A histochemical study of myofiber types in the serratus ventralis thoracis muscle of sheep during growth.

The objective of the research described herein was to describe the profile of histochemically determined myofiber types in serratus ventralis thoracis of the sheep at various stages of postnatal development. This muscle acts to suspend the trunk and to pull the anterior limbs back during locomotion. The results obtained allow comparison with other results in the literature on muscles with different functional demands of movement and postural activity. Three sheep were sacrificed at each of the following ages: birth, 2, 4, 8, 12 and 16 wk. One 52-mo-old sheep was used. The muscle was processed histochemically for a series of enzyme activities including myosin adenosine triphosphatase (ATPase). Type I myofibers reacted strongly for acid-stable myosin ATPase and negatively for alkali-stable ATPase. Type II myofibers showed the opposite reaction pattern. Various subtypes were classified on the basis of intermediate reaction patterns and on the basis of enzyme activities other than ATPase. Type II myofibers decreased greatly in proportion from birth to 4 wk of age and were essentially unchanged during further growth of the animal. Type I myofibers increased in proportion from birth to 4 wk, increased slightly from 4 to 12 wk and then underwent little further change. Intermediate types changed little from birth to 4 wk and decreased thereafter. Type II myofibers were greater in proportion than type I and intermediate myofibers were always lowest, regardless of age of animal.