[Morphological differentiation of the anorectal sphincter in the human embryo and fetus].

In order to obtain a better understanding of the anorectal continence in the newborn child, the development of the anorectal sphincter was studied in 25 embryos (3 to 60 mm CR), sectioned in the sagittal, transversal and frontal planes. A macroscopic study was also performed in 20 human embryos, completed by a light microscopic study and by a morphometric analysis of the smooth and striated components of the sphincter. The anorectal sphincter is composed of three parts: the external sphincter, which appears in the early embryos as a condensation of mesenchymal tissue around the anorectum. It receives smooth longitudinal fibers later; the m. puborectalis appears before 10 weeks and forms a sling around the anorectum; the internal sphincter, which is only well differentiated at 10 weeks and grows after the rupture of the anal membrane. This study emphasizes the importance of the anorectum in the development of its own sphincter.