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Embryo Sac

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Embryo Sac: An Introduction

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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).


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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.

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FAQs on Embryo Sac

1. Why is the development of the embryo sac called monosporic and how many eggs are present in an embryo sac?

During the megasporogenesis process, megaspore mother cells (MMC) go through the process of meiotic division in order to produce four megaspores.


After the degeneration of three megaspores, just one megaspore is left that can carry out its role. Megaspores that are capable of reproduction can develop into either female gametophytes or embryo sacs. The formation of an embryo sac from a single megaspore is an example of monosporic development. A single egg is present inside one embryo sac.

2. What is the difference between an ovule and an embryo sac?

The female gametophyte that creates the embryo sac is the ovule. At least one embryo sac is present in every ovule. The female gametes seen inside the ovule are produced by the embryo sac.


The ovule is the structure in plant biology that, upon fertilisation, transforms into a seed. A developed ovule consists of a food tissue surrounded by one or two integuments, which will serve as the seed coats in the future. The pollen tube can penetrate and release its sperm nuclei into the embryo sac, a large oval cell where fertilisation and development take place, through a small aperture (the micropyle) in the integuments. Each ovule is joined to the stalk (funiculus) bearing it by the base of the ovule.

3. What is the ploidy of cells inside the embryo sac?

All of the cells within the embryo sac or female gametophyte are haploid, meaning that there are 3 antipodals close to the chalaza end and 2 synergids at the micropylar end. At the micropylar end is the egg cell as well. However, the central polar cell—a single cell with two polar nuclei—is diploid. Finally, it has 8 nuclei and 7 cells.

After fusion,

  1. It will become triploid i.e. 3n - Primary Endosperm Nucleus (PEN) 

  2. The egg cell fusing with male gamete becomes diploid i.e. 2n


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