Anatomy

The prostate is a compound tubuloalveolar gland. It is adjacent to the bladder neck proximally and merges with the membranous urethra to rest on the urogenital diaphragm distally. The intact adult gland resembles a blunted cone, weighing approx 18 to 20 g. The gland measures about 4.4 cm transversely across its base, and it is 3.4 cm in length and 2.6 cm in anteroposterior diameter (1). The urethra enters the prostate near the middle of its base and exits the gland on its anterior surface just before the apical portion. The ejaculatory ducts enter the base on its posterior aspect and run in an oblique fashion, terminating adjacent to the verumontanum. The capsule of the prostate gland is incomplete at the apex and does not represent a true capsule (2). Fibrous septa emanate

Verumontanum Prostatic Utricle

Fig. 2. This frontal view of the prostate reveals its ductal system in continuity with the bladder and the urethra. The prostatic utricle rests atop the verumon-tanum. The majority of the prostatic ducts drain distal to the verumontanum. The bladder neck (internal sphincter) is comprised of the area extending from the trigone to the termination of the prostatic urethra. (From 3, with permission.)

Fig. 2. This frontal view of the prostate reveals its ductal system in continuity with the bladder and the urethra. The prostatic utricle rests atop the verumon-tanum. The majority of the prostatic ducts drain distal to the verumontanum. The bladder neck (internal sphincter) is comprised of the area extending from the trigone to the termination of the prostatic urethra. (From 3, with permission.)

from the capsule, pierce the underlying parenchyma, and divide it into glandular units called lobules (3). Most of these units empty their contents into the prostatic urethra near the verumontanum (4). The anatomic details are illustrated in Figs. 1 and 2.

The endopelvic fascia represents the fusion of extraperitoneal connective tissue that forms a subserous covering for the pelvic viscera and envelops its neurovascular supply. A sheetlike proliferation of this fascia contributes to the formation of the puboprostatic ligaments. They anchor the anterior and lateral aspect of the prostate to the posterior aspect of the pubis (5).

The lateral pelvic fascia, also described as the parietal layer of the endopelvic or prostatic fascia, serves as the fascial envelope to the leva-

tor ani muscle and maintains continuity with the capsule of the prostate along its anterior and anterolateral aspects. Anatomic dissections by Walsh and Donker revealed that the major neurovascular bundles to the prostate were contained posterolaterally within the lateral leaves of this fascia (5).

Neurovascular Supply

The prostatovesicular artery, the major arterial supply to the prostate and seminal vesicles, is a branch of the inferior vesical artery. It originates from the anterior division of the hypogastric artery and courses medially on the levator muscle to the bladder base. The artery has tiny branches that go to the bladder base, prostate, and tip of the seminal vesicles. These urethral and capsular branches are the prostate's main arterial supply (1). The urethral branches course along the posterolateral aspect of the vesicoprostatic junction and usually enter the bladder neck and periurethral aspect of the prostate gland at the 5 and 7 o'clock positions (Fig. 3). The anterior division of the hypogastric artery also supplies the inferior aspect of the prostate, the seminal vesicles, and the vas deferens with accessory vessels from the middle hemorrhoidal and internal pudendal arteries (1,3).

Wide, thin-walled veins on the lateral and anterior aspect of the prostate gland merge with veins of the vesical plexus and the deep dorsal vein of the penis to form the plexus of Santorini within the puboprostatic space. This confluence of veins empties into the hypo-gastric vein.

In 1982, Walsh and Donker published landmark observations descr~2ing the anatomic relationship of the pelvic (autonomic)*ylexus and the prostate gland (6). The prostate, the other pelvic organs, and the corpora cavernosa receive their autonomic innervation from the pelvic plexus, a fenestrated 4-cm long, 2.5- to 3.0-cm high rectangular plate lying retroperitoneally adjacent to the rectum (7). Both the parasympathetic and sympathetic divisions of the autonomic nervous system contribute to the plexus. Parasympathetic visceral efferent preganglionic nerve fibers from the second through fourth levels of the sacral cord enter the plexus by way of the pelvic splanchnic nerve (nervi erigentes). This nerve is a composite of five or six branches rather than a discrete entity. The sympathetic component emanates from the thoracolumbar center (T11 to L2) and courses through the hypogastric nerve.

Normal Internal Architecture

The proposed organization of the fetal, newborn, and adult prostate into discrete lobes has been regarded with skepticism (8-12). With a

Prostate Blood Supply

Fig. 3. The arterial blood supply to the prostate. The prostatovesicular artery is a terminal branch of the inferior vesical branch, arborizing into urethral and capsular tributaries. The urethral branches typically enter the bladder neck at the 5- and 7-o clock positions. The anterior division of the hypogastric artery supplies the inferior vesicle, the middle rectal, and the pudendal branches to the prostate gland. (From 12, with permission.)

