Abstract
A morphometric analysis of the ano-rectal sphincter was carried out in 45 human fetuses (8-40 weeks), completed by immunocytochemical and biochemical studies of different markers of muscle differentiation (vimentin, desmin, actin, isoforms of the myosin heavy chain (MHC), titin). The external sphincter (ES) and the m. pubo-rectalis are determined at the earliest stages and contain striated fibers after 10 weeks. After 20 weeks, the internal sphincter (IS) becomes well determined and the ES is colonized by smooth muscle from the longitudinal layer of the rectum, which represents up to one third of the total muscle. The smooth muscle components express actin, vimentin and sometimes desmin. The striated muscle fibers first express vimentin, desmin, titin and four different isoforms of the MHC - with a very immature pattern of reactivity up until 25 weeks when compared to other striated muscles at the same ages. The maturation is rapid afterwards and almost achieved at 40 weeks : the fibers then express desmin, titin and the slow or the fast MHC. Conclusions : 1/ the development of the ano-rectal sphincter is characterized by an intrication of smooth and striated muscle, 2/ the ES is partially composed of smooth fibers, 3/ the study of muscle differentiation markers is a new approach to digestive motor function in childhood.
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