[Intrication of smooth and striated muscle during the development of the ano-rectal sphincter]

The development of the foetal anorectal continence is related to the appearance of the anatomical components of the sphincter. The present study has been carried out in order to analyse the development of the smooth and striated components of the anorectal sphincter, in a series of 7 embryo and foetuses aged from 10 to 40 weeks of pregnancy. Our results indicate that the external sphincter, the puborectalis muscle and the internal sphincter are present before 24 weeks, although they obviously play variable roles in the establishment of the foetal continence. The internal sphincter becomes quantitatively important after 14 weeks, and is likely to be responsible for the establishment of the initial continence. The growth of the striated components during the foetal period corresponds to the maturation of the innervation and of the voluntary mechanisms controlling continence. Intrication of smooth and striated muscle components in the external sphincter starts after the end of the embryonic period. All the anatomical components of the anorectal sphincter are present at birth.