General Structure of the Organ
Histologically, the prostate gland is classified as a fibromuscular-glandular organ. Its structure is formed by a combination of glandular parenchyma and a robust fibromuscular stroma.
The stromal component features a radial architecture—connective tissue and smooth muscle elements radiate outward from the center of the organ (from the urethra) toward the periphery. Large septa of stroma divide the organ into lobules, while finer septa penetrate inside these lobules, surrounding individual glands. The stroma consists of loose fibrous connective tissue rich in elastic fibers and bundles of smooth myocytes. On a histological section, the combination of branching terminal units and radially diverging stroma creates a visual pattern resembling the leaf of an exotic plant.
The muscular elements of the stroma form two layers (circular and longitudinal) around the lobules and terminal units. A sheath consisting of an inner longitudinal layer and an outer circular layer also forms around the urethra as it passes through the organ.
Glandular Apparatus: Parenchyma and Classification
The glandular parenchyma consists of 30–50 mucous tubuloalveolar glands characterized by extensively branching terminal units. The excretory ducts of these glands merge into larger ducts and open into the prostatic urethra. In humans, the excretory duct system has a wide lumen (unlike dogs, for example, which have narrow ducts).
Depending on their localization, the glands are divided into three groups:
- Group I: Mucosal glands located directly within the lamina propria of the urethral mucosa.
- Group II: Submucosal glands situated in the connective tissue surrounding the urethra.
- Group III: Main prostatic glands occupying the major, peripheral portion of the organ.
Cellular Composition of Terminal Units and Urethra
The epithelium of the tubuloalveolar glands contains three cell types:
- Chief cells (exocrinocytes): Cuboidal or columnar in shape. These are mucous cells that produce the main components of the secretion.
- Basal cells: Small elements resembling reserve cells, functioning as stem (cambial) cells for epithelial regeneration.
- Endocrine cells: Rarely encountered; secrete hormones with paracrine (local) effects.
As the excretory ducts enlarge, their single-layer lining transitions into pseudostratified columnar (multiseriate) epithelium.
The prostatic urethra passes through the anterior part of the organ. It has a narrow stellate lumen and is lined with transitional epithelium (urothelium), typical of the urinary tract. Beneath the epithelium lie the lamina propria of the mucosa, the submucosa, and the smooth muscle layer.
Functions of the Prostate Gland
The prostate gland performs three key functions: exocrine, endocrine, and contractile.
Exocrine function consists of producing secretions that contribute to the liquid fraction of semen. The main components of the secretion include:
- Proteolytic enzymes (e.g., fibrinolysin): ensure the liquefaction of semen.
- Phospholipids and annexins (carrier proteins): replace cholesterol in sperm membranes with phospholipids, increasing their lability to prepare for fertilization.
- Prostasomes: small membrane vesicles that bind to spermatozoa and protect them from immune attack in the female reproductive tract.
- Prostaglandins: stimulate contractions of the smooth muscle of the female tract to transport sperm (this group of substances was historically named because they were first discovered in prostatic secretions).
Endocrine function includes the synthesis of factors released into the blood (e.g., a follicle-stimulating hormone analogue that stimulates testicular activity, and a hypothalamic sex differentiation factor). In addition, an APUD-like system of local hormones operates within the gland. Endocrine cells release serotonin, calcitonin, somatostatin, and cholecystokinin into the intercellular space (paracrine effect). Serotonin and cholecystokinin regulate myocyte contraction, while somatostatin controls glandular secretion.
Contractile function is executed by the powerful stroma. During ejaculation, muscular elements contract, actively expelling glandular secretions. Simultaneously, an involuntary urethral sphincter forms, preventing the retrograde flow of seminal fluid into the urinary bladder.
Age-Related Changes
After 50–60 years of age, age-related tissue remodeling begins in the prostate gland. Atrophy of the glandular component (parenchyma) occurs, accompanied by proliferation of the stromal connective tissue.
A characteristic sign of organ aging is the formation of prostatic concretions ( corpora amylacea / calculi). These are layered bodies localized in the lumen of the terminal glandular units and excretory ducts. These concretions consist of degraded epithelial debris, thick secretory components, and mineral substances, predominantly calcium phosphate.