Abstract
The sequence of events during the development of the mouse diaphragm was studied from gestational days(g. d) 9.25(late branchial arch period) through 12.5(cleft lip period), using graphic reconstrucitons or electron micrographs. We found that 1) Some portion of the diaphragm premuscle mass arose initially from the ventral edges of the 4th and 5th cervical somites at g. d.9.25 to 10.25(early limb bud period) and then it migrated ventrocaudally toward the transverse septum up to g. d.12.5.2) During its descent to the septum, the premuscle mass was laterally connected with the forelimb bud mesoderms. During this period of contact the cell population in the premuscle mass increased gradually. This suggests that in mice, myogenic precursor cells from forelimb mesoderms contribute to the development of the diaphragmatic muscle.3) Until g. d.12.5, by which time the premuscle mass had almost arrived at its adult position in the primitive diaphragm, myogenetically, the cellular components of the premuscle mass were present as spindle-shaped mononucleated cells. Primitive myotubes, containing myofibers exhibiting a faint z-band, initially appeared at g. d.12.5.
