Anatomy of the Urethra
LAWRENCE M. DAGROSA
E. ANN GORMLEY
The female urethra extends 3 to 5 cm from the bladder neck to the external urethral meatus. This fibromuscular tube from the lumen outward is composed of a mucosal lining, a submucosal layer, and a muscular layer. The proximal urethra is lined with transitional cell mucosa and is continuous with the bladder urothelium. Distally, the mucosa transitions to nonkeratinized stratified squamous epithelium (Fig. 36.1). The submucosa consists of abundant longitudinal and circular elastic fibers and contains a prominent venous system. The engorgement of this venous plexus decreases the luminal diameter of the urethra creating a hermetic seal, which has been reported to account for up to 33% of urethral closing pressure (1,2). Historically, it has been described that these veins are hormone-sensitive and their relative volume has been shown to decrease with age, likely adding to the age-related decrease in urethral closing pressure seen in the postmenopausal population (3). Colleselli and others (4) have shown that in the proximal third and most of the middle third of the urethra, there are three smooth muscle layers, including outer and inner longitudinal layers and a middle transverse or circular layer. These muscles are continuous with the bladder neck proximally and terminate distally in the subcutaneous tissue surrounding the external urethral meatus, intermingling with the striated muscle fibers of the rhabdosphincter. In fetal specimens, the smooth muscle layer is more defined distally, whereas this layer is thin or absent in the distal urethra of the adult (4). The density of urethral circular smooth muscle decreases with age, which may also contribute to the decrease in urethral closure pressure and incontinence associated with increasing age (5).
In the distal two-thirds of the urethra, a layer of striated muscle, the rhabdosphincter, surrounds the smooth muscle layer, on the dorsal and lateral aspects making an omegalike-shaped sphincter (4). The rhabdosphincter is composed of delicate type I (slow-twitch) fibers and consists of three distinct muscles. Proximally, the muscle forms a ring (sphincter urethrae) that encircles the urethra. Distally, the muscle (compressor urethrae) fans out laterally along the curve of the inferior border of the pubic rami to compress the urethra against the anterior vaginal wall. At the vestibule, the muscle completely surrounds the urethra and vagina to form a urethrovaginal sphincter (Fig. 36.2) (6). The three muscles work together to provide constant urethral tone (Fig. 36.3) (7). Normally, the striated sphincter plays a minimal role in resisting abdominal pressure. However, preservation of the rhabdosphincter is necessary for continence after creation of a female neobladder (4). There are numerous small periurethral mucous glands that run along the length of the urethra and open into small recesses in the mucosa. Paired periurethral (Skene) glands drain on either side of the midline just posterolateral to the urethral meatus.
The arterial supply to the female urethra is from the urethral artery, a branch of the internal pudendal artery, which in turn is a branch of the internal iliac artery (8). The venous drainage is via the pelvic venous plexus.
The smooth muscle of the urethra is innervated by parasympathetic nerves. The predominant sympathetic receptors are alpha-adrenergic (9). These receptors are responsible for urethral smooth muscle contraction and possibly engorgement of the submucosal vasculature to create a watertight seal of the urethral mucosa (10). The striated muscle fibers of the rhabdosphincter receive innervation from the pudendal and pelvic somatic nerves (11). Both somatic and autonomic nerves to the urethra travel on the lateral walls of the vagina near the urethra. Various authors have advised against dissection in this area during transvaginal surgery to prevent development of stress urinary incontinence due to intrinsic sphincter dysfunction (12).