Localization and histology of the process
Spermatogenesis takes place in the male gonads—the testes (testicles). Inside them are structural and functional units known as convoluted seminiferous tubules. It is here, within the thickness of the spermatogenic epithelium, that gametes originate and mature.
In a cross-section of a seminiferous tubule, it is evident that the cellular composition of the wall is heterogeneous. It contains both germ cells at various stages of development and somatic cells. A crucial role is played by Sertoli cells (sustentocytes), which are large supporting structures that create the necessary microenvironment for developing gametes.
The process of differentiation is strictly oriented in space: immature forms are located at the periphery (near the basement membrane), and as they mature, the cells shift toward the center, into the lumen of the tubule. From there, ready spermatozoa enter the epididymis and are subsequently transported via the vas deferens.
Phases of spermatogenesis
The entire cycle of sperm formation is divided into four consecutive stages. At each stage, the genetic set of the cells changes, where n is ploidy (chromosome set) and c is the amount of DNA molecules (chromatids).
- Multiplication phase. The initial cells of the spermatogenic tissue (2n4c) actively divide by mitosis. This results in a pool of diploid cells—spermatogonia with a 2n2c set. They are localized on the basement membrane.
- Growth phase. This stage corresponds to interphase prior to the first meiotic division. DNA replication occurs, and the cells increase significantly in size. Each spermatogonium forms a primary spermatocyte, whose genetic set doubles to 2n4c.
- Maturation phase. Includes two consecutive meiotic divisions:
- First (reductional) division: the primary spermatocyte divides, forming two cells—secondary spermatocytes. The chromosome set becomes haploid, but the amount of DNA is still doubled (n2c).
- Second (equational) division: secondary spermatocytes divide, resulting in four equal-sized round cells—spermatids. Their genetic set is nc.
- Differentiation (spermiogenesis) phase. No further divisions occur. Spermatids undergo complex cellular differentiation, grow a flagellum, and transform into fully mature spermatozoa (nc).
Structure of a mature spermatozoon
A spermatozoon is a highly differentiated cell whose cytoplasm is represented only by a thin membrane. Its anatomy is perfectly adapted for reaching and fertilizing the oocyte.
- Head. The main part of the cell. Contains the nucleus with a haploid set of chromosomes and the acrosome. The acrosome is a modified Golgi apparatus filled with enzymes necessary for dissolving the egg membrane upon contact.
- Neck. Contains centrioles. These structures connect the axial filament of the flagellum to the nucleus located in the head.
- Midpiece (intermediate part). Mitochondria are arranged spirally here. Their main task is the synthesis of ATP molecules, providing the cell with energy for active movement.
- Tail. Represented by a long flagellum (in some animal species, there may be several). The flagellum is formed by microtubules, and its beating is driven by the energy of high-energy ATP bonds.
Briefly: answers to topical questions
What are the functions of Sertoli cells (sustentocytes)?
Sertoli cells perform a number of critical tasks for spermatogenesis.
- Supporting function — they form the skeleton of the spermatogenic epithelium.
- Trophic function — providing nutrition to developing germ cells in the adluminal compartment.
- Phagocytic function — utilizing dying germ cells and digesting residual bodies.
- Exosecretory function — secreting fluid into the tubular lumen for sperm transport.
- Coordinating function — regulating the rate of germ cell development and controlling the wave of spermatogenesis initiation.
- Production of transport proteins — synthesis of androgen-binding protein (ABP), testicular albumin, and transferrin.
- Endocrine function — synthesis of the hormone inhibin, growth factors, and estrogens (via aromatization of testosterone).
Which hormones of the anterior pituitary regulate spermatogenesis?
Spermatogenesis is regulated by two pituitary gonadotropic hormones.
- Luteinizing hormone (LH) — stimulates interstitial Leydig cells in the testes, which in turn produce testosterone. Testosterone acts as the main stimulator of spermatogenesis.
- Follicle-stimulating hormone (FSH) — stimulates Sertoli cells (sustentocytes), inducing them to produce androgen-binding protein, which is necessary to maintain a high concentration of testosterone around the spermatogenic cells.
What structures form the blood-testis barrier?
The blood-testis barrier is a multilayered structure extending from the blood vessel to the tubular lumen.
The barrier includes:
- Testicular capillary endothelium (fenestrated) and its basement membrane.
- Interstitial tissue (narrow layers).
- Wall of lymphatic vessels (sinusoidal capillaries forming a "case").
- Proper sheath of the seminiferous tubules.
- Basement membrane of the spermatogenic epithelium.
- Tight junctions between Sertoli cells (zonula occludens).
The mnemonic "Goni Citatu, Citatu Spishi Srazu" helps remember the cell sequence: spermatoGONIA → PRIMARY spermatoCYTE → SECONDARY spermatoCYTE → SPermatids → Spermatozoa.
Class and exam questions
Where exactly are spermatozoa formed?
The process takes place in the walls of the convoluted seminiferous tubules located inside the paired gonads—the testes (testicles).
How does a primary spermatocyte differ from a secondary spermatocyte?
A primary spermatocyte is a diploid cell (2n4c) entering the first meiotic division. A secondary spermatocyte is a haploid cell (n2c) formed after the first division and entering the second.
What function does the acrosome perform?
The acrosome, being a modified Golgi apparatus, contains a set of enzymes. They are necessary to dissolve the protective membranes of the egg and successfully penetrate it.
Which cells create the microenvironment for developing gametes?
This function is performed by specialized somatic cells—sustentocytes, or Sertoli cells, located in the wall of the seminiferous tubule.
What else to study on this topic
- Hormonal regulation of spermatogenesis (role of testosterone)
- Morphological functions of Sertoli cells
- Differences between reductional and equational meiotic divisions
- Oogenesis: comparison of male and female gametogenesis
- Role of interstitial Leydig cells
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