Atlas of plant and animal histology

The prostate is the largest gland of the male reproductive system. In humans, it has a size and form similar to a nut and is located in the pelvis, before the rectum and just below the urinary bladder. The prostate is crossed by the initial segment of the urethra (prostatic urethra) coming from the urinary bladder and by the two ejaculatory ducts, after ducts from seminal vesicles and the ductus deferens have joined. In young humans, the prostate typically weighs about 20 g, a mass that remains mostly constant for about 20 years. Prostatic tissue is mostly glandular, divided into independent tubuloalveolar glands that release their products into the urethra through the excretory ducts. Secretory parts (or adenomeres) and excretory ducts of the glands are similar throughtout the prostate. Glands are surrounded by connective tissue stroma and muscle fibers organized in walls or septa. The prostate is covered by a layer of connective tissue known as the capsule, along with a dense layer of smooth muscle cells. Each septum extends from the capsule. In the image above, both tissue types can be readily distinguished due to the trichromic staining that results in a red color for the muscle and a bluish color for collagen fibers (see Figure A).

TThe antomical division of the prostatic parenchyma into regions has clinical relevance. However, the number of regions and how they are described have been controversial since the classification into three regions of Gil Vernet (1953) and into four regions of McNeal (1968). In routine clinical descriptions, there is a peripheral zone encompassing about 70% of the total glandular tissue. This region is more prone to inflammation and shows a higher chance of developing carcinomas. The adenomeres within this region are surrounded by smooth muscle and a smooth epithelial layer. The central and transitional regions collectively account for about 25% of the glandular tissue; although malignant neoplasms are infrequently encountered in these regions, benign prostatic hyperplasia is frequently documented. Their adenomeres are thinner, and the epithelial layer is folded. The prostate can also be divided into lobules based on the distribution of septa: anterior, posterior, medial, and two lateral lobules.

Prostatic glands are distributed in three regions of the prostate around the prostatic urethra. Those found closer to the urethra are known as mucosal periurethral glands. The submucosal periurethral glands are located in the transition part of the prostate and are connected to the urethra by short ducts. The major prostatic glands are found in the peripheral zone. From 30 to 50 tubuloalveolar glands release their contents into the prostatic urethra via 15 to 30 long excretory ducts ending at both sides of the urethral crests (or cristae urethralis).

Tubuloalveolar glands have a convoluted morphology; therefore, their secretory alveoli look irregular and show variable size. The secretory epithelium has folds, and it is mostly biseriate pseudostratified or simple columnar (Figure C). The activity of secretory cells influences the cellular height. More activity induces higher cells, and less activity makes cells cuboidal or nearly flat. As in the seminal vesicle, two secretory mechanisms have been described in the prostate: apocrine and merocrine. During the apocrine secretion, portions of the apical part of the cells are detached. These cell fragments are known as prostasomes (figure D).

Five types of acinar cells have been described in the glands in humans: microvillar cells, which show dense apical microvilli; secretory cells; perforated cells with inner cavities; crater cells with broken apical regions; and smooth cells with apical regions almost devoid of microvilli.

Prostate secretion is up to 15–20% of the total ejaculated fluid. It is a milky and fluent liquid containing a significant amount of citric acid responsible for the acid pH (around 6.6) and less than 1% in proteins. Moreover, it contains prostaglandin, acid phosphatases, and proteolytic enzymes that help make the prostatic secretion fluid. Among proteases, the serine protease is known as the prostate-specific antigen (PSA), which is important for the diagnosis of prostatic pathologies. A high concentration of zinc is also found contributing to chromatin stability, which is very condensed in the head of the spermatozoa. Prostatic secretion also contains type A immunoglobulins and polyamines, like spermine, with bacteriostatic effects. Spermine crystals, first described by Leeuwenhoek, have been used in forensic science to solve sexual crimes. In the interior of the prostatic gland, concentric laminar structures called amylaceous bodies are occasionally observed (figure C). It is thought that these bodies result from the precipitation of prostatic secretion around cellular fragments. The number of amylaceous bodies increases during aging, and they are renamed as concretions or calculus. They can be found in the semen.

The image above is from a monkey's prostate, where the regions previously described cannot be distinguished. In non-human primates, like macaques and baboons, the prostate shows a different organization with cranial and caudal areas, which are similar to the central and peripheral areas, respectively, in humans.