Postnatal development of masticatory organs in rats. I. consecutive changes in histochemical properties and diameter of muscle fibers of the M. masseter superficialis.

At birth, the diameter of muscle fibers in the M. masseter superficialis of rats has a narrow range. The diameters begin to increase rapidly from day 10 after birth, and on day 30 the diameters range between about 25 and 55 micrometers, and can be distinguished into 3 groups. SDH activity also increases in some fibers from day 10 and on day 15, 2 types of fibers can be distinguished on the basis of SDH activity, fibers of larger diameter having high activity. From day 30 to 60, 3 types of fibers can be distinguished on the basis of diameter and SDH activity. During this period, the diameter of fibers with the lowest SDH activity increases rapidly, the diameter of fibers with the highest SDH activity decrease transiently, and the diameter of fibers with intermediate SDH activity increases slightly. After day 60, the diameter of fibers with the highest SDH activity begins to increase again. Some fibers show high EDTA-insensitive ATPase activity for the first 15 days. This activity begins to decrease from day 15 after birth, and after day 25 it is no longer detectable in any fibers. No relation between SDH activity and EDTA-insensitive ATPase activity was found in serial sections of fibers. These results indicate that histochemical differences between muscle fibers of the M. masseter superficialis first appear during the late suckling period and develop gradually over a comparatively long period after weaning during use of the muscle for mastication.