The postnatal development of the inferior oblique muscle of the cat

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Abstract
Transversal sections of the inferior oblique muscle from kittens of various ages and from adult cats were stained for myofibrillar ATPase at pH 9.4 and 4.35, succinic dehydrogenase (SDH) and fat. The same muscles had been previously submitted to studies of contractional and fatigue properties. With ATPase the fibers could be differentiated into types I, II and II C. The percentages of each fiber type remained approximately the same from birth onwards. In the inner, global layer of the muscle, type I fibers showed a linear increase in size, but types II and II C fibers showed an accelerated growth after age 20 wk. In the outer, orbital layer, where no type I fibers were seen, types II and II C fibers showed the same growth pattern as in the global layer. The fiber content of SHD and fat was low at birth but increased after 2 wk of age. Type I fibers were poor and type II C fibers rich in SDH and fat. Type II fibers showed varying amounts of these substances. Provided that type I fibers are slow, type II fast and type II C intermediate in speed of contraction, as in other muscles, the findings on fiber growth and SDH content apparently support the idea that slow, fatigue resistant components in eye muscles reach maturity earlier than fast components.