Anatomy

The normal prostate weighs 20 g by early adulthood and is best thought of as having an inverted pyramid shape, with anterior, posterior and lateral surfaces, a narrow apex anteroinferiorly and a broad base superiorly which lies against the bladder neck. It is related anteriorly to the symphysis pubis, laterally to the anterior fibres of the levator ani muscle and posteriorly to the seminal vesicles and rectum, separated from the latter by Denonvilliers' fascia. The prostate is surrounded by an ill-defined fibrous capsule which blends with the pelvic fascia. Numerous neurovascular bundles are found within this connective tissue. At the apex, skeletal muscle fibres of the urethral sphincter are admixed with occasional benign prostatic glands and, at the base, fibres from the bladder detrusor muscle blend imperceptibly with the prostate capsule. At these points the boundaries of the organ are particularly obscure, rendering difficult in resection specimens the interpretation of capsular penetration by carcinoma and capsular incision during surgery. Adipose tissue is occasionally found just inside the prostatic capsule.

The prostate is composed of branching tubuloalveolar glands lined by cuboidal or columnar epithelium and invested and surrounded by fibromuscular stroma which is continuous with the prostatic capsule. The urethra transverses the full diameter of the prostate in a curved fashion, entering at the centre of the prostate base and exiting just anterior to the apex. Prostatic ducts empty into the prostatic urethra. The ejaculatory ducts, formed at the juncture of the vasa defer-entia and seminal vesicle, also secrete into the prostatic urethra.

The glandular prostatic tissue has been divided into four distinct zones, characterised by differing embryological origin, location and pathologies (Figure 30.1a). The anterior fibromus-cular stroma, composed mainly of fibromuscular tissue with very few glands, merges with the bladder neck superiorly and the external sphincter at the apex inferiorly. The preprostatic zone surrounds the urethra proximal to the ejaculatory ducts and comprises the periurethral ducts and the larger transition zone. This region commonly gives rise to benign prostatic hypertrophy and approximately 25% of adenocarcinomas. The central zone, surrounding the ejaculatory ducts, is felt to differ embryologically from the remainder of the gland and is least commonly affected by pathological abnormality. Glands in the central zone may show complex papillary infoldings and a cribriform architecture on histology. Lack of cytological atypia distinguishes them from prostatic intraepithelial neoplasia (PIN). The peripheral zone occupies approximately 70% of the normal prostate in a horseshoe shape around the posterior and lateral aspects of the organ. Glands are normally small and simple but this zone is the main site of origin for prostatic adenocarci-nomas (70%).

To simplify the concept of zones, the prostate may be considered to have significantly differing inner (transition zone) and outer (peripheral and central zones) regions.

Clinically, the prostate gland is often described as having right and left lateral lobes, a central sulcus and a middle lobe. These do not equate to any anatomically defined structures but rather

Seminal vesicle

Seminal vesicle

Urethra a

Urethra a

Superior

Superior

Figure 30.1. (a) Prostatic zones (lateral view). (b) Prostatic lobes (anterior view).

Inferior b

Figure 30.1. (a) Prostatic zones (lateral view). (b) Prostatic lobes (anterior view).

relate to palpable masses on rectal examination, usually enlargement of the transitional zone laterally and periurethral glands centrally.

For the purposes of TNM staging the prostate gland is simply divided into right and left lobes (Figure 30.1b).

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