EMBRYO SAC

The embryo sac, also known as the female gametophyte, is an oval structure found in flowering plants’ ovules. When the haploid megaspore nucleus splits, an embryo sac is said to develop. It has two haploid nuclei and six haploid cells that are devoid of cell walls.

The haploid nuclei can sometimes merge to create a single cell (endosperm motor cell). One male nucleus and one egg nucleus join during fertilization to produce a zygote, which leads to the development of the embryo. The primary endosperm nucleus and the second male nucleus fuse to produce the endosperm nucleus.

EMBRYO SAC EXPLANATION

Megaspore mother cells, also known as megaspores, are diploid cells in plants that undergo meiosis and produce four haploid megaspores. A haploid female gametophyte develops from at least one of the spores (megagametophytes). The megaspore mother cell develops from the megasporangium tissue. The megasporangium is also known as the nucellus in flowering plants, while the female gametophyte is also known as the embryo sac.

The female gametophyte of angiosperms (known as the embryo sac) is a small structure with only a few (usually eight) nuclei; the cytoplasm associated with these nuclei is not partitioned by cell walls. In sexual reproduction, one of the many nuclei of the embryo sac acts as the egg, combining with one of the two sperm nuclei provided by the pollen tube. The second sperm nucleus from the pollen tube fuses with two additional nuclei of the embryo sac. This triple-fusion nucleus is the predecessor of the endosperm, which is a multicellular food-storage layer of the seed.

Double fertilisation is the process of two nuclei from the pollen tube fusing together. This is the most distinguishing trait of angiosperms, and it is not seen in any other group. Gymnosperms, on the other hand, have a multicellular female gametophyte with hundreds, if not thousands, of cells. In this situation, there is no double fertilisation, and the female gametophyte develops into the seed’s food-storage tissue.

It is made up of two haploid nuclei and six haploid cells without cell walls (2 synergids, 3 antipodal cells, and one egg cell) (polar nuclei). The two haploid, polar nuclei can sometimes fuse together to generate a single endosperm mother cell. One male nucleus combines with the egg nucleus during fertilisation to produce a zygote, which develops into the embryo. The endosperm nucleus is formed when the second male nucleus unites with the primary endosperm nucleus. The endosperm is formed when this split.

Embryo Sac Formation:

Mega gametogenesis is the process of the female gametophyte maturing in plants, during which the megaspore emerges from megasporogenesis and develops into the embryo sac, which houses the female gamete. During the process of megasporogenesis, the growing embryo goes through meiosis, which involves many phases. The surviving megaspore continues through mitosis, resulting in a structure with two nuclei known as a binucleate embryo sac, with each nucleus migrating to opposing ends of the embryo sac. Two rounds of mitosis occur in the haploid nucleus, resulting in four haploid nuclei on either end of the embryo sac. Each set of nuclei in the embryo sac migrates to the center to create the binucleate endosperm mother cell, leaving three nuclei on the micropylar end. The portal nuclei egg cell and synergid cells are surrounded by cell plates.

The megagametophyte is produced by two separate processes from the megaspore mother cell:

Megasporogenesis: Megasporogenesis is the process of producing megaspores from a megasporocyte that has gone through meiosis. Because of the four megaspores that are formed and add value to the female gametophyte, the meiosis of the mega sporophyte nucleus results in the development of haploid megaspore nuclei, and the sequence is known as monosporic megasporogenesis. After the initial meiotic division, cytokinesis occurs, resulting in cells with two haploid nuclei in each of them.

Mega gametogenesis: The embryo sac is created when a functioning haploid megaspore undergoes mitosis to produce an 8-nucleate, 7-celled gametophyte. Out of all eight nuclei, the polar nuclei travel to the center to join and form a single diploid cell at that site (center). When this single diploid cell joins the sperm, the triploid endosperm is created. The other three nuclei develop into antipodal cells, while two develop into synergid cells that progressively degenerate.

Classification of embryo Sac: Based on the number of megaspores, embryo sacs can be divided into three types:

• Monosporic
• Bisporic
• tetrasporic 
• Monosporic: Meiosis of the diploid megaspore mother cell in the nucellus generates four haploid megaspores in the monosporic or Polygonum-type embryo sac. Three of the megaspores, generally those at the nucellus’ micropylar end, die as a result of programmed cell death, leaving only one functioning megaspore.
• Bisporic: Due to the absence of cytokinesis and cell plate development after the second meiotic division in bisporic embryo sacs, meiosis creates only two megaspores, each containing two haploid nuclei. The megaspore closest to the micropyle then dies, leaving just a single functioning megaspore with two haploid nuclei.
• Tetrasporic embryo sacs have a single four-nucleate megaspore because cell plates fail to develop during both meiotic divisions.

Note: Because bisporic and tetrasporic embryo sacs are formed from two or four separate meiotic products, their nuclei are not genetically similar to those of monosporic embryo sacs.

Cellular Anatomy of the Mature Embryo Sac: 

• Egg Cell:  The egg cell is found near the embryo sac’s micropylar end and eventually combines with a sperm nucleus to form a zygote. The egg cell is in close proximity to the two synergids, separated from them only by the plasmalemma or partia1 cell walls. Due to the existence of a huge vacuole at the micropylar end that confines the nucleus and most of the cytoplasm to the chalaza end, the distribution of cytoplasm within the egg cell is highly polarised.
• Synergids: The synergids, which are found on either side of the egg cell, are crucial in fertilisation. Before the sperm nuclei are incorporated into the egg and central cells, the pollen tube releases its contents into one of the synergids.
• Central Cell:  This cell, which is found in the heart of the embryo sac, has two nuclei, a huge vacuole, and numerous cytoplasmic organelles. The polar nuclei of the coenocytic megagametophyte begin at both the micropylar and chalaza ends and move to the center during cellularisation.
• Antipodal Cells: At the chalaza end of the embryo sac, three antipodal cells are situated opposing the egg. The antipodals have not been assigned a defined role during reproduction, however they are considered to be involved in the import of nutrients to the embryo sac.

EMBRYO SAC USE

It is the megaspore of a seed-bearing plant that forms the egg cell or nucleus from which the embryo plant grows following fertilization and gives rise to endosperm.

CONCLUSION

An integrated approach to studying the female gametophyte of angiosperms reveals fresh information on the mechanics of sexual reproduction in higher plants. To answer concerns generated by morphological investigations, researchers are using genetic, molecular, and physiological techniques. These approaches have the potential to uncover new knowledge on ovule and embryo sac development, ovule-embryo sac interactions, and the biology of the female gametophyte.