Prostate
The prostate is a partly glandular, partly fibromuscular organ which lies beneath the bladder and above the urogenital diaphragm, and is penetrated by the proximal part of the urethra. It is normally broader than it is long, approximately 4 × 3 × 2cm. Its female homologue is the small group of paraurethral glands (of Skene; see p. 307). The prostate provides about 30% of the volume of seminal fluid (most comes from the seminal vesicle).
The prostate has a base and an apex, and anterior, posterior and inferolateral surfaces. The base is the upper surface, fused with the neck of the bladder and perforated by the urethra which traverses the whole length of the gland (Fig. 5.60). The blunt apex is the lowest part, and the prostatic urethra emerges from the front of the apex to become the membranous urethra which is surrounded by the sphincter urethrae (see p. 317). The anterior surface is at the back of the retropubic space and is connected to the bodies of the pubic bones by the puboprostatic ligaments. The inferolateral surfaces are clasped by the pubourethralis parts of levator ani. The posterior surface is in front of the lower rectum but separated from it by the rectovesical fascia (see p. 294). The ejaculatory ducts pierce the posterior surface just below the bladder and pass obliquely through the gland for about 2cm to open into the prostatic urethra about halfway down. The prostate's own ducts also open into this part of the urethra (see below).
A thin strong layer of connective tissue at the periphery of the gland forms the ‘true capsule’ of the prostate, and outside this there is a condensation of pelvic fascia forming the ‘false capsule’. Between these two capsules lies the prostatic plexus of veins. The gland consists of acini of varying shapes and sizes embedded in a fibromuscular stroma—a mixture of connective tissue and smooth muscle; this is the characteristic histological feature.
The prostatic urethra, 3–4cm in length, passes through the substance of the prostate closer to the anterior than the posterior surface of the gland. It runs downwards and backwards from the internal meatus, then bends at the middle of its length and continues downwards and forwards to emerge from the anterior aspect of the apex. A midline ridge, the urethral crest, projects into the lumen from the posterior wall throughout most of the length of the prostatic urethra (Fig. 5.60). The shallow depression on either side of the crest is termed the prostatic sinus. At about the midlength of the crest the seminal colliculus, or verumontanum, forms a midline rounded eminence. The prostatic utricle, a small recess representing the fused ends of the paramesonephric (Müllerian) ducts, opens on to the middle of the verumontanum and the ejaculatory ducts open on either side of the utricle. The proximal part of the prostatic urethra, also termed the preprostatic part, is surrounded by a cylinder of smooth muscle, an extension of the circular muscle at the bladder neck; as has been noted above, this muscle contracts to prevent seminal regurgitation into the bladder during ejaculation.
The prostate is now considered to consist of a peripheral zone, a central zone and a transition zone, accounting for approximately 70%, 20% and 5% of the glandular substance, respectively, rather than being made up of lobes as previously described. The central zone is wedge-shaped and forms the base of the gland with its apex at the verumontanum (Fig. 5.60); it surrounds the ejaculatory ducts as they course through the gland. The peripheral zonesurrounds the central zone from behind and below, but does not reach up to the base; it extends downwards to form the lower part of the gland. The transition zone lies around the distal part of the preprostatic urethra, just proximal to the apex of the central zone. The ducts of the transition zone open on the verumontanum, just above where the ducts of the peripheral zone open into the prostatic sinuses. Benign prostatic hyperplasia affects the transition zone which may increase markedly in size, compressing the peripheral zone (Fig. 5.61). The peripheral zone is almost exclusively the site of origin for carcinoma of the prostate. The central zone is rarely involved in any disease process.
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Figure 5.61 Oblique axial MRI of the prostate. The transition zone is markedly enlarged by benign prostatic hyperplasia. (Provided by Dr G. Brown, The Royal Marsden Hospital, Sutton, Surrey.) |
There is very little glandular tissue anterior to the prostatic urethra, the anterior part of the prostate being mainly fibromuscular; it is overlapped from above by the detrusor muscle of the bladder and from below by the striated muscle of the urethral sphincter.
Blood supply
The main arterial supply is from the prostatic branch of the inferior vesical artery, with some small branches from the middle rectal and internal pudendal vessels. The veins run into a plexus between the true and false capsules and this joins the vesicoprostatic plexus situated at the groove between bladder and prostate. This plexus receives the deep dorsal vein of the penis, and drains backwards into the internal iliac veins.
