscles were smaller than normal but occupied segmental and transverse positions in the LS cord that generally matched those of normal embryos. These findings suggest that individual muscles within somatopleure-shifted limbs are innervated specifically and are in accord with their connective tissue (and epidermal) level of origin. Reconstructions of nerve patterns at st 28-31 suggested that LS motoneurons corrected for the shift by altering their pathways at midthigh regions. We conclude that the somatopleure, and most likely its connective tissue component, contains the information for setting up a specific axon guidance system in the developing limb.