The process of development of megaspores from megaspore mother cells is known as megasporogenesis. The megaspores are developed in the female reproductive organs.
Archesporial cells
Archesporial cells divide into several parietal cells, and one (or sometimes two) sporogenous cells grow in size and become the megaspore mother cell, resulting in the formation of thousands of megaspores (MMC). A prominent nucleus can be seen in this structure, which is located in the nucellus and partially covered by the developing integuments. Fluorescence microscopy reveals that it is surrounded by callose before the onset of meiosis.
A dyad (a large chalazal cell and a small micropylar cell) is formed when the MMC undergoes the first meiotic division. After that, the chalazal member of the dyad appears to undergo the second meiotic division, while the micropylar member appears to remain undivided, resulting in a linear triad of cells.
It is important to note that megaspore tetrads were never observed in any sample, including fluorescence samples, and that the chalazal member always becomes the functional megaspore while the other members degenerate in the presence of oxygen.
The presence of a callose envelope between the protoplasts and the original walls of the triad cells, as well as on the transverse walls between them, was observed, with the degenerating members of the micropylar pole staining more strongly than the other members.
The functional megaspore, as well as the different stages of embryo sac formation (two-, four-, and eight-nucleus phases), all retain a discontinuous and lightly stained callose wall, with its disappearance occurring gradually as the embryo sac grows larger. At the time of embryo sac initiation, degradation in the nucellar cell that comes into contact with the embryo sac is observed for the first time.
Archesporial cells division:
- Archesporial cells divide into several parietal cells, and one (or sometimes two) sporogenous cells grow in size and become the megaspore mother cell, resulting in the formation of thousands of megaspores (MMC).
- A prominent nucleus can be seen in this structure, which is located in the nucellus and partially covered by the developing integuments. Fluorescence microscopy reveals that it is surrounded by callose before the onset of meiosis.
- A dyad (a large chalazal cell and a small micropylar cell) is formed when the MMC undergoes the first meiotic division. After that, the chalazal member of the dyad appears to undergo the second meiotic division, while the micropylar member appears to remain undivided, resulting in a linear triad of cells.
- It is important to note that megaspore tetrads were never observed in any sample, including fluorescence samples, and that the chalazal member always becomes the functional megaspore while the other members degenerate in the presence of oxygen.
- The presence of a callose envelope between the protoplasts and the original walls of the triad cells, as well as on the transverse walls between them, was observed, with the degenerating members of the micropylar pole staining more strongly than the other members.
- The functional megaspore, as well as the different stages of embryo sac formation (two-, four-, and eight-nucleus phases), all retain a discontinuous and lightly stained callose wall, with its disappearance occurring gradually as the embryo sac grows larger.
- At the time of embryo sac initiation, degradation in the nucellar cell that comes into contact with the embryo sac is observed for the first time.
Development
The female flowering plant’s ovules contain a layer of sporogenous tissue, which is responsible for seed formation.
Megaspore mother cells (MMC) are cells derived from sporogenous tissue that differentiate into megaspores. It’s a large, dimorphic, ovoid diploid cell with a prominent nucleus and dense cytoplasm.
The MMC undergoes meiotic nuclear divisions, resulting in the formation of four haploid megaspores.
The linear tetrad refers to the axial arrangement of the megaspores.
Only one of the four megaspores is active, and the other three are degenerating.
At the chalazal end, there are three functional megaspores and one degenerating megaspore.
During megagametogenesis, the single functional megaspore divides mitotically to form the embryo sac, resulting in the formation of the embryo sac.
A megagametophyte is a cell that divides several times to produce eight haploid nuclei for the embryo sac.
Types of Embryo Sac Development
Monosporic
In the majority of angiosperms, the embryo sac develops from a single functional megaspore, with the remaining three megaspores becoming degenerate as they mature. Flowering plants such as Oenothera and Polygonum, for example.
Bisporic
In this type, two megaspores are involved in the development of the embryo sac. Flowering plants include Allium, Scilla, and Trillium, among others.
Tetrasporic
In this type of megaspore, all four megaspores are involved in the development of the embryo sac. Consider the situation of Peperomia.
Conclusion:
Megasporogenesis is the process by which megaspores are produced from the megasporocyte, which is a cell that has undergone meiosis. During meiosis, the megasporocyte nucleus divides and divides again, resulting in the formation of four haploid megaspore nuclei. Meiosis is followed by cytokinesis in the majority of taxa, resulting in the formation of four megaspore cells. This pattern is referred to as monosporic megasporogenesis; it is so named because only one of the four megaspores produced contributes to the development of the female gametophyte.