The mammalian myotome: a muscle with no innervation.

The segmented muscular myotome is the first muscle to form in all vertebrates. In fish and amphibian embryos, the myotome becomes innervated very early and is essential for larval swimming. Its role in birds and mammals, however, is not clear. Using immunohistochemistry on sections and whole mounts of rat embryos, we demonstrate that the mammalian myotome differentiates and develops over a period of 3 days without being invaded by the outgrowing spinal nerves. In contrast, the limb muscle masses become filled with fine nerve branches from the first time that myocyte differentiation can be detected. Additionally, we show that the mammalian myotome does not express clustered acetylcholine receptors until after embryonic day 13.5, which corresponds to the beginning of its transformation into the adult epaxial muscles, showing that there are no functional myotomal neuromuscular junctions before this age. We suggest that the mammalian myotome has entirely lost the function of neurally controlled muscular contraction: its remaining functions are likely to be as a signaling tissue, as a structural scaffold, and as an incubator for myogenic precursors of the deep back, abdominal, and intercostal muscles.