Developmental changes in hindlimb muscles and diaphragm of sheep.

In this study, plasma thyroxine, contractile and histochemical (adenosinetriphosphatase and NADH) characteristics of soleus (SOL), medial gastrocnemius (MG), and extensor digitorum longus (EDL) were examined in 140-day-gestation fetal sheep and in 2-, 5-, and 30-day-old lambs and adult ewes. Electrophoretic separation of myosin heavy chains was also done on all muscles and the diaphragm. There were no differences in the twitch contraction and relaxation times of MG and EDL at the different ages; in contrast SOL contraction times were significantly shorter in the fetus and newborn than in the adult. Fast glycolytic fibers first appeared in EDL, MG, and diaphragm at 5, 30, and 5 days after birth, respectively. The proportion of slow oxidative fibers decreased after birth and with postnatal development in EDL, whereas they increased in MG and diaphragm. Plasma thyroxine concentrations were higher in the fetus and day-old lambs than in 2-, 5-, and 30-day-old lambs or adult sheep. It is suggested that contractile specialization of the fast-twitch diaphragm, MG, and EDL is largely achieved in utero and is probably mediated by thyroid hormone. In contrast, SOL changed postnatally, probably influenced by the altered neural drive.