Histology of the connection between the vagina and levator ani muscles. Implications for urinary tract function.

The proximal urethra is a mobile structure, and voluntary control of its position is an integral part of the initiation of urination and continence. This paper describes the histology of the vagina's attachment to the medial portion of the levator ani muscles, which, because of the intimate attachment of the vagina and urethra, is responsible, in part, for control of the urethral position. A histologic examination of 1,500 serial microscopic slides from eight women, dissection of four bodies and study of whole pelvis cross-sections from two cadavers were performed. Smooth muscle, collagen and elastin fibers of the vaginal wall and paraurethral tissues directly interdigitate with the muscle fibers of the most medial portion of the levator ani, in the region of the proximal urethra. This strong connection lies at a level just below the entry of the urethra into the bladder and extends downward to the level of the perineal membrane (urogenital diaphragm) in an area corresponding to the mobile portion of the urethra. The inseparable nature of the vagina and urethra in this region makes it possible for the connection of the levator ani to the vagina to control the proximal urethral position. These observations suggest a specific role of the medial levator ani muscle in controlling vesical neck position and open the question of the specific part played by this arrangement in voiding and continence relative to other factors known to influence lower urinary tract function.