Part fifteen. Male internal genital organs
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.
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.