A Histochemical Analysis of the Development of Human Fetal Muscles

Abstract

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 lateral nucleus. We compared histochemically the development of the psoas muscle as a trunk muscle with that of the quadriceps muscle as a limb muscle. Four legally abortioned fetuses and 16 autopsied preterm or full-term infants served as subjects. Type I fibers began to appear around the 18th week gestation in both muscles. Before the 30 th week, both muscles comprised a great number of smaller type II C 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 30 th week, a great number of small type I fibers increased greatly in the psoas, and gradually in the quadriceps. Type II A and II B fibers appeared suddenly around the 37 th week in both muscles. Type II C fibers decreased markedly in the last 5 weeks of pregnancy in both muscles. The histochemical difference in development in the two muscles innervated by the motoneurons in the different nuclei was emphasized.