Megasporangium and embryo sac (female gametophyte)

The ovule (megasporangium) inside the ovary is where the female gametophyte — the embryo sac — develops. Understanding its 7-celled, 8-nucleate structure is central to this chapter.

The ovule (megasporangium)

An ovule is attached to the placenta by a stalk, the funicle; the point of attachment to the body is the hilum. It has one or two protective integuments leaving a small opening, the micropyle, at one end, and a basal chalaza at the other. Inside is the nucellus, a mass of nutritive cells enclosing the embryo sac.

Megasporogenesis and the embryo sac

A single megaspore mother cell (MMC) in the micropylar region of the nucellus undergoes meiosis to form four megaspores — this is megasporogenesis. Usually only the chalazal megaspore stays functional; the other three degenerate. The functional megaspore develops into the embryo sac (monosporic development). Its nucleus divides mitotically three times to give eight nuclei (2 → 4 → 8), arranged as a 7-celled, 8-nucleate structure: at the micropylar end an egg apparatus of one egg cell and two synergids (bearing the filiform apparatus that guides the pollen tube); at the chalazal end three antipodal cells; and a large central cell with two polar nuclei.

Exam Tricks & Tips

• 🎯 Embryo sac = 7 cells, 8 nuclei — the central cell holds two polar nuclei, so 8 nuclei but only 7 cells.
• 🎯 The chalazal (not micropylar) megaspore is the functional one; the other three degenerate.
• 🎯 Egg apparatus = 1 egg + 2 synergids; the synergids' filiform apparatus guides the pollen tube in.
• 🎯 Development is monosporic — the whole embryo sac forms from a single functional megaspore.
• 🎯 Three antipodals at the chalazal end — often asked as 'how many antipodal cells?'
• ❌ Common mistake: writing '8 cells'. There are 8 nuclei but only 7 cells because the two polar nuclei share one central cell.

Quick recap

Ovule: funicle, hilum, integuments, micropyle, chalaza, nucellus. MMC → meiosis → 4 megaspores; chalazal one functional (monosporic). Three mitoses → 8 nuclei, 7 cells: egg apparatus (egg + 2 synergids), 3 antipodals, central cell with 2 polar nuclei.

Cover the answer, recall, then check. 11 cards on the megasporangium and embryo sac.

Q1. By what stalk is the ovule attached to the placenta?
A1. The funicle; the point of attachment to the ovule body is the hilum.

Q2. What is the micropyle?
A2. A small opening at the tip of the ovule formed by the integuments, through which the pollen tube usually enters.

Q3. What is the nucellus?
A3. A mass of nutritive cells within the ovule that encloses the embryo sac.

Q4. What is megasporogenesis?
A4. Formation of megaspores from the megaspore mother cell by meiosis.

Q5. How many megaspores form, and how many are functional?
A5. Four megaspores form; usually only the chalazal one is functional, the other three degenerate.

Q6. Why is embryo sac development called monosporic?
A6. Because the whole embryo sac develops from a single functional megaspore.

Q7. How many nuclei and cells does the mature embryo sac have?
A7. Eight nuclei but seven cells (the central cell has two polar nuclei).

Q8. What makes up the egg apparatus?
A8. One egg cell and two synergids at the micropylar end.

Q9. What is the role of the filiform apparatus of the synergids?
A9. It guides the entry of the pollen tube into the embryo sac.

Q10. How many antipodal cells are there and where?
A10. Three antipodal cells at the chalazal end.

Q11. What does the central cell contain?
A11. Two polar nuclei.

If the pollen grain is a compact male plant, the embryo sac is an even stranger female one — a seven-celled, eight-nucleate structure hidden inside the ovule, holding the single egg that will become the next generation.

Core idea: The ovule (megasporangium) houses the female gametophyte — the embryo sac — which develops by megasporogenesis followed by megagametogenesis.

The ovule (megasporangium)

Structure

The ovule is attached to the placenta by a stalk, the funicle; the junction is the hilum. Its parts:

• Nucellus — central nutritive parenchyma where the embryo sac lies.
• Integuments — one or two protective envelopes that later become the seed coat.
• Micropyle — a small pore at one end (where pollen tube usually enters).
• Chalaza — the base, opposite the micropyle.

Megasporogenesis

The formation of megaspores from the megaspore mother cell (MMC):

A single MMC (2n) in the nucellus undergoes meiosis → four haploid megaspores in a linear tetrad.

Usually three degenerate and only one functional megaspore survives (near the chalazal end). This is monosporic development.

Megagametogenesis — forming the 7-celled, 8-nucleate embryo sac

The functional megaspore enlarges and its nucleus divides by mitosis three times (free-nuclear divisions) → 8 nuclei, then walls form:

• Egg apparatus at the micropylar end = 1 egg cell + 2 synergids (synergids bear the filiform apparatus that guides the pollen tube).
• 3 antipodal cells at the chalazal end.
• 2 polar nuclei in the large central cell.

So: 8 nuclei, 7 cells (the central cell is binucleate). This is the typical (Polygonum-type) embryo sac.

Worked example

Q: How many meiotic divisions produce one typical embryo sac, and how many mitotic divisions follow?
A: One meiotic division of the MMC (giving 4 megaspores, 3 degenerating), then three successive mitotic (free-nuclear) divisions of the functional megaspore → 8 nuclei. Total nuclei in mature sac = 8; cells = 7.

Real-world / exam application

Counting nuclei/cells is a favourite exam trap. Because the egg is single and deep inside, hand-pollination and controlled crossing target it indirectly. In apomixis (asexual seed formation) the diploid embryo sac forms without meiosis — building on this structure.

Exam tricks & mnemonic

• 8 nuclei, 7 cells, 4 types: egg (1), synergids (2), antipodals (3), central cell (1 with 2 polar nuclei).
• Mnemonic for the micropylar egg apparatus: "Two Synergids Sandwich the Egg."
• Filiform apparatus is the synergids' 'GPS' that guides the pollen tube.

Worked Example

Problem: Describe the formation of the embryo sac (female gametophyte) from the megaspore mother cell, and give the cellular organisation of a typical mature embryo sac.

Solution:
Step 1 — Start at the megasporangium (ovule). The nucellus of the ovule contains a single diploid megaspore mother cell (MMC).

Step 2 — Describe megasporogenesis. The MMC undergoes meiosis to produce four haploid megaspores. Typically three degenerate and only one functional megaspore remains.

Step 3 — Describe megagametogenesis. The functional megaspore enlarges and its nucleus divides mitotically three times (without cell wall formation) to give eight nuclei, which then organise into the embryo sac.

Step 4 — Give the organisation of the mature embryo sac. It is 7-celled and 8-nucleate: at the micropylar end an egg apparatus of one egg cell and two synergids; at the chalazal end three antipodal cells; and a large central cell with two polar nuclei.

Answer: Meiosis of the megaspore mother cell gives one functional megaspore, which by three mitotic divisions forms the 7-celled, 8-nucleate embryo sac (egg + 2 synergids, 3 antipodals, and a central cell with 2 polar nuclei).