Sperm, Egg, Ovaries, and Spermatogenesis

Sperm and Egg Characteristics

Sperm are male gametes, representing highly specialized cells. These cells are small and mobile and consist of three parts:

• Head: The most voluminous part of the sperm. It contains the nucleus, with 23 chromosomes, and the acrosome, a cellular organelle that secretes substances that participate in the fertilization of the egg.
• Tail: Consists of a flagellum, a protein structure that allows the movement of the sperm.
• Neck: Lies between the head and tail. It contains numerous mitochondria around the base of the flagellum; these cellular organelles provide the energy needed to travel to the egg. It also contains a centriole, an organelle responsible for controlling movement.

Eggs have 23 chromosomes and have a very different look from sperm:

• The egg cell is a large, round cell; it is at the limit of visibility to the human eye.
• It is immobile.
• It has reserve substances to feed the embryo during the first days after fertilization.

Ovaries and Follicle Development

Ovaries are two bodies, the size and shape of an almond, housed in the abdominal cavity. Each ovary has an internal and external part. Circular structures, called ovarian follicles, are located inside, where eggs are formed. The size of ovarian follicles increases the closer they are to the surface: the most external and largest, known as Graafian follicles, contain eggs that are ready to be expelled. The ovaries also produce female sex hormones.

Spermatogenesis: Sperm Formation

Spermatogenesis, the formation of sperm, takes place in the wall of the seminiferous tubules of the testes, starting from cells called spermatogonia. The whole process takes about two months and, after puberty, occurs almost continuously throughout life. Spermatogenesis takes place in three stages:

1. Initial growth of cells and increase in their numbers.
2. Meiosis.
3. Maturation: In this stage, the four cells obtained by meiosis, which have half the chromosomes of other cells, undergo a differentiation process that transforms them into sperm. The changes that occur are: enlargement, reduction of cell volume, concentration in a small front of the nucleus, and the formation of an organelle that secretes substances, and a flagellum.