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Spermatogenesis
This MCAT post discusses spermatogenesis, the process by which sperm, the male gamete cell, is formed. This process starts at puberty and continues until the male dies. Note that this fact is in contrast with oogenesis that occurs cyclically in females from puberty to menopause.
Sperm formation requires two conditions. First, the temperature for sperm production must be lower than body temperature. Thus, the scrotum and testicles are not located directly inside the body, and can rise or drop depending on body temperature. The second condition is that spermatogenesis requires high concentrations of testosterone.
The Role of Sertoli Cells, Leydig Cells, and Spermatogonia
The testes, the site of spermatogenesis, contain several types of cells. In the walls of the seminiferous tubules, which make up a majority of the testes, are the Sertoli cells. The Sertoli cells secrete androgen binding protein, which binds testosterone, helping concentrate the testosterone to the high levels necessary for spermatogenesis. Also, scattered in between the seminiferous tubules are the Leydig, or interstitial, cells. The Leydig cells secrete the testosterone that androgen binding hormone will eventually bind. Lastly, there are the spermatogonia, which are the stem cell precursors to sperm. These cells are constantly renewed in males, allowing for spermatogenesis throughout the entire duration of the male’s life from puberty onward.
Process of Spermatogenesis
The process of spermatogenesis (Figure 1) begins with a single spermatogonium, which is a diploid cell containing 46 chromosomes. The spermatogonium undergoes mitosis to form two diploid, identical daughter cells. One of the newly created daughter cells will continuously undergo mitotic division as a spermatogonium, ensuring a constant supply of stem cells. The other cell will become the primary spermatocyte and undergo meiosis. At the end of meiosis I, the diploid primary spermatocyte splits into two haploid secondary spermatocytes, which each contain 23 chromosomes. The secondary spermatocytes then undergo meiosis II, resulting in four haploid daughter cells called spermatids. These spermatids are immature sperm cells and are not yet motile. They will be stored in the epididymis while they mature. Once the maturation process is complete, the mature sperm cells are referred to as spermatozoa (singular spermatozoon). The spermatozoa are haploid, mature sperm cells that are motile, fully functional, and capable of fertilization.
