Pre-fertilization in Plant: Formation of Embryo Sac

Pre-fertilization in Plant: Formation of Embryo Sac

1. Formation of Embryo sac

• Megagametogenesis is the process of forming the embryo sac within a flowering plant's ovule. It begins with three rounds of mitotic division, resulting in an eight-nucleate structure.
• The mature sac consists of seven cells: one egg cell, two synergid cells, one central cell, and three antipodal cells. The egg and central cells are crucial for fertilization. 

2. Female gametophyte

• The female gametophyte, commonly called the embryo sac in angiosperms, develops from the megaspore within the ovule.
• It is haploid and contains several specialized cells, including the egg cell, which participates in fertilization.
• The central cell houses two polar nuclei, which fuse with one sperm nucleus to form the triploid endosperm after fertilization.
• The synergids assist in guiding the pollen tube, while the antipodal cells are believed to play a nutritional role in early development.
• The female gametophyte is crucial for sexual reproduction in plants.

3. Development of Female Gametophyte

• Megaspore mother cells undergo meiosis to form linear tetrads of haploid cells, with the lowest one remaining functional and acting as the first cell of female gametophyte.  
• They undergo three successive nuclear mitotic divisions, forming eight nuclei, four at each pole. Polar nuclei migrate towards the center, while egg apparatus consists of a large central egg cell and two supporting haploid synergid cells.  
• The synergid has hair-like projections called filiform apparatus, which guide the pollen tube towards the egg.
• Antipodal cells are three cells at the chalazal end, fused to form a diploid secondary nucleus before fertilization.  
• This seven celled and eight nucleated structure is called an embryo sac. Monosporic development is the method of embryo sac development from a single megaspore.
• Female gametophyte in angiosperms is endosporous within the megaspore, while female gametophyte is colorless and endosporic.

Development of female gametophyte