Gametogenesis: Spermatogenesis and Oogenesis

It is the process by which gametes form in the gonads.Oogenesis and Spermatogenesis are the two forms.

• The process of spermatogenesis
• It is the process by which sperm (spermatozoa) develop in the testis’s seminiferous tubules.
• There are two phases to it:
• A. Spermatid formation: In this, spermatids are produced by sperm mother cells, also known as male germ cells or Spermatogonia.
• B. spermiogenesis: Spermatoid transformation into sperm.

Schematic representation of spermatogenesis

• From each primary spermatocyte, four spermatids are produced.
• Sperm heads are lodged in Sertoli cells to get nutrition after spermiogenesis.
• Following that, they are discharged into the seminiferous tubule lumen. We refer to it as spermiation.
• Hormones’ Function in Spermatogenesis
• Gonadotropin-releasing hormone (GnRH) is released by the hypothalamus.
• GnRH causes the anterior pituitary gland to release two gonadotropins, including follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
• LH promotes androgen secretion by acting on Leydig cells.
• Spermatogenesis is stimulated by androgens.
• FSH increases the release of certain spermiogenesis-related components via acting on Sertoli cells.

Structure of spermatozoa (Sperm)

• About 60 µ (0.06 mm) is the length of a mature sperm.
• A plasma membrane surrounds the entire sperm body.
• Three areas make up a sperm:
• a. Head: Round in form.
• composed of an acrosome and a nucleus.
• Golgi complex forms the acrosome. Lytic enzymes are present in it.
• There’s a neck behind the head.
• b. Middle section: Consists of an axial filament encircled by cytoplasm and mitochondria.
• Sperm motility is powered by energy produced by mitochondria.
• c. Tail: consists of an axial filament in the center.
• The sperm’s tail moves in an undulating manner within the female vaginal canal and aqueous medium.
• During a coitus, a man ejaculates 200–300 million sperm.
• A minimum of 60% of sperm must be of a regular size and shape for normal fertility.
• At least 40% of them need to move vigorously.

2.Oogenesis

• It is the process by which an ovum forms and matures.
• Graafian follicles are the location of it.
• Oogenesis begins in the embryonic stage, when each ovary forms two million oogonia, or egg mother cells.
• After birth, no more oogonia are created or added.
• Oogonia proliferate to generate first oocytes.
• They go into meiosis’s prophase I and are momentarily stopped there.
• To produce a primary follicle, a layer of granulosa cells surrounds each primary oocyte.
• During the period from infancy until puberty, many main follicles deteriorate. As a result, each ovary only contains 60,000–80,000 primary follicles during adolescence.
• In order to create secondary follicles, granulosa cells and a new theca surround primary follicles.
• A tertiary follicle develops from the secondary follicles. Its antrum, or hollow, is filled with fluid. An outside theca externa and an inner theca interna are formed by the theca layer.
• In a tertiary follicle, the primary oocyte develops and goes through its first unequal meiotic division to create a tiny first polar body (n) and a big secondary oocyte (n).
• Thus, the nutrient-rich main oocyte cytoplasm is retained in the subsequent oocyte.
• Whether the initial polar body degenerates or divides further is uncertain.
• The mature follicle (also known as the Graafian follicle) develops from the tertiary follicle.
• The zona pellucida, or secondary oocyte, creates a new membrane.
• Now, the Graafian follicle bursts, releasing the ovary’s secondary oocyte, or ovum. We refer to this as ovulation.

Schematic representation of oogenesis

Structure of ovum (egg)

• Both non-motile and spherical. roughly 0.2 mm across.

• Three membranes surround the ovum:
• a. Plasma membrane: Innermost layer.
• b. Zona pellucida: Outer to the plasma membrane.
• c. Corona radiata: Outer layer formed of follicle cells.