[Dorso-ventral gradient of maturation for puborectalis muscle]

Foetal maturation of the skeletal fibers in the anorectal sphincter and in the vesico-urethral sphincter follows a dorso-ventral gradient, in parallel with the development of their motor innervation. The anatomical situation of the m. puborectalis, which early development plays an important role in the establishment of the foetal anal continence, suggests that this muscle should also be submitted to this gradient. We have studied in parallel the development of both the ventral and the dorsal parts of the m. puborectalis in human foetuses aged between 19 and 40 weeks of pregnancy. The results indicate that the transversal thickness of the muscle increases progressively during pregnancy, with a dorsal part being always thicker than the ventral one. More interestingly, the increase in the diameter of the muscle fibers becomes significantly more important at the level of the dorsal part between 25 and 40 weeks of pregnancy. This last result confirms the existence of a dorso-ventral gradient of growth and maturation of the m. puborectalis, as part of the global mechanisms involved in the development of the striated musculature of the pelvic organs.