Fertilisatsioon

Fertilisatsioon, or fertilization, is the biological process by which a male gamete (sperm) fuses with a female gamete (egg) to form a zygote, initiating development in sexually reproducing organisms. It restores the diploid chromosome number and activates embryogenesis. Fertilization can occur internally, within the female’s body, or externally in the environment.

In animals, fertilization typically involves recognition of compatible gametes, sperm penetration of protective layers around the egg, fusion of membranes, and union of genetic material. In mammals, the acrosome reaction enables sperm to penetrate the zona pellucida, followed by egg membrane fusion and sperm genome delivery. Cortical granule exocytosis helps prevent polyspermy. The zygote then undergoes cleavage divisions to become a multicellular embryo.

In plants, fertilization occurs after pollen reaches the stigma and germinates a pollen tube that grows toward the ovule. Two sperm cells are delivered: one fuses with the egg to form the zygote; the other fuses with the central cell to form the endosperm, a tissue that nourishes the developing embryo (double fertilization). In angiosperms, this process ensures seed formation.

Natural fertilization can be external (e.g., many fish and amphibians) or internal (mammals, birds, many reptiles). Assisted reproductive technologies, such as in vitro fertilization, involve bringing gametes together outside the body to achieve fertilization.