Embryo Sac: An Introduction
The embryo sac, also known as the female gametophyte, is an oval structure found in the ovule of flowering plants. When the haploid megaspore nucleus divides, an embryo sac is said to form. It has two haploid nuclei and six haploid cells with no cell walls. The two haploid, polar nuclei can sometimes fuse to form a single endosperm mother cell.
One male nucleus fuses with one egg nucleus during fertilisation to form a zygote, which develops into the embryo. The endosperm nucleus is formed when the second male nucleus fuses with the primary endosperm nucleus. The endosperm is formed when this divides. In this article, we will dive deep into the structure and role of various structures formed in the embryo sac.
Embryo Sac
Embryo Sac is the female gametophyte of a seed plant, consisting of a thin-walled sac within the nucellus which contains the egg nucleus and other nuclei that produce endosperm upon fertilisation.
Seven cells and eight nuclei make up the mature embryo sac.
There are many different types of embryo sac development. Polygonum type embryo sac development is the most typical type.
Since one of the four megaspores is produced by meiosis, the Polygonum type of embryo sac is monosporic.
When a megaspore is monosporic, its nuclei go through three sequential mitosis cycles, producing an eight-nucleated embryo sac. The seven-celled embryo sac is organised from the eight-nucleate cell.
Megasporogenesis
Megasporogenesis is the process through which megaspores develop inside megasporangia (integument ovules). Megaspores are produced when the megaspore mother cell, i.e., the megasporocyte, undergoes meiotic division. Megaspores that are haploid form a linear tetrad when the megasporocyte divides during meiosis. Three of the four megaspores degenerate in most angiospermic families, but one usually remains functional near the chalaza end (polygonum type).
Megaspore production and the development of the embryo sac occur in the ovule that is emerging from the ovary's placenta (female gametophyte). The following primary components make up the ovule most frequently: Funiculus, the stalk linking the ovule with the placenta; one or two integuments (hence, unitegmic and bitegmic ovules) enclosing the nucellus; and the nucellus, the central body with vegetative cells surrounding the sporogenous cells.
The chalaza, a vaguely defined region, is the area where the nucellus, integuments, and funiculus converge. There is still an aperture where the inner integument curves over the nucellus, which is called the micropyle. The development of the micropyle may involve either one or both integuments. In the ovules, the nucellus size varies. Ovules with larger nucelli are referred to as crassinucellate, whereas those with smaller nuclei are referred to as tenuinucellate.
The nucellus is where archesporium first begins to form. The nucellus's sole hypodermal cell enlarges and changes into the primary archesporial cell. This cell undergoes periclinal division to create the primary parietal cell and the primary sporogenous cell. The main sporogenous cell performs the role of the megaspore mother cell, going through meiosis to create megaspores.
Structure of Embryo Sac
The egg apparatus, which comprises the egg and two synergids, is made up of the three cells at the micropylar end. Three antipodal cells can be found at the other end of the embryo sac. The enormous central cell, which sits between the two groups of cells, is home to two polar nuclei, one of which was derived from each of the two groups of four nuclei.
A diploid secondary nucleus is created when the two polar nuclei eventually unite and fuse. Consequently, the mature embryo sac is a seven-celled entity. Except for the centre cell, which is diploid, all of the cells in the embryo sac are haploid (due to the fusion of two polar nuclei).
Mature Embryo Sac Diagram
Role of Synergids, Antipodal Cells and Egg
Synergids control the expansion of the pollen tube by secreting a number of chemo-tropically active compounds. One of the synergids receives the content from the pollen tube. In the event that the egg is not functioning, it also serves as a prospective egg cell.
The role of antipodal cells is haustorial. They abstain from fertilisation. Antipodal cells deteriorate after conception.
The fertilisation process includes the egg. One male gamete joins the egg nucleus during fertilisation to create a zygote. The zygote splits twice to create an embryo, which is a multicellular structure.
Conclusion
This article gives an insight into the important process of megasporogenesis, the structure of the embryo sac and the role of synergids, antipodal cells and eggs. The embryo sac of angiosperm is very small and has only a few nuclei, usually eight. When combined with one of the two sperm nuclei transported by the pollen tube, one of the numerous embryo sac nuclei acts as the egg in sexual reproduction.
The second sperm nucleus from the pollen tube fuses with two more embryo sac nuclei. This triple-fusion nucleus is a typical precursor of the endosperm, a multicellular food-storage tissue in the seed. Double fertilisation is the process through which the pollen tube's two nuclei merge.
FAQs on Embryo Sac
1. What exactly is an embryo sac in flowering plants?
An embryo sac is the female gametophyte found within the ovule of a flowering plant. It is a microscopic, oval-shaped structure that contains all the necessary cells for reproduction, including the female egg cell. Its main purpose is to host the process of double fertilization, which ultimately leads to the formation of a seed.
2. What is the main function of the embryo sac?
The primary function of the embryo sac is to enable double fertilization. It holds the egg cell, which fuses with a male gamete to form the zygote (the future embryo), and the central cell, which fuses with the second male gamete to form the endosperm (the nutritive tissue for the embryo).
3. How is a typical embryo sac structured?
A mature embryo sac is commonly described as a 7-celled, 8-nucleate structure. It is organised as follows:
- At one end (the micropylar end), there is an egg apparatus, which includes one egg cell and two supporting cells called synergids.
- At the opposite end (the chalazal end), there are three antipodal cells.
- In the middle lies a large central cell that contains two nuclei, known as the polar nuclei.
4. What is the difference between an ovule and an embryo sac?
The key difference is that the ovule is the larger structure that houses and protects the embryo sac. After fertilization, the entire ovule develops into a seed. The embryo sac is the female gametophyte that develops *inside* the ovule and contains the egg cell. You can think of the ovule as the house and the embryo sac as a crucial room inside it where fertilization happens.
5. Why is the development of an embryo sac called 'monosporic'?
It is called monosporic development because the entire embryo sac develops from a single functional megaspore. During its formation, a diploid cell undergoes meiosis to produce four haploid megaspores. Typically, three of these degenerate, and only one megaspore survives to grow and divide, eventually forming the complete embryo sac.
6. What is the ploidy of the different cells inside an embryo sac?
Before fertilization, the cells within the embryo sac have different ploidy levels. The egg cell, synergids, and antipodal cells are all haploid (n). The large central cell is unique; it contains two separate haploid polar nuclei, which fuse during fertilization. Therefore, the embryo sac contains both haploid cells and a binucleate central cell.
7. How does a plant's embryo sac differ from its embryo?
An embryo sac exists *before* fertilization; it is the female gametophyte structure that contains the unfertilized egg. An embryo is what forms *after* fertilization. It is the young, multicellular organism that develops from the zygote when the egg cell is fertilized by a male gamete. Essentially, the embryo sac is the parent structure, and the embryo is the offspring.
8. Do humans or other animals have an embryo sac?
No, embryo sacs are structures found exclusively in flowering plants as part of their reproductive cycle. The term is sometimes mistakenly used in animal biology. In humans, the structure that contains the egg cell is the ovarian follicle, and the developing offspring is called an embryo, but this is completely unrelated to the plant's embryo sac.
