Hierarchy of Hormonal Control
The regulation of spermatogenesis occurs top-down through a multi-level scheme. The highest center is the hypothalamus: its arcuate nuclei secrete gonadotropin-releasing hormone (GnRH).
Under the influence of releasing hormones, the anterior pituitary begins to produce gonadotropic hormones, which act selectively on different targets within the testis:
- Pituitary LH cells secrete luteinizing hormone (LH). In males, it is often referred to as ICSH (interstitial cell-stimulating hormone).
- Pituitary FSH cells secrete follicle-stimulating hormone (FSH), which acts on sustentocytes.
Leydig Cells: The Testosterone Factory
Testicular endocrinocytes, or Leydig cells (Glandulocytus testis), are located in the connective tissue strictly between the seminiferous tubules. They have a rounded shape and characteristic oxyphilic cytoplasm.
The primary task of Leydig cells is the synthesis of testosterone. This process depends directly on hydroxylation and microsomal oxidation reactions. For this reason, at the ultrastructural level, glandulocytes feature a heavily developed smooth (agranular) endoplasmic reticulum.
An interesting feature of hormone transport is that a "case" of lymphatic sinusoidal capillaries forms around each seminiferous tubule. A significant portion of newly synthesized testosterone enters not directly into the bloodstream, but into an extensive network of lymphatic sinuses. From there, via efferent lymphatic vessels, the hormone collects into the thoracic duct and only then enters the systemic circulation of the greater circulation.
Mechanisms of Testosterone Action
Testosterone secreted by Leydig cells operates in two main directions:
- Systemic action: The hormone is distributed to distant target organs while simultaneously exerting negative feedback—high levels of testosterone directly inhibit LH production by the pituitary gland.
- Local action: Part of the hormone remains inside the testis. Testosterone penetrates through the basement membrane of the convoluted seminiferous tubules and enters Sertoli cells, stimulating the synthesis of specific proteins. Subsequently, it enters the lumen of the tubule, where it critically influences the maturation of spermatocytes and spermatids.
The Role of Sertoli Cells (Sustentocytes)
Sertoli cells act as the main intermediaries between hormonal signals and developing germ cells. Their activity is controlled simultaneously by pituitary FSH and local testosterone.
Sertoli cells possess rich secretory activity:
- ABP (Androgen-Binding Protein): Binds testosterone in the tubular lumen, creating a locally high concentration of the hormone around spermatogenic cells, which is necessary for their development.
- Activin and Interleukin-1: Serve as growth factors stimulating the division of early cells—spermatogonia.
- Inhibin: A factor that, via a feedback mechanism, inhibits FSH production in the anterior pituitary.
Additionally, local steroid metabolism occurs within Sertoli cells. Under the action of enzymes, a small fraction of testosterone is converted into the female sex hormone—estradiol. It diffuses back into the interstitium and exerts a paracrine (local) inhibitory effect, preventing Leydig cells from synthesizing excessive amounts of testosterone.
Morphology of Spermatogenesis on a Histological Slide
The spermatogenic epithelium functions cyclically, forming the so-called "wave" of spermatogenesis. With standard hematoxylin-eosin staining, different stages of cell development can be identified on a cross-section of a seminiferous tubule based on their position:
- In the basal compartment (near the membrane) lie spermatogonia and spermatocytes.
- Closer to the lumen of the tubule are early spermatids, which have rounded nuclei.
- At the very edge (in the lumen) are late spermatids: they are easily recognized by their elongated dark nuclei and actively forming flagella. These cells are already preparing to enter the tubular lumen.