Testosterone-induced development of the rat levator ani muscle.

The perinatal development of the levator ani (LA) muscle in male and female rats was investigated by measuring the total number of muscle units (MU) (i.e., mononucleate cells, clustered or independent myotubes, and muscle fibers) in transverse semithin sections of the entire muscle and the MU cross-sectional area in 22-day-old fetuses (F22), 1-day-old (D1 = day of birth), 3-day-old (D3), and 6-day-old (D6) newborns. Male muscle contained 350 +/- 64 MU on F22, twice that of the female. The number of MU increased markedly in males from F22, but changed little in females; the number of MU in males was 760% that of females on D6. The MU cross-sectional area was greater in males on F22 (120.8 micron(s)2 +/- 7.5) and D1 (155.2 micron(s)2 +/- 64.8) than in females (F22: 89.2 micron(s) +/- 14.2, D1: 64.1 micron(s)2 +/- 19.7) and dropped to about 30 X micron(s)2 in both sexes on D6. Female rats given a single injection of testosterone propionate (TP) before D7 showed a significant increase in the number of fibers, but no increase in cross-sectional area. TP given after D7 had no effect on the fiber number, but increased the average cross-sectional area. The increase in fiber number induced by postnatal TP treatment was a permanent effect, still quantifiable in 15-month-old females. We conclude that the sexual dimorphism of the rat LA muscle is principally due to a dramatic increase in the MU number in male muscles during the perinatal period, rather than to involution of the fibers in female muscles as it is widely accepted. This increase seems to be, at least partly, under the control of testosterone.