 The peripheral zone comprises 70% of the glandular tissue of the prostate. It surrounds the central zone and occupies posterior and lateral parts of the gland. Most prostatic carcinomas arise from the peripheral zone of the prostate gland. The peripheral zone is palpable during digital examination of the rectum. This zone is also the most susceptible to infammation.
 The transitional zone surrounds the prostatic urethra; it comprises about 5% of the prostatic glandular tissue and contains the mucosal glands. In older individuals, the parenchymal cells of this zone frequently undergo extensive division (hyperplasia) and form nodular masses of epithelial cells. Because this zone is proximate to the pro-static urethra, these nodules can compress the prostatic urethra, causing diffcult urination. This condition is known as benign prostatic hyperplasia (BPH) and is discussed in Folder 22.4 (page 811).
FIGURE 22.26 • Photomicrograph of human seminal vesicle a. This low-magnification photomicrograph shows part of an H&E–stained section of a human seminal vesicle. This gland is a tortuous tubular structure and in a section exhibits what appear to be a number of isolated lumina. In actuality, there is only one lumen. The mucosa is characterized by extensive folding (arrows). It rests on a thick smooth muscle (SM) investment that is organized in two layers: an inner circular layer and an outer longitudinal layer. 20. b. This higher magnification shows the mucosal folds surfaced by a pseudostratified epithelium. Arrows indicate the basal cells. 500.
FIGURE 22.27 • Schematic drawing of the zones of the human prostate gland. This drawing illustrates the relative location, by color, of the four zones of the prostate gland and anterior fibromuscular stroma of the gland.
 The periurethral zone contains mucosal and submucosal glands. In later stages of BPH, this zone may undergo pathologic growth but mainly from the stromal components. Together with the glandular nodules of the transitional zone, this growth causes increased urethral compression and further retention of urine in the bladder.
In addition, fbromuscular stroma occupies the anterior surface of the prostate gland anterior to the urethra and is composed of dense irregular connective tissue with large amount of smooth muscle fibers.
The growth of the prostatic glandular epithelium is regulated by the hormone dihydrotestosterone.