Regarding the urethra, which of the following is TRUE?
A. Transitional cell epithelium lines the length of the urethraAnswer E
The urethra extends from the bladder neck to the external meatus. It is longer in men (up to 20 cm) than in women (4 cm). In men, it is broadly divided into the anterior and posterior urethra. The latter is further subdivided into prostatic and membranous while anteriorly it is bulbar and pendulous. There are bends at the junctions between the membranous and bulbar and the bulbar and pendulous urethra. The membranous urethra is short and narrow as it passes through the urogenital diaphragm. It is surrounded by the striated external urethral sphincter whose inner smooth muscle is continuous with that of the proximal sphincter at the bladder neck. The outer muscle extends from the bladder neck and anterior prostate. The sphincter has a horseshoe configuration on cross section due to a broad anterior and a deficient posterior surface. Some of the fibres also attach to the perineal body causing the urethra to be pulled backwards when the sphincter contracts. The sphincter is also suspended from the pubic bones through attachments to the puboprostatic and suspensory ligament of the penis. Neural supply is from the somatic pudendal nerve and an autonomic branch from the sacral plexus. The anterior urethra is longer (15 cm), beginning at the level of the perineal membrane and transversing the whole length of the spongiosum to end at the narrow external meatus. It is dilated at the bulbar urethra and fossa navicularis in the glans. Mucus is secreted into the urethra by bulbourethral glands in the base of the external urethral sphincter and the glands of Littre in the submucosa. Proximally, the anterior urethra is lined by columnar epithelium but distally within the glans penis it is lined by stratified squamous epithelium. In contrast, the prostatic urethra is lined by transitional cell epithelium. The female urethra opens into the vestibule at the external meatus. On either side of the meatus are two ducts that drain mucus from the paraurethral (Skene) glands. It is surrounded by a striated external urethral sphincter. At the proximal and distal ends, the slow twitch muscle fibres completely surround the urethra whilst the fibres in the middle do not meet posteriorly and instead attach to the vagina. The urethra is pulled against the vagina when the sphincter contracts. The urethra is suspended beneath the pubic bone by the suspensory ligament of clitoris and the pubourethral ligament. It is lined proximally by transitional cell epithelium, which becomes stratified squamous epithelium distally. Somatic nerve innervation is similar to that in the male.