Human Reproduction - Gametogenesis (Spermatogenesis and Oogenesis)

Gametogenesis is the process of formation of haploid gametes ($n$) from diploid germ cells ($2n$) in the gonads.

Spermatogenesis occurs in the seminiferous tubules of the testes and begins at puberty due to the increased secretion of $GnRH$.

The sequence of spermatogenesis: Spermatogonia ($2n$) $\xrightarrow{mitosis}$ Primary spermatocytes ($2n$) $\xrightarrow{meiosis-I}$ Secondary spermatocytes ($n$) $\xrightarrow{meiosis-II}$ Spermatids ($n$) $\xrightarrow{spermiogenesis}$ Spermatozoa ($n$).

Spermiogenesis is the transformation of non-motile spermatids into motile spermatozoa, while spermiation is the release of sperm from the seminiferous tubules.

Oogenesis is the process of formation of a mature female gamete. Unlike spermatogenesis, it is initiated during the embryonic development stage.

Primary oocytes ($2n$) are arrested at Prophase-I of meiosis until puberty. At puberty, only $60,000$ to $80,000$ primary follicles are left in each ovary.

Secondary oocyte ($n$) is formed along with the first polar body after the completion of Meiosis-I within the tertiary follicle.

Meiosis-II in oogenesis is only completed upon the entry of a sperm into the secondary oocyte, resulting in an ootid ($n$) and a second polar body.

Hormonal control: $LH$ acts on Leydig cells to stimulate androgens; $FSH$ acts on Sertoli cells to stimulate spermiogenesis. In females, $LH$ surge induces ovulation of the secondary oocyte.