The female gametophyte, often called the embryo sac, is the microscopic structure containing the female reproductive cells in flowering plants, or angiosperms. It is a highly reduced, haploid generation within the plant life cycle that is entirely dependent on the surrounding maternal tissue. This structure is the physical location where the egg cell resides and is prepared for fertilization. The proper development of the female gametophyte is necessary for the plant to successfully complete sexual reproduction and produce a viable seed.
Structure and Location
The mature female gametophyte is composed of seven cells that contain a total of eight nuclei. This specific organization is known as the Polygonum type, which is the most common form found across diverse flowering plant species. The entire structure is physically located deep inside the ovule, a small, enclosed structure that is itself housed within the ovary of the flower.
The cells are arranged with a distinct polarity, divided between the micropylar end, where the pollen tube enters, and the opposite chalazal end. At the micropylar end, a group of three cells forms the egg apparatus, consisting of one egg cell flanked by two synergid cells. The opposite pole contains three antipodal cells, which are thought to play a role in nutritional support before eventually degenerating.
The remaining single, large cell occupies the center of the embryo sac, known as the central cell. This central cell is unique because it contains two separate nuclei, called the polar nuclei, which migrate from opposite ends of the cell. The arrangement of one egg cell, two synergids, three antipodal cells, and one central cell confirms the characteristic seven-celled structure containing eight nuclei.
The Process of Formation
The development of the female gametophyte begins with a single, diploid cell called the megaspore mother cell, or megasporocyte, located within the ovule. This cell initiates the first phase of development, known as megasporogenesis, a process involving meiotic division. The megasporocyte undergoes meiosis, resulting in the production of four haploid megaspores.
In the majority of flowering plants, three of these four haploid megaspores degenerate, leaving only one functional megaspore. This functional megaspore then begins the second developmental phase, called megagametogenesis. The nucleus of the functional megaspore undergoes three consecutive rounds of mitosis.
These mitotic divisions are initially free nuclear, meaning the nucleus divides, but a cell wall does not immediately form to separate the daughter nuclei. This results in an eight-nucleate structure before the formation of cell walls partitions the cytoplasm. The final positioning and wall formation around six of the eight nuclei establish the mature, seven-celled female gametophyte.
Essential Roles in Reproduction
The female gametophyte’s overarching function is to coordinate the complex reproductive events that lead to seed formation. Its primary role involves housing the female gamete, the haploid egg cell, which is the cell that will ultimately fuse with a male gamete to form the new sporophyte generation. The egg cell is positioned within the egg apparatus, ready to receive one of the two sperm cells delivered by the pollen tube.
The female gametophyte also actively participates in guiding the incoming male gamete toward the egg. The two synergid cells adjacent to the egg contain a structure known as the filiform apparatus, which is a mass of finger-like projections near the pollen tube entrance. This apparatus is responsible for secreting chemical signals that attract the growing pollen tube and direct it precisely to the micropylar opening of the ovule.
Once the pollen tube has been successfully guided, it penetrates one of the synergid cells, which then rapidly degenerates, and releases its two sperm cells into the embryo sac. This delivery initiates the unique reproductive event of angiosperms: double fertilization. Double fertilization involves two separate fusion events occurring almost simultaneously.
In the first event, one of the two sperm cells travels to and fuses with the egg cell, resulting in the formation of a diploid cell known as the zygote. This zygote will undergo cell division and differentiation to become the embryo, which is the rudimentary plant within the seed. The second sperm cell then fuses with the two polar nuclei located within the large central cell.
This second fusion event, often termed triple fusion, creates a triploid cell, meaning it contains three sets of chromosomes. This triploid cell is called the primary endosperm cell, which develops into the endosperm, a specialized tissue. The endosperm’s function is to provide nutrient storage, such as starches, oils, and proteins, that will nourish the developing embryo during its growth and upon seed germination.