fertilisatsiooni

Fertilisation is the union of haploid gametes to form a diploid zygote, initiating embryonic development. It occurs across various kingdoms, including animals, plants and fungi, though the mechanisms differ among groups.

In animals, fertilisation can be external or internal. External fertilisation takes place when gametes are released into the environment, as seen in many fish and amphibians, and success depends on environmental conditions. Internal fertilisation occurs within the female reproductive tract, typically following mating, and provides protection for the developing offspring.

The general sequence involves recognition between sperm and egg, penetration of protective layers by the sperm (often via the acrosome reaction), and fusion of the gamete membranes. In many species, the egg completes meiosis after sperm entry, producing a pronucleus that fuses with the sperm pronucleus to form a diploid zygote. The zygote then enters embryogenesis through cleavage divisions.

In plants, fertilisation usually involves pollen delivering sperm cells to the ovule via a pollen tube. In flowering plants (angiosperms) fertilisation is accompanied by double fertilisation: one sperm fertilises the egg to form the zygote, while another fertilises the central cell to form the triploid endosperm, which nourishes the developing embryo.

Fertilisation is central to sexual reproduction and contributes to genetic variation, enhancing adaptability. Errors in gamete formation or fusion can lead to infertility, embryonic loss, or congenital abnormalities. In medicine, assisted reproductive technologies (ART), such as in vitro fertilisation, are used to treat infertility and support related research.