The morphogenesis of human sphincter urethrae muscle.

The morphogenesis of the sphincter urethrae muscle was studied in human ontogeny. Muscles of 65 embryos and fetuses, 7 newborns, 3 children and 3 adults of both sexes were examined histologically and by means of microdissection. Three developmental stages can be recognized in terms of morphogenetic events, histogenesis and development of sexual dimorphism. In the sexually indifferent stage (up to approximately 50 mm crown-rump length), the sphincter urethrae primordium is formed by a shallow arch apposed only to the ventrolateral wall of the urethra. The primordium extends from the level of the urogenital diaphragm up to the vesicourethral transition. It consists of a condensation of mononuclear cells. Myotubes appear in fetuses of 30 mm crown-rump length. During the second stage (until birth) sexual dimorphism develops in conjunction to the formation of the prostate and vagina. In this stage, the sphincter urethrae muscle fibres gradually extend to the posterior wall of the urethra. At the same time cranially situated muscle fibres project to the lateral wall of the prostate, whereas in females caudally located muscle fibres attach to the lateral wall of the vagina. In this way the sphincter achieves the sexually distinct form. The definitive arrangement develops in the third morphogenic stage (after birth), in which a complete muscle ring is formed by encircling the urethra in the infraprostatic part in males and in the upper, larger part of the sphincter in females. The sphincter urethrae muscle is located inside the sling of the puborectalis muscle in both sexes, but no muscle fibres connect them to one another.(ABSTRACT TRUNCATED AT 250 WORDS)