The embryo sac, also known as the female gametophyte, is the structure that houses the female reproductive cells within the ovule of a flowering plant. It is the site of fertilization. A mature embryo sac is characterized by seven cells containing a total of eight nuclei before fertilization occurs. This arrangement ensures the two distinct fertilization events, which form both the plant embryo and its food source, can be successfully carried out.
The Physical Structure and Polarity of the Embryo Sac
The embryo sac is located deep inside the ovule. This sac exhibits a clear organizational polarity, meaning it has two distinct ends that serve different functional roles. One end is designated the micropylar pole, which faces the micropyle, a small pore where the pollen tube enters. The end directly opposite is called the chalazal end, considered the base of the structure. The arrangement of the seven cells is regulated along this defined axis. This polarity results from the megaspore undergoing three rounds of mitotic division, establishing the eight nuclei before cell walls form around them.
Defining the Seven Cells: Identities and Arrangement
The mature seven-celled structure is a combination of three distinct groups of cells, each positioned according to the sac’s established polarity.
The Egg Apparatus
The micropylar end contains the three cells of the egg apparatus, which are directly involved in fertilization. This apparatus consists of the single, large egg cell flanked by two smaller synergid cells, forming a triangular group. The egg cell is the haploid female gamete, ready to fuse with one of the incoming male sperm nuclei. The two synergid cells assist the pollen tube’s entry and discharge of the sperm. Within the synergids, a complex wall thickening known as the filiform apparatus is present, which is thought to increase the surface area for molecular exchange and guidance.
The Antipodal Cells
The chalazal end, opposite the egg apparatus, is occupied by the three antipodal cells, which are also haploid. These cells are highly variable in appearance and persistence across different plant species. They are often the first cells to degenerate, sometimes even before fertilization is complete.
The Central Cell
The final and largest cell is the central cell, which occupies the vast majority of the embryo sac’s middle volume. This single cell is unique because it contains two separate, haploid nuclei, known as the polar nuclei. These two nuclei do not fuse until fertilization, meaning the central cell is counted as one cell but contributes two of the total eight nuclei.
Functions of the Cells in Double Fertilization
The seven cells of the embryo sac participate in double fertilization, a process unique to flowering plants that forms both the embryo and its nutrient supply. The synergid cells play the initial role in guiding the pollen tube to the egg apparatus. They secrete chemical attractant molecules that direct the growth of the pollen tube toward the micropyle. The pollen tube penetrates one of the synergid cells, which then rapidly degenerates, releasing the two male sperm cells into the embryo sac. This targeted delivery ensures the two sperm cells are positioned correctly for the subsequent fusion events.
One of the two haploid sperm cells then fuses with the haploid egg cell in a process called syngamy. This fusion results in a diploid (2n) cell, the zygote, which will undergo mitosis to develop into the plant embryo. The remaining, second haploid sperm cell travels to the central cell to initiate the second fertilization event.
This second sperm cell fuses with the two haploid polar nuclei, a phenomenon termed triple fusion. The result is a triploid (3n) nucleus, which rapidly divides to form the endosperm, a nutrient-rich tissue that serves as the food source for the developing embryo and, later, the seedling. The antipodal cells are thought to provide some nutritional support to the sac early in its development before they typically degenerate.