Development of Female Gametophyte in Angiosperms
Embryo sac is the female gametophyte of a plant. It develops from megaspore, which is formed inside the megasporangium or ovule.
The process of formation of female gametophyte from megaspore is called megagametogenesis.
During this process, the functional megaspore divides three times by mitotic division.
First mitotic division results in formation of two nuclei which migrate to opposite poles, forming 2-nucleate embryo sac.
Further, mitotic divisions lead to the formation of 4-nucleate and B-nucleate stages of the embryo sac. One nucleus from each pole reaches to the centre to function as polar nuclei. The two polar nuclei fuse to form a definitive nucleus. The three nuclei located at the chalazal pole are called antipodals. On the top three cells form the egg apparatus which consists of two synergids and a round egg cell or ovum. In these mitotic divisions, nuclear division is not followed by cell division within a functional megaspore. After the 8-nucleate stage, cell walls are laid down and a typical female gametophyte or embrya sac is formed. This type of development is called as endospermic. When only one megaspore participates in embryo sac formation, the development is called monosporic.
