Gametogenesis is a process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid
gametes. Depending on the biological life cycleof 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.
Gametogenesis in animals
Animals produce gametes directly through meiosis in organs called
gonads. 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 endodermof the yolk sacmigrate along the hindgut to the gonadal ridge. They multiply by mitosisand once they have reached the gonadal ridge they are called gametogonia. Gametogonia are not longer the same between males and females.
From gametogonia, male and female gametes develop differently - males by
spermatogenesisand females by oogenesis. However, by convention, the following pattern is common for both:
Gametogenesis in gametangia
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:
Gametogenesis 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
antheror after pollinationand growth of the pollen tube. The female gamete is produced inside the embryo sacof the ovule.
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