Gametogenesis

Gametogenesis:

Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic division of diploid gametocytes into various gametes, or by mitosis.

The testis and ovary produce the male and female gametes respectively by gametogenesis (spermatogenesis in males and oogenesis in females).

Spermatogenesis:

Diagrammatic sectional view of a seminiferous tubule

Ø In males, sperms are produced by the spermatogonia (immature germ cells), which are present in the inner walls of the seminiferous tubules.

Ø Spermatogonia increase in number by mitosis. These are diploid.

Ø Some of the spermatogonia called primary spermatocytes periodically undergo meiosis.

Ø After the first meiotic division, two haploid and equal secondary spermatocytes are formed.

Ø These further undergo meiosis to give rise to four haploid spermatids.

Ø These spermatids are converted into sperms by spermiogenesis.

Ø The sperm head gets embedded in the Sertoli cells after spermiogenesis and is released from the seminiferous tubules by spermiation.

Ø Spermatogenesis starts at puberty by the action of the gonadotropin releasing hormone (GnRH), which in turn causes the release of two gonadotropins called Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH).

Ø LH acts on Leydig cells and causes them to release androgens, which stimulate the process of spermatogenesis while the FSH acts on the Sertoli cells, which help in spermiogenesis.

Structure of a Sperm:

• A mature sperm consists of:
• Head
• Neck
• Middle piece
• Tail

Sperm cell

Ø The whole sperm is enclosed in a plasma membrane.

Ø The head consists of a haploid nucleus and a cap-like acrosome, which contains enzymes that aid in fertilisation.

Ø The middle piece contains several mitochondria, which produce energy for the motility of the sperm.

Ø Sperms released by the seminiferous tubules are transported by the accessory ducts.

Ø Secretions of epididymis, vas deferens, seminal vesicles, and prostate are essential for maturation and motility of sperms.

Oogenesis:

Ø The ovum is formed by the process of oogenesis.

Ø It starts during embryonic growth and millions of gamete mother cells (oogonia) are formed in the foetal ovary.

Ø These cells undergo meiosis, but get temporarily arrested at the prophase and are called primary oocytes.

Ø Before reaching puberty, a large number of primary oocytes degenerate and the remaining ones get surrounded by layers of granulosa cells and new theca and are called secondary follicles.

Ø The secondary follicles are then converted into tertiary follicles that have characteristic fluid-filled cavity called antrum. At this stage, the primary oocyte present within the tertiary follicle completes meiosis, which results in the formation of haploid secondary oocyte and a tiny polar body.

Ø This tertiary follicle further changes into the Graafian follicle. The secondary oocyte is surrounded by the zone pellucida.

Ø Then the Graafian follicle ruptures to release the ovum by ovulation.