Differences in the histogenesis of EDL and diaphragm in rat

We have examined the histogenesis of the diaphragm and extensor digitorum muscle in rat embryos, with the aim of defining differences in developmental patterns that can be related to the functional requirements of these muscles during and after development. Patterns of interactions between myotubes and other cells, and frequency of gap junctions are quite different in the two muscles. In diaphragm, primary myotubes (at day 16 in utero) are closely associated with each other, forming parallel sheets or palisades and communicating by gap junctions. Secondary myotubes have formed by day 18, but are immature, and the frequency of gap junctions is lower. The arrangement in palisades is maintained even after fibers are separated from each other by their individual basal lamina. In EDL primary fibers at day 16 have fewer gap junctions, and the peak in communication occurs after the appearance of secondary myotubes (day 18 and 21). Secondary myotubes are more mature than in diaphragm at day 18.