The embryo sac, also called the female gametophyte or megagametophyte, is the structure that contains the female reproductive cells within the ovule of a flowering plant. This specialized tissue is central to sexual reproduction in angiosperms, providing the environment for fertilization and the initial development of the seed. It is the haploid stage in the plant’s life cycle, meaning its cells contain a single set of chromosomes, setting the stage for creating both the plant embryo and its food source.
The Cellular Structure of the Mature Embryo Sac
The fully developed embryo sac is a highly organized, microscopic structure defined by a unique arrangement of seven cells containing eight nuclei. This configuration is the most common pattern found across flowering plants. The structure is polarized, with distinct groups of cells located at opposite ends.
At the micropylar end, where the pollen tube enters, is the egg apparatus, consisting of three cells. The single egg cell is the female gamete, positioned between two smaller synergids. Synergids possess a finger-like wall thickening called the filiform apparatus, which aids in guiding the incoming pollen tube.
The center of the embryo sac is occupied by a single, massive central cell, which uniquely contains two haploid polar nuclei. These two nuclei typically fuse just before fertilization. At the opposite, chalazal end are three small antipodal cells, which typically degenerate shortly after fertilization.
The Process of Embryo Sac Formation
The development of the embryo sac is a two-phase process that begins with a diploid cell inside the ovule called the megaspore mother cell, or megasporocyte. The first phase is megasporogenesis, which involves meiosis, a cell division that reduces the chromosome number by half. The megasporocyte undergoes meiosis, producing four haploid megaspores.
In the most common type of development, known as the Polygonum type, three of these four megaspores degenerate, leaving only one functional megaspore. The second phase, megagametogenesis, is the maturation of this single functional megaspore into the multicellular embryo sac. This process involves three successive rounds of mitosis.
The nucleus of the functional megaspore divides to form two nuclei, which migrate to opposite ends of the developing sac. These two nuclei then divide twice more, resulting in eight haploid nuclei, four at each pole. Cell wall formation then organizes six of the eight nuclei into the distinct cells of the mature embryo sac. The two remaining nuclei move to the center to form the large central cell.
Function in Double Fertilization
The embryo sac’s primary function is to house the cells necessary for double fertilization, a reproductive process unique to flowering plants. This process begins when a pollen tube, guided by chemical signals secreted by the synergid cells, enters the embryo sac through the micropyle. The synergids direct the pollen tube to discharge its two sperm nuclei.
The two sperm nuclei participate in two separate fusion events. The first sperm nucleus fuses with the egg cell, resulting in the formation of a diploid zygote, which will develop into the plant embryo. Simultaneously, the second sperm nucleus migrates to the central cell and fuses with the two polar nuclei, a process known as triple fusion.
This triple fusion creates a triploid cell, which develops into the primary endosperm nucleus. This nucleus then divides repeatedly to form the endosperm, a nutrient-rich tissue that surrounds and nourishes the developing embryo. The embryo sac is the physical location where the two products of fertilization—the embryo and the endosperm, the seed’s food source—are simultaneously initiated.