Gametogenesis – Spermatogenesis and Oogenesis

Gametogenesis is a process of producing male and female gametes, carried out by all sexually reproducing organisms. The process involves various multiple stages of division and differentiation and is highly regulated under hormonal control.

Gametogenesis

Gametogenesis produces male and female gametes from the primordial germ cells present inside the fetus’s body. The process involves several stages of division, which are regulated under the influence of sex hormones. The process takes place inside the primary sex organs, i.e., the testis in males and ovaries in females.

What is Spermatogenesis?

Spermatogenesis starts at the age of puberty. These are the marked changes that are under the control of the following hormones.

GnRH (Gonadotropin releasing hormone): At the time of puberty, a significant increase in the level of the hypothalamic hormone GnRH. The increased level of GnRH acts as a signal on the anterior pituitary gland, ultimately producing two gonadotropin hormones:
- Luteinising hormone (LH)
- Follicle-stimulating hormone (FSH)
Luteinising Hormone (LH): LH is known to act on the Leydig cells, which results in the secretion of androgens that ultimately stimulates the process of spermatogenesis.
Follicle-stimulating hormone (FSH): The secretion of FSH results in the release of various factors by Sertoli cells (nurse cells) which helps in the process of Spermiogenesis.

The process of the formation of sperm starts with the division of diploid cells (2n or 46 chromosomes), which ultimately results in the formation of a haploid (n/ 23 chromosomes) spermatozoa (sperm). This process completes by the completion of the following two processes.

Spermatogenesis

The diploid spermatogonia (singular-spermatogonia) cells are present inside the seminiferous tubules and multiply by the process of mitotic division and increase their number. Some of the spermatogonia cells get mature and become primary spermatocytes(2n).

The primary spermatocytes(2n) periodically undergo the meiotic division and produce the haploid secondary spermatocytes(n) after completing the first meiotic division.
The secondary spermatocytes(n) complete the second meiotic division and produces four haploid cells known as spermatids.

Spermiogenesis

It is the process of transformation of a spermatid into mature spermatozoa, also known as sperm cells.

After completion of both the Spermatogenesis and spermiogenesis, the sperm heads become embedded in the Sertoli cells, and they finally get released into the seminiferous tubules by the process called spermiation.

What is Oogenesis?

The process initiates at the embryonic stage of the fetus with the formation of million gamete mother cells, also known as oogonia, with each fetal ovary. The oogonia cells start division and enter into prophase-I of the meiotic division and get temporarily arrested at that stage, known as a primary oocyte.

The primary oocyte becomes the primary follicle after getting surrounded by a layer of granulosa cells.
Till the time of birth, a large number of primary follicles gets degenerated, and at puberty, only 60000-80000 are left in the ovary.
The primary follicle gets surrounded by more layers of granulosa, and new theca gets generated, resulting in the formation of the secondary follicle.
The secondary follicle then transforms into a tertiary follicle which this by a cavity filled with fluid called an antrum.
The tertiary follicle has two layers of theca; the outer theca externa and the inner theca interna.
Inside the tertiary follicle, the primary oocyte grows in size and completes its first meiotic division (first unequal division), resulting in the formation of a secondary oocyte and a tiny polar body that gets degenerated. Since it is an unequal division, the secondary oocyte retains a large amount of the cytoplasm.
Furthermore, the tertiary follicle gets mature and becomes a Graafian follicle.