UEU-co logo

//PROSTATE

PROSTATE

The prostate is a pyramidal fibromuscular gland which surrounds the prostatic urethra from the bladder base to the membranous urethra. It has no true fibrous capsule, but is enclosed by visceral fascia containing neurovascular tissue (Fig. 75.10A,B). The fascia is firmly adherent to the gland and is continuous with a median septum and with numerous fibromuscular septa which divide the glandular tissue into indistinct lobules.

  

Fig. 75.10  A, The endopelvic fascia. The fascia covers the pelvic viscera and continues posteriorly over the lateral aspect of the prostate, as the lateral prostatic fascia and on the lateral aspect of the rectum as the lateral rectal fascia. Its relationship with Denonvilliers may be visualized as an ‘H’. The deep dorsal vein and lateral branches run deep to the endopelvic fascia and the lateral prostatic fascia, although communicating with perforators to the pelvic side wall. Denonvilliers is adherent to the prostate in the posterior midline, but like the lateral prostatic fascia is separated from the prostatic tissue by the neurovascular structures and fatty tissues elsewhere. Between Denonvilliers and the rectum is a fatty potential plane, the prerectal space. B, The prostatic capsule (PC) is a condensation of connective tissue on the surface of the gland (green) and encloses the neurovascular bundle (NVB). The prostatic fibromuscular stroma and glands lie beneath the capsule.

The muscular tissue within the prostate is mainly smooth muscle. Anterior to the urethra a layer of smooth muscle merges with the main mass of muscle in the fibromuscular septa; it blends superiorly with vesical smooth muscle. Anterior to the layer of smooth muscle a transversely crescent-shaped mass of skeletal muscle is continuous inferiorly with the external urethral sphincter in the deep perineal pouch. Its fibres pass transversely internal to the capsule, and are attached to it laterally by diffuse collagen bundles; other collagen bundles pass posteromedially, merging with the prostatic fibromuscular septa and the septum of the urethral crest. This muscle, supplied by the pudendal nerve, probably compresses the urethra but it may pull the urethral crest back and the prostatic sinuses forwards, dilating the urethra. Glandular contents may be expelled simultaneously into the urethra when it has expanded in this way, so that it contains 3–5 ml seminal fluid prior to ejaculation.

The prostate lies at a low level in the lesser pelvis, behind the inferior border of the symphysis pubis and pubic arch (Figs 75.2A, 75.11) and anterior to rectourethralis and the rectal ampulla, through which it may be palpated. Being somewhat pyramidal, it presents a base or vesical aspect superiorly, an apex inferiorly, and posterior, anterior and two inferolateral surfaces. The prostatic base measures about 4 cm transversely. The gland is 2 cm in anteroposterior and 3 cm in its vertical diameters, and weighs about 8 g in youth, but almost invariably enlarges with the development of BPH: it usually weighs 40 g, but sometimes as much as 150 g or even more, after the first five decades of life. The small prostate without BPH is described as a croissant shape (short anterior commissure, prominent apical notch and posterior lip of prostatic tissue), and the enlarged gland is more doughnut-shaped. The shape of the prostate affects the relationship of the prostatic apex to the external urethral sphincter. This relationship is important when removing the prostate at radical prostatectomy for cancer, and anastomosing the bladder to the urethra to maintain sphincter integrity. The external urethral sphincter is flush to a large doughnut-type gland so a perpendicular incision will separate the prostate and external urethral sphincter accurately. In a small prostate the external urethral sphincter fills the defect in the anterior aspect of the prostate, so a perpendicular incision at the level of the posterior lip of the croissant shaped gland will excise much external urethral sphincter and leave the patient incontinent.

  

Fig. 75.11  Sagittal section, male pelvis. The pelvic plexus lies with its midpoint level with the tip of the seminal vesicles. It gives branches to the prostate, bladder urethra, seminal vesicles, rectum and corpora cavernosa via the cavernous nerves. The cavernous nerves are the continuation of the neurovascular bundles, lying posterolateral to the prostate. These bundles may be damaged at radical prostatectomy resulting in impotence.

Superiorly the base is largely contiguous with the neck of the bladder. The apex is inferior, surrounding the junction of the prostatic and membranous parts of the posterior urethra. The apical posterior relation of the prostate and external urethral sphincter is rectourethralis, a Y-shaped muscle that originates from the outer longitudinal coat of the rectum. The upper limbs of the Y extend from the lateral rectal wall to the midline where rectourethralis inserts into the perineal body at the anorectal junction.

The anterior surface lies in the arch of the pubis, separated from it by the dorsal venous complex (Santorini’s plexus) and loosely attached adipose tissue. It is transversely narrow and convex, extending from the apex to the base. Near its superior limit it is connected to the pubic bones by the puboprostatic ligaments. The urethra emerges from this surface anterosuperior to the apex of the gland. The anterior part of the prostate is relatively deficient in glandular tissue and is largely composed of fibromuscular tissue. The anterior and lateral aspects of the prostate are covered by a layer of fascia derived from the endopelvic fascia on each side, called the lateral prostatic fascia. This is adherent medially to the prostate, continues posteriorly over the lateral aspect of the prostate, neurovascular bundles and rectum (lateral rectal fascia) and passes distally over the urethra (Fig. 75.10A). The prostatic venous plexus (Fig. 75.11) lies between this extension of the endopelvic fascia and the prostate. Anteroinferiorly the parietal and visceral fasciae of the prostate merge and blend with the puboprostatic ligaments. The anterior surface of the prostate and associated vascular plexus is covered by the detrusor apron.

