Gametogenesis is the development of germ cells into gametes. Gametes are large nonmotile oocytes in females, and small motile sperm in males. Gametogenesis occurs in the gonads: ovaries in females and testes in males. The two main differences between oogenesis and spermatogenesis: prophase arrest; and unequal division.
| Step | Start | Result | Overview |
|---|---|---|---|
| Germ Cell | Germ Cell | Germ cells originate in the earliest embryonic cell divisions and remain distinct. | |
| Mitosis 1 | Germ Cell | 2N Gamete | Germ cells migrate to newly formed gonads and proliferate mitotically into diploid gametes. |
| Mitosis 2 | 2N Gamete | 1° Gamete | Diploid gametes divide mitotically into diploid primary oocytes and primary spermatocytes. |
| Meiosis | 1° Gamete | 1N Gamete | Meiosis reduces the chromosomes to haploidicity. |
| Meiosis 1 | Primary spermatocytes undergo the first meiotic division into haploid secondary spermatocytes. |
|---|---|
| Meiosis 2 | Secondary spermatocytes under the second meiotic division into four haploid spermatids per primary spermatocyte. |
| spermatogonium |
|---|
| ↓ |
| proliferation |
| ↓ |
| 1° spermatocyte |
| ↓ |
| meiosis I |
| ↓ |
| 2° spermatocyte |
| ↓ |
| meiosis II |
| ↓ |
| spermatid |
| ↓ |
| differentiation |
| ↓ |
| sperm |
In spermatogenesis: microtubule-based flagellum are built (for motility); ribosomes and mRNA are lost; and the nucleus is condensed (to stop transcription).
Mammalian spermatocytes are connected by cross-bridges of cytoplasm whilst dividing. This is due to asymmetry of sex chromosomes in males. Half of secondary spermatocytes receive an X chromosome, and the other half receive a Y chromosome.
However, some gene products essential for spermatocyte development are found only on the X chromosome. Cytoplasmic contact allows all 4 secondary spermatocytes to share X chromosome gene products.
| Arrest | Oogenesis begins the first meiotic division but is arrested in prophase for days, months or years. |
|---|---|
| Growth | As Prophase 1 arrest ends, the primary oocyte uptakes yolk from blood and synthesizes proteins, maternal mRNAs, ribosomes, organelles and localized cytoplasmic determinants. This stocks all RNA needed for the first embryonic divisions, and all the embryo’s nutrients until the placenta forms or it self-feeds. |
| Meiosis 1 | The primary oocyte divides meiotically such that one daughter cell receives most cytoplasm (the secondary oocyte) and the other daughter cell receives almost none (1st polar body). |
| Arrest | In many species, the 2nd meiotic division does not occur until the egg is fertilized. |
| Meiosis 2 | The secondary oocyte undergoes a second asymmetrical meiosis divides to produce a large haploid ootid and a 2nd polar body. |
| Mature | The polar bodies degenerate. The large haploid ootid is a mature egg. |
