Question

Describe the process of megasporogenesis up to fully developed embryo sac formation in an angiosperm.

Answer 1

The process of formation of megaspores from the megaspore mother cell is called megasporogenesis. The megaspore mother cell undergoes mitosis to form two nuclei that migrate to opposite poles, forming a 2-nucleate embryo sac.
Further mitotic divisions lead to the formation of 4-nucleate and 8-nucleate stages of the embryo sac. In these mitotic divisions, a nuclear division is not followed by cell division. After the 8-nucleate stage, cell walls are laid down and a typical female gametophyte or embryo sac is formed.
Among the eight nuclei, six are enclosed by cell walls and organized into cells, while the remaining two nuclei (called polar nuclei) are situated below the egg apparatus in a large central cell. Out of the six cells, three are grouped at the micropylar end and constitute the egg apparatus made up of two synergids and one egg cell. The other three cells are located at the chalazal end and are called antipodals. Thus, a typical angiosperm embryo sac after maturity is 8-nucleated and 7-celled.