Oogenesis begins in the ovary during the embryonic period at around six to eight weeks. Gametes are derived from primordial germ cells.

After rapid mitotic division of the primordial germ cells, the number of oocytes reach up to 6–7 million by 16–20 weeks. This number falls to 1–2 million by birth. The process of atresia continues and, until the onset of puberty, this number reduces to 300 000 to 500 000.

In the next 35–40 years, 400 to 500 will be selected to ovulate and only a few hundred will remain until menopause.

This process is called apoptosis (programmed cell death).

At birth, all the primary oocytes enter the prophase of first meiotic division and remain suspended.

At puberty around 15 to 20 follicles are recruited each month.

The flat cells surrounding the oocyte proliferate, forming stratified epithelium called granulosa cells.

Granulosa cells are surrounded by a basement membrane separating them from the surrounding the ovarian tissue called theca folliculi.

As the follicle continues to grow, these theca folliculi differentiate into theca interna cells and an outer fibrous capsule called theca externa.

Oocyte and granulosa tissues also secrete a layer of glycoprotein on the surface of the oocyte called the zona pellucida.

Under the influence of hormones, the follicle continues to grow, leading to ovulation.

meiosis I is completed just before ovulation, forming a secondary oocyte and polar body.

The cell then enters into meiosis II and arrests in the metaphase.

meiosis II is completed only after fertilisation, giving rise to a fertilised ovum and a second polar body. See Figure 10.1.

The germ cells in a male infant are situated in the sex cord of the testis.

At puberty, primordial germ cells give rise to spermatogonial stem cells.

At regular intervals, type A spermatogonia are developed from these stem cells. These further undergo several mitotic divisions, giving rise to type B spermatogonia, which in turn divide and give rise to primary spermatocyte.

Primary spermatocyte undergoes meiosis I to form secondary spermatocyte, which further undergoes second meiotic division, forming haploid spermatids.

This spermatid undergoes several changes to form mature spermatozoa.

The approximate time taken for development of spermatozoa from spermatogonia is 74 days. An additional 12–20 days are spent in travelling through the epididymis. See Figure 10.2.