The embryo sac, also known as the female gametophyte, is a microscopic structure found within the ovules of flowering plants. It serves as the site for fertilization and the initial development of the plant embryo. This structure is fundamental for angiosperm reproduction, containing the egg cell and other associated cells.
Anatomy and Placement
A mature embryo sac consists of seven cells and eight nuclei. At the micropylar end, an egg apparatus contains one egg cell and two synergid cells. The egg cell is the female gamete, while the synergids assist in guiding the pollen tube.
Opposite the micropylar end, at the chalazal end, are three antipodal cells. The function of these cells is not entirely understood, but they may be involved in nutrient absorption. A large central cell occupies the central position, containing two polar nuclei that participate in fertilization.
The embryo sac is housed within the ovule, a small structure located inside the ovary of a flower. The ovule is enclosed by protective layers called integuments, providing a protected environment for reproduction.
Development of the Embryo Sac
The development of the embryo sac involves two main stages: megasporogenesis and megagametogenesis. Megasporogenesis is the initial process where a diploid cell, known as the megaspore mother cell or megasporocyte, undergoes meiosis within the ovule. This meiotic division produces four haploid megaspores, typically arranged in a linear tetrad.
In most flowering plants, specifically in the common Monosporic or Polygonum type of development, three of these four megaspores degenerate, leaving only one functional megaspore. This single functional megaspore then undergoes megagametogenesis, which involves three successive rounds of mitotic nuclear divisions without immediate cell wall formation. This results in an eight-nucleate structure within the single megaspore.
Following these nuclear divisions, cytokinesis (cell wall formation) occurs, leading to the formation of the seven-celled, eight-nucleate mature embryo sac. The two polar nuclei migrate to the center of the central cell, while the egg cell and synergids differentiate at the micropylar end, and the antipodal cells form at the chalazal end. Other types of embryo sac development, such as Bisporic and Tetrasporic, differ in how many megaspores contribute to the final embryo sac structure and the pattern of subsequent nuclear divisions. In bisporic development, two megaspore nuclei contribute, while in tetrasporic development, all four megaspore nuclei participate in forming the embryo sac, leading to variations in the number of initial nuclear divisions or the fate of the megaspores.
Function in Plant Fertilization
The embryo sac plays a direct role in plant fertilization, guiding the pollen tube and facilitating gamete fusion. When a pollen grain lands on the stigma of a flower, it germinates and forms a pollen tube that grows through the style towards the ovule. The synergid cells within the embryo sac secrete chemical signals that guide the pollen tube towards the micropyle, an opening in the ovule.
Once the pollen tube reaches the micropyle, it enters a synergid, which then degenerates. The pollen tube releases its two male gametes (sperm cells) into the embryo sac. Double fertilization then occurs: one male gamete fuses with the egg cell, forming a diploid zygote that develops into the plant embryo. Simultaneously, the second male gamete fuses with the two polar nuclei in the central cell, forming a triploid primary endosperm nucleus. This event forms both the embryo and its nutritive tissue, preparing the ovule for seed development.
Contribution to Seed Formation
Following double fertilization, the structures within the embryo sac undergo transformations, directly contributing to seed formation. The diploid zygote, formed from the fusion of the egg cell and one male gamete, begins to divide mitotically. This series of divisions leads to the development of the embryo, a miniature plant complete with rudimentary root, stem, and leaf structures, contained within the developing seed.
Simultaneously, the triploid primary endosperm nucleus, resulting from the fusion of the second male gamete and the central cell’s polar nuclei, also undergoes rapid cell divisions. This develops into the endosperm, a nutrient-rich tissue that serves as the primary food source for the growing embryo during its development and initial germination. The endosperm provides carbohydrates, proteins, and lipids, sustaining the young plant until it can photosynthesize independently.
The integuments surrounding the ovule harden and develop into the protective seed coat. This outer layer shields the embryo and endosperm from environmental damage. Seed formation, facilitated by the embryo sac, is important for the propagation of flowering plants and is also a major source of food for many organisms, including humans.