The inferolateral surfaces are related to the muscles of the pelvic sidewall: the anterior fibres of levator ani embrace the prostate in the pubourethral sling or pubourethralis. These muscles are separated from the prostate by a thin layer of connective tissue.

The posterior surface of the prostate is transversely flat and vertically convex. Near its superior (juxtavesical) border is a depression where it is penetrated by the two ejaculatory ducts. Below this is a shallow, median sulcus, usually considered to mark a partial separation into right and left lateral lobes. It is separated from prerectal fat in the prerectal space (Figs 75.2A, 75.10A), and rectum by Denonvillier’s fascia, a condensation of pelvic fascia which develops by obliteration of the rectovesical peritoneal pouch, and by loose yellow fatty areolar tissue. The rectovesical pouch is obliterated from below upwards as fetal life progresses, forming Denonvillier’s fascia: at birth this fascia separates the prostate, the seminal vesicles and the ampullae of the vasa deferentia from the rectum. The superior limit of Denonvillier’s fascia is the peritoneum of the rectovesical pouch. Laterally Denonvillier’s fascia fuses with the lateral pelvic fascia: anterior to Denonvillier’s fascia, the fascia is called the lateral prostatic fascia, and posterior to Denonvillier’s fascia it is called the lateral rectal fascia.

The prostate is traversed by the urethra and ejaculatory ducts, and contains the prostatic utricle. The urethra enters the prostate near its anterior border and usually passes between its anterior and middle thirds. The ejaculatory ducts pass anteroinferiorly through its posterior region to open into the prostatic urethra (p. 1250).

ZONAL ANATOMY OF THE PROSTATE

The prostate gland was initially thought to be divided into five anatomical lobes, but it is now recognized that five lobes can only be distinguished in the fetal gland prior to 20 weeks’ gestation. Between then and the onset of BPH, only three lobes are recognizable, two lateral and a median lobe. Clinicians refer to left and right ‘lobes’ when describing either what can be felt on rectal palpation, or endoscopically visible abnormalities in the diseased state when prostatic anatomy is distorted by BPH.

From an anatomical, and particularly from a morbid anatomical perspective, the glandular tissue may be subdivided into three distinct zones (Figs 75.12–75.14), peripheral (70% by volume), central (25% by volume), and transitional (5% by volume). Non-glandular tissue (fibromuscular stoma) fills up the space between the peripheral zones anterior to the preprostatic urethra. The central zone surrounds the ejaculatory ducts, posterior to the preprostatic urethra, and is more or less conical in shape with its apex at the verumontanum. The transitional zone lies around the distal part of the preprostatic urethra just proximal to the apex of the central zone and the ejaculatory ducts. Its ducts enter the prostatic urethra just below the preprostatic sphincter and just above the ducts of the peripheral zone. The peripheral zone is cup-shaped and encloses the central transitional zone and the preprostatic urethra except anteriorly, where the space is filled by the anterior fibromuscular stroma. Simple mucus-secreting glands lie in the tissue around the preprostatic urethra, above the transitional zone and surrounded by the preprostatic sphincter. These simple glands are similar to those in the female urethra and unlike the glands of the prostate.

  

Fig. 75.12  Ultrasound of the prostate. A, Axial view showing the hypoechoic transition zone (TZ) and the more echogenic peripheral zone (PZ). Their interface is the surgical capsule (*). B, Sagittal view showing the hypoechoic transition zone (TZ) and the more echogenic peripheral zone (PZ).

  

Fig. 75.13  MRI of the prostate. A, T2-weighted MRI scan showing the normal high signal of the peripheral zone and the intermediate signal of the central and transitional zones and the verumontanum in the central gland. B, T2-weighted coronal MRI scan of the prostate showing the zonal anatomy.

The zonal anatomy of the prostate is clinically important because most carcinomas arise in the peripheral zone, whereas BPH affects the transitional zone, which may grow to form the bulk of the prostate. BPH begins as micronodules in the transitional zone which grow and coalesce to form macronodules around the inferior margin of the preprostatic urethra, just above the verumontanum. Macronodules in turn compress the surrounding normal tissue of the peripheral zone posteroinferiorly, creating a ‘false capsule’ around the hyperplastic tissue which coincidentally provides a plane of cleavage for its surgical enucleation. As the transitional zone grows, it produces the appearance of ‘lobes’ on either side of the urethra. In due course, these lobes may compress or distort the preprostatic and prostatic parts of the urethra and produce symptoms. The central zone surrounding the ejaculatory ducts is rarely involved in any disease. It shows certain histochemical characteristics which differententiate it from the rest of the prostate: it is thought to be derived from the Wolffian duct system (much like the epididymi, vasa deferentia and seminal vesicles), whereas the rest of the prostate is derived from the urogenital sinus (p. 1323).

On magnetic resonance (MR) imaging the prostate gland has a zonal anatomy on T2-weighted images (Fig. 75.13). The normal peripheral zone has high signal intensity, as does fluid within the seminal vesicles. The central and transitional zones have relatively low signal and are often referred to as the ‘central gland’. The verumontanum may be seen as high signal within the central gland.

The relationship of the zones of the gland normally changes with age. The central zone atrophies, and the transitional zone enlarges secondary to BPH. This often produces a low signal band at the margin of the hypertrophied transitional and compressed peripheral zones, the surgical pseudocapsule, which is well seen on T2-weighted MR images.

Leave a Reply


Time limit is exhausted. Please reload the CAPTCHA.

Categories

apply_now Pepperstone Group Limited