megagamete

Megagamete is the larger form of the two gametes in species that exhibit anisogamy, and it is the female gamete in many organisms. It is typically haploid after meiosis and contains substantial cytoplasm and nutrients to support early development of the zygote.

In animals, megagametogenesis occurs by oogenesis in the ovaries. Oogenesis involves asymmetric meiotic divisions that produce one large cell—the egg (secondary oocyte after meiosis I, then the mature ovum after meiosis II)—and smaller polar bodies that are usually discarded. The egg remains stationary in the body, while microgametes (sperm) are produced separately and often are motile. Fertilization by a sperm cell restores diploidy, and the egg contributes most of the cytoplasm and organelles to the early embryo.

In plants, the megagamete is the egg cell within the female gametophyte (embryo sac). It is formed through megasporogenesis and megagametogenesis inside the ovule contained in the ovary. The mature embryo sac typically contains the egg cell along with supportive cells such as synergids and the central cell with polar nuclei. After pollination, sperm delivered by a pollen tube fertilizes the egg to form the zygote, while a second sperm often fuses with the polar nuclei to form the triploid endosperm in angiosperms, a process known as double fertilization.

The megagamete’s large size and resource-rich cytoplasm contrast with the typically small, mobile microgamete, reflecting different strategies for providing resources to the new organism.