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 mitotic division of haploid gametogenous cells. For example, plants produce gametes through mitosis in gametophytes. The gametophytes grow from haploid spores after sporic meiosis. The existence of a multicellular, haploid phase in the life cycle between meiosis and gametogenesis is also referred to as alternation of generations.
Animals produce gametes directly through meiosis in organs called gonads (testicle in males and ovary in females). Males and females of a species that reproduces sexually have different forms of gametogenesis
However, before turning into gametogonia, the embryonic development of gametes is the same in males and females.
Gametogonia are usually seen as the initial stage of gametogenesis. However, gametogonia are themselves successors of primordial germ cells. During early embryonic development, primordial germ cells (PGCs) from the dorsal endoderm of the yolk sac migrate along the hindgut to the gonadal ridge. They multiply by mitosis and once they have reached the gonadal ridge in the late embryonic stage, they are called gametogonia. Gametogonia are no longer the same between males and females.
From gametogonia, male and female gametes develop differently - males by spermatogenesis and females by oogenesis. However, by convention, the following pattern is common for both:
|Cell type||ploidy/chromosomes in humans||DNA copy number/chromatids in human[Note 1]||Process|
|gametogonium||diploid (2N)/46||2C before replication, 4C after / 46 before, 46 X 2 after||gametocytogenesis (mitosis)|
|primary gametocyte||diploid (2N)/46||2C before replication, 4C after / 46 before, 46 X 2 after||gametidogenesis (meiosis 1)|
|secondary gametocyte||haploid (1N)/23||2C / 46||gametidogenesis (meiosis 2)|
|gametid||haploid (1N)/23||1C / 23|
|gamete||haploid (1N)/23||1C / 23|
Fungi, algae and primitive plants form specialized haploid structures called gametangia where gametes are produced through mitosis. In some fungi, for example zygomycota, the gametangia are single cells on the end of hyphae and acting as gametes by fusing into a zygote. More typically, gametangia are multicellular structures that differentiate into male and female organs:
In flowering plants
In flowering plants, the male gamete is produced inside the pollen grain through the division of a generative cell into two sperm nuclei. Depending on the species, this can occur while the pollen forms in the anther or after pollination and growth of the pollen tube. The female gamete is produced inside the embryo sac of the ovule.
- Sources are mixed when using this system of numbering. Some sources use the chromatid number when writing "n" rather than the ploidy number. Hence gametogonium and primary gametocyte would be "4n" rather than "2n" with "4c" for copies. The system used below has been determined by wikipedia consensus and should not necessarily be used as the definitive source on the issue.