Lymph drainage
The lymphatics of the prostate pass across the pelvic floor mainly to internal iliac nodes; a few may reach external iliac nodes.
Nerve supply
The acini receive parasympathetic (cholinergic) inner-vation from the pelvic splanchnic nerves (see p. 311) via the inferior hypogastric plexus. The muscle fibres of the stroma, which contract to empty the glands during ejaculation (see p. 322), are under sympathetic (adrenergic) control from the inferior hypogastric plexus (see p. 311).
Development
The pelvic part of the endodermal urogenital sinus (see p. 29) gives rise to lateral epithelial buds which become the prostatic acini of the peripheral and transition zones. Dorsal outgrowths from above the level of entry of the mesonephric ducts form the acini of the central zone. The fibromuscular stroma develops from the surrounding mesenchyme.
Surgical approach
Most operations for benign prostatic hyperplasia are now carried out by the transurethral route, with the resectoscope, the area of resection being restricted to above the verumontanum so that the external urethral sphincter, which is distal to it, is not damaged during the procedure. An approach through an abdominal suprapubic incision into the retropubic space gives exposure for a total removal of the organ for prostatic carcinoma, which can also be achieved laparoscopically, or through a perineal approach. The bladder neck is anastomosed to the membranous urethra.
Vas deferens and seminal vesicle
The origin of the vas deferens as the continuation of the epididymis has been considered on page 231. It enters the abdomen at the deep inguinal ring and passes along the side wall and floor of the pelvis to reach the back of the bladder. In its course no other structure intervenes between it and the peritoneum.
After hooking around the interfoveolar ligament and inferior epigastric artery at the deep inguinal ring, it crosses the external iliac artery and vein, obliterated umbilical artery and the obturator nerve, artery and vein, lying on the obturator fascia (Fig. 5.65). It curves medially and forwards, crosses above the ureter and approaches its opposite fellow. The two ducts now turn downwards side by side (Fig. 5.62) and each dilates in fusiform manner. This dilatation is the ampulla, the storehouse of spermatozoa. The proximal part of the vas absorbs fluid produced by the seminiferous tubules of the testis, and the ductus itself makes only a small contribution to the volume of seminal fluid. The ampullae lie parallel and medial to the seminal vesicles; at their lower ends each loses its thick muscle wall and joins with the outlet of the seminal vesicle to form the ejaculatory duct. Each ejaculatory duct passes obliquely through the prostate to open on the verumontanum (Fig. 5.60).
The seminal vesicle is a thin-walled, elongated sac, like a lobulated, blind-ending tube much folded on itself. The pair produce about 60% of the seminal fluid, and are applied to the base of the bladder above the prostate (Fig. 5.62). The rectovesical fascia lies behind them and their tops are just covered by the peritoneum of the rectovesical pouch. Each lies lateral to the ampulla of the vas deferens of its own side, and at the lower end of the ampulla behind the prostate the duct of the seminal vesicle joins the vas to form the ejaculatory duct.
Blood supplies. The artery to the vas deferens is a branch of the superior vesical (or sometimes the inferior vesical) artery. It accompanies the ductus to the lower pole of the epididymis and anastomoses with the testicular artery (see p. 229). The seminal vesicles are supplied by branches from the inferior vesicle and middle rectal arteries.
Lymph drainage. Lymphatics accompany the blood vessels to the nearest iliac nodes.
Nerve supplies. The smooth muscle of the vas and seminal vesicles receives fibres from the inferior hypogastric plexus. They are mainly sympathetic motor fibres from the first lumbar ganglion via the hypogastric plexuses; their division produces sterility, for the paralysed muscle cannot contract to expel the stored secretion and spermatozoa, i.e. there is no emission or ejaculation (see p. 322).
Structure
The striking histological feature of the vas deferens is the thickness of the muscular wall compared with the small size of the lumen. The smooth muscle of the vas is arranged as inner and outer longitudinal and a middle circular layers. The mucous membrane is columnar with stereocilia (elongated microvilli).
The muscle coat of the seminal vesicle is thinner than that of the vas. Although a single tube it is much convoluted and so appears in sections as a number of tubules, with mucosa that is very folded giving a glandular appearance. The epithelium is columnar.
Development
The vas deferens is a main derivative of the mesonephric duct (see pp. 231 and 286), and at the back of the prostate a diverticulum from the duct forms the seminal vesicle.
