Structural and physiological changes associated with the growth of avian fast and slow muscle.

Abstract

AbstractElectron microscope examination of the myofibrillar material in the avian latissimus dorsi anterior (tonic) and posterior (phasic) muscles revealed that the characteristic felderstruktur arrangement of the tonic muscle fibers develops during growth. Fibers of embryonic and young latissimus dorsi anterior muscles up to 87 days after hatching exhibited a fibrillenstruktur arrangement. Unlike the phasic muscle fibers in which the myofibril mass splits into discrete and regularly shaped myofibrils, the myofibrils in the tonic muscle become felderstruktur in appearance because of incomplete splitting of the myofibrils. The incomplete splitting of the myofibrils and the less extensive sarcoplasmic reticulum in the tonic muscle were related to its slower rate of tension development. The isometric contractile tension of the muscles was measured and it was found to increase considerably during growth. The tension increase was directly related to the increase in the fiber size, and the myofibril content. The rate of contraction of both the latissimus dorsi anterior and posterior muscles was found to increase very rapidly just prior to hatching. After hatching, the rate of contraction of the anterior muscle decreases differentiating into a tonic muscle at one month. The latissimus dorsi posterior muscle remains fast throughout development.