Embryo Sac: Formation and Classification

The embryo sac, which is the female gametophyte of a seed plant, is a hollow sac inside the nucellus that contains the egg nucleus and other nuclei that produce endosperm when an egg is fertilized. 

• During fertilization, one male nucleus and one egg nucleus combine to create a zygote, which later transforms into the embryo. 
• Megasporogenesis or meiosis-mediated megaspore production in the megasporangium (nucellus) occurs. 
• Megagametogenesis is the process by which a megaspore develops into a megagametophyte that houses gametes.

Read More: Internal Formation of Zygote

Keywords: Embryo Sac, Nucellus, Megasporogenesis, Gametes, Zygote, Cytoplasm, Endosperm. 

---

What is an Embryo Sac? 

[Click Here for Sample Questions]

The female gametophyte, or embryo sac, is an oval component of the ovules of flowering plants. 

• It is believed that an embryo sac forms when the haploid megaspore nucleus breaks. 
• Six haploid cells without cell walls and two haploid nuclei make up this organism.
• Sometimes, the haploid nuclei can combine to form a single cell (the endosperm motor cell). 
• During fertilization, one male nucleus and one egg nucleus combine to form a zygote, which is where the embryo develops.

Read More: Origin and Evolution of Man

---

Explanation of Embryo Sac

[Click Here for Sample Questions]

In a few simple steps, the whole process of embryo sac development can be described –

• Plants that produce four haploid megaspores by meiosis are said to have four megaspore mother cells, also referred to as megaspores. At least one of the spores matures into a haploid female gametophyte (megagametophytes). The megasporangium tissue gives rise to the megaspore mother cell. In flowering plants, the megasporangium is sometimes referred to as the nucellus, while the female gametophyte is also referred to as the embryo sac.
• The cytoplasm surrounding the few (often eight) nuclei found in the female gametophyte of angiosperms, also known as the embryo sac, is not divided by cell walls. One of the many embryonic nuclei serves as the egg in sexual reproduction by fusing with one of the two sperm nuclei supplied by the pollen tube. 
• Two more embryo sac nuclei merge with the second sperm nucleus from the pollen tube. The endosperm, a multicellular layer of the seed that serves as a food storage compartment, is the forerunner of this triple-fusion nucleus.
• The process of double fertilization involves the union of two pollen tube nuclei. The most distinctive characteristic of angiosperms, which is unique to this group. On the other hand, gymnosperms have a multicellular female gametophyte that contains hundreds or even thousands of cells.
•  A single endosperm mother cell can occasionally be created by the fusion of the two haploid, polar nuclei. A zygote, which later becomes the embryo, is created when one male nucleus fuses with one egg nucleus during fertilization. When the secondary male nucleus joins the primary endosperm nucleus, the endosperm nucleus is created. This divide causes the endosperm to develop.

---

Formation of Embryo Sac

[Click Here for Sample Questions]

The embryo sac formation occurs in two stages:

1. Megasporogenesis, in which the haploid megaspore tetrad develops as a result of the meiosis of a single diploid mother cell, takes place in the first stage. Three of them disintegrate, leaving one to endure and later grow into the embryo sac.

Megaspores are created during a process known as megasporogenesis from a megasporocyte that has undergone meiosis. The meiosis of the mega sporophyte nucleus results in the creation of haploid megaspore nuclei, and the sequence is known as monosporic megasporogenesis. This is because the four megaspores that are produced contribute value to the female gametophyte.

2. In the second step, known as megagametogenesis, an 8-nucleate, 7-celled gametophyte is produced after the functioning haploid megaspore undergoes mitosis. Out of the eight nuclei, the polar nuclei travel to the center where they join to form a single diploid cell. The sperm and this single diploid cell then combine to form the triploid endosperm.

The two nuclei change into synergid cells, which gradually degenerate, while the other three expand into antipodal cells.

Read More: Multicellular Organisms

---

Classification of Embryo Sac

[Click Here for Sample Questions]

The embryo sac can be divided into three different categories – 

1. Monosporic: The monosporic or Polygonum-type embryo sac contains four haploid megaspores that are produced during meiosis of the diploid megaspore mother cell in the nucellus. As a result of programmed cell death, three of the megaspores, usually those towards the micropylar end of the nucellus, perish, leaving just one megaspore alive and functional.
2. Bisporic: Meiosis only produces two megaspores, each containing two haploid nuclei, in bisporic embryo sacs because there is no cytokinesis or cell plate growth following the second meiotic division. One functioning megaspore with two haploid nuclei is all that remains once the closest megaspore dies.
3. Tetrasporic: Due to cell plates failing to form during both meiotic divisions, tetrasporic embryo sacs only contain one four-nucleate megaspore.

Also Read:

---

Things to Remember

• The female gametophyte, or embryo sac, is an oval structure present in the ovule of flowering plants.
• During fertilization, one male nucleus and one egg nucleus combine to produce a zygote, which later transforms into the embryo.
• The mature embryo sac is made up of seven cells and eight nuclei.
• The growth of an embryo sac can take many different forms. The most frequent type of embryo sac development is the polygonum type.
• The process through which megaspores (integument ovules) form inside megasporangia is known as megasporogenesis.
• The nucleus, integuments, and funiculus converge in the chalaza, an ill-defined region. The micropyle, or point where the inner integument curls over the nucellus, still has an opening.
• The two polar nuclei finally fuse to form a diploid secondary nucleus.