Megaspore
The formation of megaspores from a megaspore mother cell by reductional division, is termed megasporogenesis. These megaspores then undergo megagametogenesis, a process of cell growth and specialization, ultimately becoming multicellular female gametophytes.
Some pteridophytes, gymnosperms, and angiosperms, which exhibit a heterosporous nature. Heterosporous plants produce two distinct types of spores—microspores and megaspores—via their diploid sporophytes.
Microspores, the smaller of the two types, are generated through microsporogenesis. Diploid microspore mother cells (MMCs) undergo meiotic division to give rise to haploid microspores. These microspores then embark on microgametogenesis, evolving into male gametophytes.
Conversely, megaspores, the larger counterparts, emerge from the process of megasporogenesis. Diploid megaspore mother cells (MMCs) similarly undergo meiotic division, resulting in the formation of haploid megaspores. Subsequently, megaspores undergo megagametogenesis
1.0Structure Of Ovule Or Megasporangium
The ovule, or integumented megasporangium, attaches to the placenta through a thin stalk called the funicle. The ovule and funicle unite at the hilum. The main part of the ovule, the nucellus, is composed of parenchymatous cells rich in reserve food. Within the nucellus is the embryo sac or female gametophyte. The nucellus is enveloped by one or two protective integuments, leaving a small opening called the micropyle at the tip. The chalaza, opposite the micropyle, represents the ovule's base. As a crucial structure in seed plants, the ovule holds the potential for seed development through its intricate components.
2.0Megasporogenesis
In the beginning of this megasporogenesis. Any one cell of nucellus differentiates and increases in size. It becomes different from the rest of the cells due to the presence of a distinct nucleus and acts as a megaspore mother cell (At micropylar region). i.e. a single MMC is differentiated in the micropylar region of nucellus during ovule development. MMC is a large cell with dense cytoplasm and a prominent nucleus. MMC divides meiotically to form four haploid megaspores.
The four haploid megaspores are generally arranged in linear tetrad. In a majority of flowering plants the lowermost or chalazal megaspore remains functional out of four megaspores and the other three which lie towards the micropyle, degenerate. The functional megaspore produces female gametophyte (embryo sac)
3.0Megagametogenesis
The megaspore functions as the initial cell of the female gametophyte and grows by deriving nutrition from the nucellus. The nucleus of the functional megaspore undergoes mitotic division, resulting in two nuclei that move to opposite poles, forming the 2-nucleate embryo sac. Subsequent mitotic divisions lead to the 4-nucleate and then the 8-nucleate stages of the embryo sac. Importantly, these divisions are free nuclear, meaning they aren't immediately followed by cell wall formation.
Two nuclei, one from each pole, migrate towards the center to become polar nuclei. Post the 8-nucleate stage, cell walls are formed, organizing the typical female gametophyte or embryo sac. New cell walls encase six of the eight nuclei, defining individual cells within the structure. After 3 mitosis in megaspore, seven celled and eight nucleated structures are formed. This eight nucleated and seven celled structure is called female gametophyte or embryo sac of Angiosperms.