Postnatal development of limb motor innervation in the opossum Monodelphis domestica: immunohistochemical localization of acetylcholine.

The development of limb motor innervation was studied in the opossum Monodelphis domestica, a marsupial born with immature mobile forelimbs and immobile hindlimbs. Choline acetyltransferase (ChAT), the synthesis enzyme of acetylcholine, was evidenced on sections of the spinal enlargements, and the protein that transports acetylcholine (VAChT) on limb sections. In newborn, ChAT immunolabeling occurred in small, undifferentiated neurons of the ventral horn, presumably motoneurons, and intermediate and dorsal gray matter, and in the presumptive white matter, all less abundant at lumbosacral than brachial levels. Scant immunolabeling for VAChT marked small terminal-looking profiles, presumably growth cones or immature neuromuscular junctions, decreasing proximodistally in each limb and being less abundant in hindlimbs than forelimbs; it was absent distally in the foot where no muscle tissue was formed. ChAT labeling disappeared from the white matter within 1 week while cholinergic neurons increased in number and size. Motoneurons segregated in a medial and lateral group by 4-5 weeks. VAChT-labeled profiles increased in number and size and they flattened along a proximodistal gradient within each limb, but later in the hindlimbs than in the forelimbs. Labeling appeared in distal foot muscle at 1 week. The density, size, and shape of terminals became comparable in all segments of a given limb by 3-4 weeks. Their number and size increased, and by 8 weeks, they clustered in 3 or 4 along muscle fibers. Thus, limb motor innervation develops largely postnatally in the opossum, along rostrocaudal and proximodistal gradients. Its timecourse is compared to the development of motor behaviors.