Q. What type of cell division takes place in the functional megaspore initially in angiosperms?
Solution:
Ovule is integumented megasporangium in Angiosperm. The Megaspore Mother Cell (MMC) undergoes meiotic division to form four Megaspores.
Out of the four Megaspores, single megaspore towards the micropylar region remains active (so called Monosporic Embryo Sac).
Other three towards the Chalazal end degenerates. The functional megaspore develops into the female gametophyte or the embryo sac. This method of embryo sac formation is termed as monosporic development.
The nucleus of functional megaspore divides mitotically to form two nuclei which move towards the opposite poles, forming the two nucleate embryo sac. Two more sequential mitotic divisions take place resulting in formation of 4-nucleate, and then 8-nucleate stages of the embryo sac. These divisions are completely free nuclear, i.e, nuclear division is not followed by cell wall formation immediately.
After 8-nucleate structure, cell wall formation takes place leading to organization of embryo sac. 6 of 8 nuclei are covered by cell wall. Three cells move towards the micropylar end and constitute the egg apparatus; it includes the egg cell, the female gamete, and two synergids, that have cellular thickenings at the micropylar tip called filiform apparatus, which plays a vital role in guiding pollen tube into the synergids. Three cells are at chalazal end and are called antipodals. The large central cell has two polar nuclei. Thus, a typical mature angiosperm embryo sac has a 7-celled 8-nucleate structure.
Thus, the division occurring in the functional megaspore is homeotypic (equational or mitotic) and only accomplish the karyokinesis and not cytokinesis.
Out of the four Megaspores, single megaspore towards the micropylar region remains active (so called Monosporic Embryo Sac).
Other three towards the Chalazal end degenerates. The functional megaspore develops into the female gametophyte or the embryo sac. This method of embryo sac formation is termed as monosporic development.
The nucleus of functional megaspore divides mitotically to form two nuclei which move towards the opposite poles, forming the two nucleate embryo sac. Two more sequential mitotic divisions take place resulting in formation of 4-nucleate, and then 8-nucleate stages of the embryo sac. These divisions are completely free nuclear, i.e, nuclear division is not followed by cell wall formation immediately.
After 8-nucleate structure, cell wall formation takes place leading to organization of embryo sac. 6 of 8 nuclei are covered by cell wall. Three cells move towards the micropylar end and constitute the egg apparatus; it includes the egg cell, the female gamete, and two synergids, that have cellular thickenings at the micropylar tip called filiform apparatus, which plays a vital role in guiding pollen tube into the synergids. Three cells are at chalazal end and are called antipodals. The large central cell has two polar nuclei. Thus, a typical mature angiosperm embryo sac has a 7-celled 8-nucleate structure.
Thus, the division occurring in the functional megaspore is homeotypic (equational or mitotic) and only accomplish the karyokinesis and not cytokinesis.