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Gametogenesis

Gametogenesis

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gametogenesis
The process by which gametes (aka, sex cells) mature.
1. Origin and migration 2. Mitosis 3. Meiosis 4. Maturation
1. Origin and migration Primordial germ cells migrate from the yolk stalk to the posterior body wall during week 4.
    • They move through the dorsal mesentery, and travel to the body wall.
    • Here, they bilaterally give rise to the gonadal ridges.
    • The gonadal ridges differentiate to become testes or ovaries.
2. Mitosis At the gonadal ridges, the primordial germ cells proliferate via mitosis.
    • Diploid primordial germ cell divide, produce 2 diploid daughter cells, it has 2 sets of chromosomes (2n).
During fetal months 2-5, the ovaries produce millions of germ cells, called oocytes; most of these cells will degenerate over time. This makes reproductive sense: women ovulate a single oocyte at a time, and ovulation itself is rare in populations where women spend most of their reproductive years pregnant or breastfeeding.
The testes produce sperm cells throughout life; millions of sperm cells are lost with each ejaculation, and only one is needed for fertilization.
3. Meiosis Comprises two rounds of cell division and is unique to the gametes, has two key functions:
    • It reduces the number of chromosomes when diploid cells give rise to haploid cells.
    • It produces genetic diversity via independent assortment of chromosomes and crossing over.
    • Single diploid germ cell undergoes two rounds of cell division (MI and MII), and produces four haploid germ cells.
4. Maturation
Oogenesis
1. Oocyte nests
    • These nests nourish and support them in this early stage.
2. Ovarian Follicle: Follicular cells surround the individual gametes when the oocyte nests break apart.
    • The oogonia enter meiosis I, and become primary oocytes.
    • The granulosa follicular cells produce estrogen (the physiology of the ovarian follicle is addressed in detail, elsewhere).
3. FSH "recruitment" facilitates transformation to the antral stage
    • The antrum develops within the follicular cells, creating a space within the follicular cells.
    • Outer thecal cell layers are established (the exact mechanisms of recruitment and dominant follicle selection remain unknown).
4. LH surge
    • The growing antral follicle continues to release increasing quantities of estrogen; above a given threshold, estrogen triggers release of luteinizing hormone (LH) from the anterior pituitary.
    • LH initiates the final stages of maturation, including the resumption of meiosis.
5. Secondary oocyte
    • Upon completion of meiosis I, the gamete is referred to as a secondary oocyte, which will remain arrested in meiosis II unless fertilization occurs.
6. Ovulation occurs when the oocyte, along with some of the surrounding follicular cells, break free from the ovarian follicle and are expelled from the ovary.
Spermatogenesis
1. Testes cords Comprise primordial germ cells surrounded by Sertoli, aka, nurse cells.
2. Meiosis I The male gametes enter meiosis I at the onset of puberty under the influence of retinoic acid. Type B spermatogonia give rise to primary spermatocytes. 4. Meiosis II Completion of meiosis I produces secondary spermatocytes, which undergo meiosis II to give rise to spermatids. 5. Spermatids transform and become immature sperm cells, with definitive heads and tails. The head holds the DNA and other cell machinery; the tail provides the cell with motility.
Summary of meiotic events
Female:
    • From birth (or sometime before birth) till puberty, the immature gametes enter, then pause, meiosis I. The DNA in these primary oocytes has replicated, but cell division has not occurred, so the cells are designated as 2n, 4 c (two sets of chromosomes, with double the DNA contents).
    • At puberty, FSH recruitment of follicles initiates completion of meiosis I and entry into meiosis II, which is again paused;
    • The product of meiosis I is the secondary oocyte (for simplicity, we ignore the polar bodies, which are addressed elsewhere).
    • The secondary oocyte contains one set chromosomes with double the DNA content: 1n, 2c.
    • Fertilization triggers metabolic activation and completion of meiosis II; the oocyte is now a true haploid with 1 set of chromosomes with their DNA content.
Male:
    • Male fetal gametes, the spermatogonia, are arrested in the pre-meiotic stage; because DNA has replicated, the chromosome number and DNA content is 2n, 4c.
    • Type B spermatogonia give rise to primary spermatocytes, which, after puberty, enter meiosis I.
    • Secondary spermatocytes are characterized by 1 set of chromosomes with twice the DNA content (1n, 2c).
    • After meiosis II completes, the resulting sperm cells are true haploids: 1 n, 1c.

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