Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes.

Gametogenesis, the process of forming gametes (sex cells), has two main types: spermatogenesis (formation of sperm) and oogenesis (formation of eggs). These processes occur in the gonads (testes and ovaries, respectively) of sexually reproducing organisms.

Spermatogenesis

Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testicle.

1. Spermatocytogenesis (Proliferation & Differentiation):

• This phase involves mitotic divisions of spermatogonia (stem cells) to increase the number of germ cells. 

• Spermatogonia differentiate into primary spermatocytes, which are still diploid (containing two sets of chromosomes). 

2. Meiosis:

• Primary spermatocytes undergo meiosis I, resulting in two haploid secondary spermatocytes (containing one set of chromosomes). 

• Secondary spermatocytes then undergo meiosis II, producing four haploid spermatids. 

3. Spermiogenesis (Differentiation of Spermatids): 

• This is the transformation of round, relatively undifferentiated spermatids into mature, motile spermatozoa (sperm cells). 

• This process includes:

  • Golgi phase: Formation of the acrosome (a cap-like structure on the sperm head). 

  • Cap phase: The acrosome covers the nucleus. 

  • Tail formation: Development of the flagellum (tail). 

  • Maturation: Shedding of excess cytoplasm and final shaping of the sperm cell. 

• Mature spermatozoa are then released into the lumen of the seminiferous tubules (spermiation). 

Oogenesis

Oogenesis or ovogenesis is the differentiation of the ovum into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated before birth during embryonic development.

Multiplication Phase: 

• Oogonia (diploid cells) are formed from primordial germ cells in the developing ovary via mitosis.

• These oogonia then divide mitotically, increasing their number.

Growth Phase: 

• Oogonia transform into primary oocytes.

• The primary oocyte grows in size, accumulating cytoplasm and yolk.

• This phase involves significant nuclear and cytoplasmic growth.

• The nucleus of the primary oocyte is referred to as the germinal vesicle.

Maturation Phase:

• The primary oocyte undergoes meiosis I, resulting in two unequal cells: a large secondary oocyte and a smaller first polar body. 

• Both cells contain a haploid (n) number of chromosomes. 

• The secondary oocyte then undergoes meiosis II, but it arrests at metaphase II. 

• Meiosis II is only completed if fertilization occurs, producing a mature ovum and a second polar body.