Postnatal development of masticatory organs in rats II. Effects of hormones on the postnatal development of the M. masseter superficialis.

The effects of L-thyroxine (T4; 0.2 microgram/g body weight) an hydrocortisone (HC 5 microgram/g body weight) on the M. masseter superficialis in developing rats were investigated. Administration of T4 and/or HC to rats for 5 consecutive days from day 5 after birth induced precocious increase in diameter of the muscle fibers, increase of SDH activity and differences of the activity in various fibers and precocious disappearance of fibers with EDTA-insensitive ATPase activity. The effects of these hormones on the muscle gradually decreased after the 10th postnatal day. In thyroidectomized-adrenalectomized rats. T4 increased the diameter and SDH-activity of the muscle fibers and induced differences in SDH activity between fibers, but HC did not. Thus HC may act permissively on the SDH activity and the diameter of the muscle fibers. On the other hand, muscle fibers with EDTA-insensitive ATPase activity did not disappear on treatment with T4 or HC alone, but did disappear on treatment with T4 plus HC. These results suggest that increases in the circulating levels of T4 and HC in suckling rats are involved in development of the M. masseter superficialis.