embryosack

Embryosack, or embryo sac, is the female gametophyte of flowering plants. It forms within the ovule's nucellus from a megasporocyte that undergoes meiosis to produce four megaspores, typically only one surviving. The surviving megaspore enlarges and, through successive mitotic divisions, produces eight nuclei arranged into seven cells. In the typical Polygonum-type embryo sac, this includes three antipodal cells at the chalazal end, two synergids near the micropyle, one egg cell, and a central cell containing two polar nuclei. The embryo sac is bordered by the integuments and communicates with the outside via the micropyle, the entry point for the pollen tube.

The embryo sac functions as the site of fertilization and early seed development. When pollen tubes release sperm into the embryo sac, one sperm fertilizes the egg to form the zygote, while another fuses with the two polar nuclei in the central cell to form triploid endosperm—a phenomenon known as double fertilization unique to angiosperms. The zygote then develops into the embryo, and the endosperm provides nourishment during seed formation. After fertilization, the embryo sac generally disintegrates as the seed tissues develop, though remnants may persist in some species.

Variations exist in embryo sac development. The Polygonum type is most common, but some plants exhibit monosporic, bisporic, or tetrasporic patterns, and the number of antipodal cells can vary. Despite these differences, the embryo sac consistently serves as the key organ of female gametophyte function in angiosperms.