Embryologically, the prostate, seminal vesicles, and ductus (vas) deferens originate from 2 separate structures. The prostate arises from a budding collection of tissue in the urogenital sinus. The seminal vesicles and the ductus deferens are formed from the mesonephric duct.
The prostate develops from epithelial outgrowths form the prostatic segment of the urethra that grows into the surrounding mesenchyme. This outgrowth and branching start at week 10 during embryo growth; by week 12, there are 5 groups of tubules that form the lobes of the prostate. The first group makes up the middle lobe; the second and third groups make up the right and left lateral lobes; the fourth group is the posterior lobe that starts from the floor of the urethra; and the fifth group is the anterior lobe.
The following image depicts the posterior view of the prostate gland.
A normal prostate gland is approximately 20 g in volume, 3 cm in length, 4 cm wide, and 2 cm in depth. As men get older, the prostate gland is variable in size secondary to benign prostatic hyperplasia. The gland is located posterior to the pubic symphysis, superior to the perineal membrane, inferior to the bladder, and anterior to the rectum (see the image below). The base of the prostate is in continuity with the bladder and the prostate ends at the apex before becoming the striated external urethral sphincter. The sphincter is a vertically oriented tubular sheath that surrounds the membranous urethra and prostate.
The prostate is enclosed by a capsule composed of collagen, elastin and large amounts of smooth muscle. The prostate is covered by 3 distinct layers of fascia on the anterior, lateral, and posterior aspects. The anterior and anterolateral fascia is in direct continuity of the true capsule; this is the location of the deep dorsal vein of the penis and its tributaries. Laterally, the fascia fuses with the levator fascia. The outer longitudinal fibers of the detrusor muscle fuse and blend with the fibromuscular tissue of the capsule. The posterior aspect is covered by the rectovesical (Denonvilliers) fascia.
The rectovesical fascia is a connective tissue that is located between the anterior wall of the rectum and posterior aspect of the prostate. This fascial layer covers the prostate and seminal vesicles posteriorly and extends caudally to terminate as a fibrous plate just below the urethra at the level of the external urethral sphincter. This is described as a median fibrous raphe which has a distal extension to the level of the central tendon of the perineum.
The gland is supported anteriorly by the puboprostatic ligaments and inferiorly by the external urethral sphincter and perineal membrane. The puboprostatic ligaments are actually pubovesical ligaments; however, with the growth of the prostate from puberty, these ligaments have the appearance of terminating into the prostate.
The prostate is surrounded by the puborectal portion of the levator ani. The seminal vesicles lie superior to the prostate under the base of the bladder and are approximately 6 cm in length. Each seminal vesicle joins its corresponding ductus deferens to form the ejaculatory duct before entering the prostate.
Historically, the prostate has been divided into 3 zones: (1) transition zone, (2) central zone, and (3) peripheral zone. The transition zone accounts for 10% of the prostatic glandular tissue and 20% of the adenocarcinomas. The prostate consists of approximately 70% glandular tissue and 30% fibromuscular stroma.
The prostatic urethra courses the length of the prostate from the level of the bladder neck to the level of the membranous urethra. The epithelium consists of transitional cells similar to bladder epithelium. This transitional zone is where benign prostatic hyperplasia occurs and can lead to bladder outlet obstruction when an adenoma grows to a significant size. When the adenoma grows large enough, it can compress the fibromuscular band surrounding this zone, creating a surgical capsule.
The transitional zone is often described as having 2 lateral lobes and a median lobe that lead to the symptoms of the lower urinary tract symptoms. A urethral crest runs along the posterior midline and disappears at the membranous urethra. On both sides of the urethral crest, there is a grove where the prostatic sinuses exist and drain all of the glandular elements.
The urethral crest widens and protrudes from the posterior wall as the seminal colliculus (verumontanum). A small midline pit, the prostatic utricle, is found at the apex of the seminal colliculus. On either side of the utricular orifice, the small slitlike openings to the ejaculatory duct can be found.
The central zone is the area surrounding the ejaculatory ducts. This zone consists of 25% of the glandular tissue. Very few adenocarcinomas are found in this region and can represent as little as 1-5% of these tumors in the prostate.
