Longitudinal growth and rate of new sarcomere formation in porcine muscle.

Abstract

Fascicular length and muscle length were measured in sartorius and peroneus longus muscles of 34 pigs from 10 to 304 days postnatal. Mean sarcomere length was measured from myofibrillar fragments examined in polarized light. Fascicular and muscle lengths were adjusted to a resting length proportional to a sarcomere length of 2.5 μm in order to compensate for differences in degree of contraction occurring during sampling. The two muscles examined were known to differ in their fascicular structure with the sartorius but not the peroneus longus having intrafascicularly terminating myofibers. In both muscles the measured fasciculi were approximately parallel to the long axis of the muscle. Muscle weights and animal live weights were considered as cubic roots in order to enable a linear regression of length against weight. Longitudinal growth: of whole sartorius (r = .90) and peroneus longus (r = .92) muscles was closely related to live weight. Longitudinal growth of fasciculi was less closely related (r = .85 and r = .80, respectively). This difference was considered to be idue partly to variation in fascicular arrangement within muscles. One example of this was identified in the sartorius where the fascicular/ whole muscle length fraction was proportional to the degree of contraction as judged from sarcomere length. The correlation between whole muscle length and muscle weight was higher in the sartorius (r = .91) than in the peroneus longus (r = .82). The correlation between fascicular length and muscle weight was also higher in the sartorius than in the peroneus longus (r = .87 and r = .44, respectively). Despite these differences and a relatively low correlation between the weights of the two muscles (r = .65), fascicular lengths in the two muscles were correlated, r = .81. It was concluded that the absence of intrafascicularly terminating myofibers was not linked to an early postnatal cessation of longitudinal growth similar to that occurring in the gastrocnemius muscle of the rabbit. In the peroneus longus muscle, fascicular length was considered to be equal to myofiber length because of the absence of intrafascicularly terminating myofibers. The original fascicular length (= myofiber length) was divided by the mean sarcomere length to estimate the number of sarcomeres along the myofiber length. From the increase in number of sarcomeres in progressively older animals it was calculated that a new sarcomere was added to the myofiber length approximately every 20 minutes.