Fig. 3. The arterial blood supply to the prostate. The prostatovesicular artery is a terminal branch of the inferior vesical branch, arborizing into urethral and capsular tributaries. The urethral branches typically enter the bladder neck at the 5- and 7-o clock positions. The anterior division of the hypogastric artery supplies the inferior vesicle, the middle rectal, and the pudendal branches to the prostate gland. (From 12, with permission.)

focus on the development of benign prostatic hypoplasia (BPH), Franks conceptualized a prostate with an inner (urethral) and outer glandular configuration (13,14). McNeal (15) argued, as did Lowsley (8), that the urethral (inner) glands should be considered separately from the prostate and its intrinsic architecture. However, the major physiologic and biochemical similarities of these glands and those of the prostatic parenchyma weigh against this concept.

McNeal has proposed and promoted acceptance of the theory of anatomic subdivisions with probable pathophysiologic significance in the adult prostate (4,15). In his studies, McNeal emphasized the use of coronal and oblique coronal sections of prostates obtained between puberty and the third decade of life to study normal anatomy. Tisell and Salander, who used meticulous dissection techniques, observed subdivisions of the prostate gland that had several similarities to those reported by McNeal, but they interpreted these as evidence for the existence of prostatic lobes (16).

McNeal observed that the urethra separates the prostate into ventral (fibromuscular) and dorsal (glandular) portions. Approximately midway between the apex and base, the posterior wall of the urethra undergoes an acute 35° ventral angulation that segregates the urethra into proximal and distal segments. The verumontanum and ejaculatory duct orifices exist exclusively within the distal segment. McNeal separated the glandular prostate into four distinct regions: peripheral zone, central zone, transition zone, and periurethral gland region (Fig. 4).

The peripheral zone constitutes approx 75% of the glandular prostate. Its ductal system enters the urethra along the posterolateral recesses of the urethra and extends from the verumontanum distally to the pro-static apex. The wedge-shaped central zone, the base of which is positioned superiorly at the bladder neck, occupies approx 20% of the glandular prostate. Its ductal network closely follows the ejaculatory ducts to the urethra and empties adjacent to orifices of the ejaculatory ducts on the apex of the verumontanum. The transition zone, accounting for 4-5% of the adult glandular prostate, is not well defined in the prepubertal prostate (17). It consists of two modest lobules of paraurethral tissue anterior to the peripheral zone. Its ducts empty in the posterior lateral recess of the urethra just proximal to peripheral zone ducts. The transition zone is lateral to McNeal's preprostatic sphincter, a smooth muscle cylinder enveloping the proximal urethra from the bladder neck to the base of the verumontanum. The last anatomically discrete area within the glandular prostate is the periurethral gland region, which represents less than 1% of the total volume of the glandular prostate. Its ductal network represents a more proximal extension of the networks of the peripheral and transition zone areas. These regions have differing acinar, stromal, and cellular configurations. McNeal postulated that the anatomic and histologic similarities of the peripheral and transition zones and periurethral gland region were attributable to a common urogenital sinus embryonic origin.

The anterior fibromuscular stroma forms an apron that extends distally, covers the entire anterolateral aspect of the glandular prostate, and is responsible for the anterior convexity of the prostate gland. It represents approximately one-third of the tissue within the prostate capsule (4). This unusually distinct area, composed predominantly of smooth-muscle fibers, maintains continuity proximally with the detrusor muscle fibers of the bladder neck.

Prostatic stroma consists predominantly of smooth muscle cells and fibroblasts arranged in close proximity to the distinct basal lamina of the epithelium. The fibroblasts, however, tend to be organized parallel to the long axis of these tubulosaccular glands and form a more predict-

Prostate Zones Mcneal

Fig. 4. This sagittal diagram of the prostate demonstrates its distinct zones: central zone (CZ), peripheral zone (PZ), and transitional zone (TZ). Its urethral segments, distal (UD), proximal (UP), and ejaculatory ducts (E) are illustrated along with nonglandular tissues [bladder neck (bn), anterior fibromuscular stroma (fm), preprostatic sphincter (s), distal striated sphincter (s)]. C and OC delineate the coronal and oblique coronal planes, respectively. (From 17, with permission.)

Fig. 4. This sagittal diagram of the prostate demonstrates its distinct zones: central zone (CZ), peripheral zone (PZ), and transitional zone (TZ). Its urethral segments, distal (UD), proximal (UP), and ejaculatory ducts (E) are illustrated along with nonglandular tissues [bladder neck (bn), anterior fibromuscular stroma (fm), preprostatic sphincter (s), distal striated sphincter (s)]. C and OC delineate the coronal and oblique coronal planes, respectively. (From 17, with permission.)

able relationship with the basement membrane (18). The smooth muscle surrounds individual glands and is thought to play a pivotal role in the release of glandular secretions. Contraction of the circular smooth muscle of the bladder neck and preprostatic sphincter assists in the elimination of secretions within the prostatic urethra; this smooth muscle probably forms the major working element of the internal urethral sphincter. The anterior and anterolateral aspects of the prostate contain smooth and skeletal muscle, joining the fibers of the external sphincter and augmenting urinary control (19).

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