Gamete Formation: Spermatogenesis and Oogenesis Details

The Formation of Gametes: Gametogenesis

The process of gamete formation is called gametogenesis. It develops in distinct stages:

• Phase of Proliferation or Multiplication: The generative cells divide by mitosis to form successive stem cells (diploid, 2n) of future male and female gametes, called oogonia and spermatogonia.
• Phase of Growth: The spermatogonia and oogonia grow and become primary spermatocytes and oocytes, which are still diploid (2n).
• Meiotic or Maturation Phase: The primary spermatocytes and oocytes enter meiosis.
• Phase of Differentiation: The final sperm and ova are formed. Although the process shares similarities, distinct differences exist between spermatogenesis (sperm formation) and oogenesis (egg formation).

Spermatogenesis

In men, the proliferative phase of spermatogenesis occurs throughout their entire reproductive life. Spermatogonia continuously enter the growth phase and become primary spermatocytes. After the first meiotic division, each primary spermatocyte forms two secondary spermatocytes (n). Each secondary spermatocyte then forms four spermatids (n).

Sperm Structure

The cytoplasm of the spermatid degenerates as the nucleus enlarges to form the sperm head. At its tip, a prominence derived from the Golgi apparatus forms the acrosome, which contains lytic enzymes necessary to dissolve the egg membrane. The tail consists of a long flagellum, containing abundant mitochondria that provide the energy for movement.

Oogenesis

In women, oogenesis occurs during early embryonic development. The primary oocytes are surrounded by follicular cells, forming primordial follicles, which remain inactive until puberty. From each primary oocyte forms one secondary oocyte (n), which receives most of the maternal cytoplasm, and a degenerative cell, the first polar body. The second meiotic division results in a single mature egg (n) and a second polar body. Only the secondary oocyte enters the maturation phase periodically.

Egg Structure

The mature egg retains most of the cytoplasm, containing a complex mixture of substances in the yolk reserves and small vesicles derived from Golgi cortical granules. Around the plasma membrane is a transparent envelope, the zona pellucida, surrounded by a crown formed by follicular cells (the corona radiata). The nucleus is located at one pole, forming the germinal vesicle.