Cell that is only half the genetic material required to make a complete creature, such as a gamete, sexual, or reproductive cell; a cell that contains only one set of different chromosomes; (i.e., haploid). Meiosis (reduced division) is the process by which gametes are produced. During this process, a germ cell undergoes two fissions, culminating in the generation of four gametes. A diploid zygote (having paired chromosomes) is produced when the male and female gametes fuse during fertilisation. Fertilization is the process by which male and female gametes fuse.
Gametes may be identical in form (isogamy), as in certain species of algae, fungi, and protozoans, or they may be heterogeneous in form (heterogamy, or anisogamy), as in many green algae of the genus Chlamydomonas. In some cases, gametes may be identical in form (isogamy). Animal gametes, some algae and fungus, and all higher plants exhibit an advanced form of heterogamy known as oogamy, which is a type of heterogamy that has evolved over time. In oogamy, one of the gametes (the sperm) is small and motile, whereas the other gamete (the egg) is huge and nonmotile (the egg).
Gamete development
The mechanisms of generation, migration, proliferation, differentiation, and ovulation/spermiation, which are all comparable in fishes, are all involved in gamete development. The majority of these events occur before meiosis begins, although meiosis is halted in prophase I after the oocyte and spermatocyte are created.
During the meiotic phase, germ cells undergo considerable structural and functional changes (differentiation). Spermatogenesis is named cystic in most species because the entire process takes place inside cysts, where the germ cells develop in lockstep until the cysts open and release the spermatozoa into the testis lumen. Other species have cysts that open and release developing germ cells into the lumen before they mature into spermatozoa (semi-cystic spermatogenesis).
The development of female gametes from oogonia to eggs, known as oogenesis, is divided into two stages: main and secondary growth. During the individual immature phase, when gamete growth is gonadotropin independent, only oocytes in the primary growth stage are present. As a result, oocytes can persist for several years at this stage, at least the entire juvenile phase, however they can even be found in mature fish in iteroparous species.
The gonadotropin-dependent oocyte growth characterises secondary growth, which is separated into three developmental stages: cortical alveoli (CA), vitellogenic (VIT), and oocyte maturation (OM). The presence of cortical alveolar vesicles marks the first stage; however, not all species develop these vesicles.
During this stage, the oocyte grows in size and prepares the organelles needed to sequester vitellogenin in preparation for future vitellogenesis (the yolk precursor). The oocyte grows in size during the vitellogenic stage as the cytoplasm fills up with yolk granules or globules. Vitellogenesis is the most energy-intensive and time-consuming of the secondary growth stages, lasting anywhere from a few days in species with abundant energy (e.g., Merluccius merluccius, Sardina pilchardus) to several months in species with unfavourable periods (e.g., Merluccius merluccius, Sardina pilchardus) (e.g., winter in cold waters, as in Gadus morhua). Vitellogenesis does not occur in matrotrophic viviparous animals because embryo sustenance is provided by the mother throughout the pregnancy rather than from stored yolk.
Finally, oocyte maturation (when meiosis resumes) is a fast (less than one day) process that signals ovulation is approaching and includes two events: germinal vesicle migration (GVM) and germinal vesicle breakdown (GVB) (GVBD). The production of huge oil droplets or lipid coalescence, yolk coalescence, and hydration may also be part of OM in some species.
In marine conditions, a considerable increase in volume due to rapid water uptake, is prevalent in pelagophils (species that produce pelagic eggs). The follicle (the nursing layers encircling the oocyte) ruptures and collapses during ovulation, allowing mature oocytes to be released into the ovarian lumen and fertilised. Because ovulated eggs are only viable for a short time and are fertilised externally to the female reproductive organs, spawning (the releasing of eggs) occurs soon after ovulation in oviparous animals. The term “spawning” isn’t used in viviparous fish, because ovulation is followed by copulation for internal fertilisation.
Postovulatory follicles (POFs) are ruptured follicles that remain in the ovary after ovulation and resorb at temperature-dependent rates. The energy investment becomes irreversible after the oocyte reaches its final maturation stage. Follicular atresia, a degenerative disease that primarily affects vitellogenic and fully mature oocytes but has been seen in previtellogenic oocytes, can resorb oocytes before that time. The occurrence of atresia is frequently linked to poor fish health as a result of malnutrition.
When iteroparous species reach maturity, they all go through reproductive cycles, while in semelparous species, the life cycle finishes with spawning. For most species, these cycles involve numerous phases of gonad development, a spawning capable phase (synonymous with the spawning period), a regressing phase, and finally a regenerative phase, which indicates reproductive dormancy. If the fish lives, it will begin the next breeding season (ripening) in the developing stage, rather than the immature stage, because maturation only happens once in a lifetime.
Conclusion
Gametes are the reproductive cells of an organism. They’re also known as sex cells. Sperm refers to male gametes, whilst ova or egg cells refers to female gametes. Gametes are haploid cells, meaning they have only one copy of each chromosome. These reproductive cells are created through meiosis, a type of cell division.
During meiosis, a diploid parent cell with two copies of each chromosome undergoes one round of DNA replication followed by two cycles of nuclear division, yielding four haploid cells. These cells have the potential to develop into sperm or ova. Male sperm develops in the testes, while female ova form in the ovaries. Each sperm cell is a small, mobile sperm cell known as a spermatozoon. A spermatozoon’s flagellum is a tail-shaped structure that helps the cell propel itself and move. On the other hand, each egg cell, or ovum, is quite large and non-motile. During fertilisation, a spermatozoon and an ovum combine to form a new diploid organism.