How is the embryo sac formed?

The embryo sac, also known as the female gametophyte, forms within the ovule of a flowering plant. The process of its development is called megagametogenesis.

Megasporogenesis

• Initiation: It starts with a diploid cell called the megaspore mother cell (also known as megasporocyte) located within the ovule.
• Meiosis: The megaspore mother cell undergoes meiosis to produce four haploid megaspores.
• Selection: Out of these four megaspores, typically three degenerate, leaving one functional megaspore.

Megagametogenesis

• Development: The single functional megaspore then undergoes a series of mitotic divisions without cytokinesis.
• Divisions: It undergoes three rounds of mitosis, resulting in an eight-nucleate structure.
• Organization: These eight nuclei are strategically arranged within the developing embryo sac:
  • Micropylar End: At the end near the micropyle (opening of the ovule), three nuclei form two synergid cells and one egg cell.
  • Chalazal End: At the opposite end, three antipodal cells are formed.
  • Central Cell: In the center of the embryo sac, two polar nuclei are present which eventually fuse to form a diploid central cell.

Structure and Function

• Embryo Sac: The mature embryo sac consists of seven cells with eight nuclei: one egg cell, two synergids, three antipodal cells, and one central cell with two fused polar nuclei.
• Role in Fertilization: The egg cell and the central cell play crucial roles during fertilization. The egg cell fuses with one of the sperm cells to form the zygote, while the central cell fuses with another sperm cell to form the triploid endosperm, which nourishes the developing embryo.

Relevant Links:

• Difference Between Embryo Sac and Endosperm
• Megasporogenesis – An Overview
• What is Embryology?
• Double Fertilization: Process & Significance
• Fertilization in Plants
• What is Triple Fusion?
• Embryo Development