Spermatogenesis stands for the formation of spermatozoa. It takes place within the testes, in particular, in a round-shaped structure of the testicles called seminiferous tubules. Once the spermatozoids are formed, they are released to the center of the tubule in order to be transported to the epididymis (upper part of the testicle), where the sperm development process will come to its final stage.
Humans require from 60 to 75 days to complete a whole spermatogenesis cycle.
Spermatozoids are male gametes, i.e. male sexual, reproductive cells. They are haploids (containing half of the genetical information). During the spermatogenesis, the 46 chromosome somatic cells will be transformed into 23 chromosomes sexual cells.
For spermatogenesis to occur, an specific hormone control of the hypothalamic-pituitary-adrenal axis (HPA or HTPA axis) is required, favoring thus sperm production. It occurs for the very first time as puberty begins in men. From that moment on, testicles will constantly produce spermatozoids due to the beginning of the waterfall effect among hormones.
There are three fundamental stages in sperm formation: the proliferative phase, the meiotic phase, and the spermiogenesis.
Also known as spermatogonic phase. Type A spermatogonia are formed from a germ mother cell that will divide mitotically to form the type B spermatogonia. Once men reach puberty, these cells will split multiple times to form a cell type called spermatocyte. Throughout these splits some cellular changes happen.
This phase is known as proliferative phase due to the quantity of mitosis (cellular division) that take place. The main goal is to produce as many sperm precursor cells as possible, that is to say, a high number of sperm.
Also known as spermatocytegenesis. In this stage, a different cellular division takes place. Such division is the meiosis, which cuts the genetic information to the half, producing haploid cells called spermatids.
Meiosis can be divided into two main substages:
- Meiosis I: primary spermatocytes divide into two secondary spermatocytes.
- Meiosis II: each secondary spermatocyte divides into two spermatids; therefore, four spermatids (haploid) are obtained from the first primary spermatocytes (diploid).
These cells are increasingly beginning to resemble spermatozoa; they even have a small flagellum.
It's the final stage of the sperm formation process. The maturation of spermatids takes place at this stage.
The tail becomes larger, giving rise to the flagellum that will allow its development. Conversely, the head of the sperm becomes smaller and paddle-shaped, a feature that occurs because of the cytoplasm reduction, the nucleus enlargement, and the acrosome formation.
By the time spermiogenesis finishes, spermatids are fully developed into spermatozoa and are released into the seminiferous tubule.
Hormonal regulation of spermatogenesis
Negative feedback control the spermatogenic progression. The hypothalamus, the pituitary gland, and the testes are the ones operating this mechanism. Hormones intervening in the sperm formation process are mentioned hereafter:
- Testosterone: it is produced by Leydig cells or interstitial cells, which are located within the testicle. Among many other functions, testosterone activates genes that promote spermatid differentiation.
- FSH (follicle-stimulating hormone): it is produced by the pituitary gland and acts directly onto the testicle, specifically on testicular germ cells known as Sertoli cells or "nurse" cells. These cells provide nutritional support to spermatozoa throughout their development.
- LH (luteinizing hormone): it is produced by the pituitary gland as well. Its main function is to activate testosterone production by Leydig cells.
- Inhibin: it is produced in the Sertoli cells in the testis and acts directly on the pituitary gland by inhibiting the synthesis of FSH and thus preventing spermatogenesis to occur.
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