Development of human fetal muscles: a comparative histochemical analysis of the psoas and the quadriceps muscles

The development of human muscle can be divided into three distinct phases on the basis of the enzyme histochemistry (Dubowitz 1965). On the other hand, muscles can be classified into trunk muscles innervated by the motoneurons in the anteromedial nucleus of the anterior horn and limb muscles innervated by those of the lateral nucleus. We compared histochemically the development of the psoas muscle as a trunk muscle with the quadriceps. Four legally aborted fetuses and sixteen autopsied preterm or full-term infants with conceptional ages ranging from sixteen to fourty-two weeks, served as subjects. Serial crossly cut frozen sections of the psoas and the quadriceps were stained with HE, modified Gomori-trichrome, NADH-TR, myofibrillar ATPase (routine, preincubation at pH 4.6, 4.3), and analyzed quantitatively. In the psoas and the quadriceps, all muscles consisted of undifferentiated type IIC fibers before the eighteenth week of gestation. Type I fiber began to appear around the eighteenth week in both muscles. Before the thirtieth week, both muscles comprised a great number of smaller type IIC fibers and scattered large type I fibers. The psoas had already a little more type I fibers than the quadriceps in this phase. After the thirtieth week, the percentage of type I fibers increased greatly to about 50% at term in the psoas, gradually to 20-23% at term in the quadriceps. Type IIA and IIB fibers appeared suddenly around the thirty-seventh week. Total percentage of these two type fibers increased at term to about 50% and 70% in the psoas and the quadriceps, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)