Megasporogenesis and the embryo sac (female gametophyte)
A megaspore mother cell divides by meiosis; one megaspore survives and, by three free-nuclear mitoses, forms a 7-celled, 8-nucleate embryo sac. This is monosporic development.
Draw it: from MMC to embryo sac
Megasporogenesis and development of the embryo sac
- 1The ovule forms a single megaspore mother cell (MMC) in the micropylar region of the nucellus.
- 2The MMC divides by meiosis into four haploid megaspores. This is megasporogenesis.
- 3In most flowering plants, one megaspore is functional and three degenerate. Since one megaspore forms the embryo sac, this is monosporic development.
- 4Its nucleus divides by mitosis, and the two nuclei move to opposite poles: the 2-nucleate embryo sac.
- 5Two more mitotic divisions give the 4-nucleate and then the 8-nucleate stages. These are free-nuclear divisions: no cell walls form right after them.
- 6★Walls now form around six nuclei. Three at the micropylar end form the egg apparatus; three at the chalazal end form the antipodals. The other two polar nuclei lie in the large central cell.
★ key step
Cells of a mature embryo sac
| Cell | Number | Position | Later role |
|---|---|---|---|
| Egg cell | 1 | Micropylar end, in the egg apparatus | Fuses with a male gamete to form the zygote |
| Synergids | 2 | Micropylar end, beside the egg | The filiform apparatus guides the pollen tube; one synergid receives the male gametes |
| Antipodal cells | 3 | Chalazal end | No direct part in fertilisation |
| Central cell | 1 cell with 2 polar nuclei | Middle of the sac | Fuses with the second male gamete to form the triploid primary endosperm nucleus |
Do not confuse: 8 nuclei but 7 cells
Every nucleus in the embryo sac is haploid, because the embryo sac grows from a haploid megaspore by mitosis.
How to draw a mature embryo sac
- Draw a long oval sac. Mark the chalazal end at the top and the micropylar end at the bottom.
- At the micropylar end, draw two pear-shaped synergids with thickened tips; label the tips filiform apparatus.
- Draw the egg just above and between the synergids.
- At the chalazal end, draw three small antipodal cells.
- In the middle, draw two nuclei close together and label them polar nuclei; label the large cell around them central cell.
- Title: Mature embryo sac (7-celled, 8-nucleate).
Every grain of rice or wheat on your plate began as one ovule with one embryo sac. Its egg gave the embryo, and its central cell gave the starchy endosperm that we eat.
5 marks: “Describe the development of the female gametophyte from the megaspore mother cell in a flowering plant. Draw a labelled mature embryo sac.” MMC (2n) in micropylar nucellus → meiosis → four megaspores; one functional (monosporic); three free-nuclear mitoses give 2, 4 and 8 nuclei; walls form: egg apparatus, three antipodals, central cell with two polar nuclei; labelled 7-celled, 8-nucleate sac.
2 marks: “Why is a typical angiosperm embryo sac called 8-nucleate but 7-celled?” It has eight nuclei (egg, two synergids, three antipodals, two polar nuclei), but the two polar nuclei share one central cell, so there are only seven cells.
1 marks: “What is the ploidy of the antipodal cells?” Haploid (n).
3-2-1-2 from top to bottom: 3 antipodals, 2 polar nuclei, then 1 egg and 2 synergids. That is 8 nuclei in 7 cells.