The peripheral zone of the prostate constitutes 70% of the glandular tissue. This zone covers the posterior and lateral aspects of the prostate. The peripheral zone is the area that is palpated on digital rectal examination (DRE) and represents the area where 70% of adenocarcinomas are found. This area is also the location most commonly affected by chronic prostatitis.
The arterial supply to the prostate is primarily from the inferior vesical artery, which originates from the anterior division of the internal iliac (hypogastric) artery. The inferior vesical artery then branches into 2 main arterial branches to feed the prostate. The prostatic vessels and the autonomic innervations run between the layers of the lateral prostatic fascia and the prostate. The inferior vesical artery supplies the base of the bladder, the distal ureters, and the prostate.
The first arterial branch is the urethral artery that enters the prostatovesical junction posterolaterally and travels inward perpendicular to the urethra toward the bladder neck at approximately the 5 o'clock and 7 o'clock meridian. The urethral artery then turns caudally and parallel to the urethra to supply the transition zone. This artery is the main arterial supply for the adenomas in benign prostatic hyperplasia.
The capsular artery is the second main branch of the prostate. It runs posterolateral to the prostate with the cavernous nerves. This artery enters the prostate at right angles to supply the glandular tissue.
The arterial blood supply to the seminal vessels and ductus deferens comes from the deferential artery or artery of the ductus, a branch from the superior vesical artery.
The venous drainage of the prostate starts with the deep dorsal vein, which leaves the penis under the deep penile (Buck) fascia between the corpora cavernosa and then under the pubic arch. This vein then passes anterosuperior to the perineal membrane and divides into 3 major branches, the superficial branch and the right and left branches.
The superficial branch travels between the puboprostatic ligaments and lies on top of the prostate and bladder neck. The superficial branch is outside the anterior prostatic fascia in the retropubic fat and pierces the fascia to drain into the dorsal venous complex. The common trunk of the dorsal venous complex and the lateral venous plexuses are covered by the anterior prostatic fascia and the endopelvic fascia. The lateral plexuses travel posterolaterally and communicate with the pudendal, obturator, and vesical plexus. These veins then communicate with the internal iliac vein.
Neural and Lymphatic Anatomy
The autonomic innervations of the prostate arise from the pelvic plexuses formed by the parasympathetic, visceral, efferent, and preganglionic fibers that arise from the sacral levels(S2-S4) and the sympathetic fibers from the thoracolumbar levels (L1-L2). The pelvic plexus is located beside the rectum approximately 7 cm from the anal verge, with its midpoint located at the level of the tips of the seminal vesicles.
The sympathetic and parasympathetic fibers that come from the pelvic plexuses travel to the prostate via the cavernous nerves. The cavernous nerves run posterolateral to the prostate in the lateral prostatic fascia. The parasympathetic nerves end at the acini and lead to prostatic secretion. The sympathetic nerves lead to contraction of the smooth muscle of the capsule and the stroma.
The pudendal nerve is the major nerve supply leading to somatic innervations of the striated sphincter and the levator ani. The preprostatic sphincter and the vesicle neck or internal sphincter is under alpha-adrenergic control.
The lymphatic drainage of the prostate primarily drains to the obturator and the internal iliac lymphatic channels. There is also lymphatic communication with the external iliac, presacral, and the para-aortic lymph nodes.
Role in Continence
The mechanisms controlling urinary continence require several coordinated events. The first mechanism is an intact bladder neck with functional internal sphincter. Proximal to the bulbar urethra and distal to the seminal colliculus, the urethral wall is composed of smooth muscle and elastic tissue along with the external urethral sphincter.
The third component of continence is achieved by the levator ani. The major contributing muscles are the puborectalis complex. This complex forms the urogenital hiatus that flanks the sphincteric urethra. The active continence is controlled by the voluntary contraction of the levator ani musculature which surrounds the apex of the prostate and membranous urethra and the external urethral sphincter. The external urethral sphincter is a cylindrical structure that extends from the external surface of the prostate to the perineal membrane and consists of predominantly slow-twitch striated muscle. The internal sphincter is an extension of the circular smooth muscle of the vesicle neck that ends at the seminal colliculus.