Structure of Megasporangium (Ovule)

Definition

Diagram

Parts of Megasporangium

1. Funiculus (Funicle)

• A stalk-like structure that attaches the ovule to the placenta of the ovary wall.
• It contains a vascular strand that transports nutrients to the developing ovule.
• The point where the funiculus joins the body of the ovule is called the hilum.

2. Integuments

• One or two protective outer coverings (layers) that surround the nucellus.
• In most angiosperms, two integuments are present (bitegmic ovule): inner integument and outer integument.
• They do not completely enclose the ovule — they leave a small opening called the micropyle.
• After fertilization, integuments develop into the seed coat (testa and tegmen).

3. Micropyle

• A small pore or opening at one end of the ovule, left by the integuments.
• It serves as the entry point for the pollen tube during fertilization.
• Also involved in water absorption during seed germination.

4. Nucellus

• The central mass of parenchymatous tissue enclosed by the integuments.
• It is the actual sporangial tissue — the megaspore mother cell (MMC) is housed within it.
• Provides nutrition to the developing embryo sac.
• In crassinucellate ovules, the nucellus is well-developed (thick); in tenuinucellate ovules, it is reduced.

5. Megaspore Mother Cell (MMC) / Megasporocyte

• A diploid (2n) cell located in the nucellus.
• Undergoes meiosis to produce four haploid (n) megaspores.
• Only one functional megaspore survives (at the chalazal end); the remaining three degenerate.
• The functional megaspore develops into the embryo sac (female gametophyte).

6. Chalaza

• The basal region of the ovule, opposite to the micropyle, where the integuments and nucellus merge.
• It is the point of attachment of the integuments to the nucellus.

7. Embryo Sac (Female Gametophyte)

• Develops from the functional megaspore by three successive mitotic divisions.
• A mature 7-celled, 8-nucleate embryo sac (Polygonum type) contains:
  • Egg cell (1) + Synergids (2) — at the micropylar end
  • Central cell with 2 polar nuclei (1 cell)
  • Antipodal cells (3) — at the chalazal end

Types of Ovules (based on orientation)

Summary Table

Megasporogenesis — Process of Megaspore Formation

Process Flow Diagram

Step-by-Step Process

Step 1 — Differentiation of Archesporial Cell

• In the nucellus (tissue of the ovule), a hypodermal (subepidermal) cell at the micropylar end becomes enlarged, with a dense cytoplasm and a prominent nucleus.
• This cell is called the archesporial cell.
• The archesporial cell either:
  • Directly functions as the Megaspore Mother Cell (MMC), OR
  • Divides periclinally (parallel to the surface) to form:
    • An outer parietal cell → divides further to form parietal tissue (nutritive)
    • An inner sporogenous cell → becomes the MMC

In tenuinucellate ovules: parietal cell is absent; MMC lies directly below the nucelllar epidermis. In crassinucellate ovules: parietal cell is present and forms several layers.

Step 2 — Megaspore Mother Cell (MMC)

• The MMC is a large, diploid (2n) cell with:
  • A dense, granular cytoplasm
  • A conspicuous nucleus with a prominent nucleolus
  • Thick callosic walls (β-1,3-glucan) that isolate it from surrounding cells
• The callose wall is significant — it cuts off plasmodesmatal connections, allowing the MMC to develop independently.
• Only one MMC is differentiated per ovule (controlled by genetic and epigenetic mechanisms).

Step 3 — Meiosis I (Reductional Division)

• The diploid MMC (2n) undergoes the first meiotic division (Meiosis I).
• This is a reductional division — homologous chromosomes separate.
• Result: Two haploid (n) cells called the megaspore dyad.
• A cell wall is laid between the two cells (though in some taxa it may be absent at this stage).
• At this stage, crossing over occurs during prophase I, contributing to genetic recombination.

Step 4 — Meiosis II (Equational Division)

• Each of the two cells in the dyad undergoes the second meiotic division (Meiosis II).
• This is an equational division — sister chromatids separate.
• Result: Four haploid (n) megaspores.
• These are arranged in a linear tetrad (one above the other, in a row along the micropyle-chalaza axis).
• The arrangement is called a linear or axial tetrad.

In some species, a T-shaped or isobilateral tetrad can also form depending on the plane of the second division.

Step 5 — Linear Tetrad Formation

[ Micropylar end ]
    ┌──────────┐
    │Megaspore 1│  ← degenerates
    ├──────────┤
    │Megaspore 2│  ← degenerates
    ├──────────┤
    │Megaspore 3│  ← degenerates
    ├──────────┤
    │Megaspore 4│  ← FUNCTIONAL (chalazal)
    └──────────┘
[ Chalazal end ]

• Each megaspore is surrounded by a callose wall.
• The four megaspores are all genetically distinct (due to meiotic recombination).

Step 6 — Degeneration of Three Megaspores

• Three megaspores (the three toward the micropylar end) degenerate.
• Their degeneration is controlled by programmed cell death (PCD).
• The callose walls dissolve around the functional megaspore, which is then exposed to the nucellus for nutrition.
• The chalazal-most megaspore survives as the functional megaspore in most angiosperms (Polygonum-type development).

Why the chalazal megaspore? It is closest to the nutrient-supplying chalazal tissue and vascular bundle, giving it a nutritional advantage.

Step 7 — Functional Megaspore

• The surviving functional megaspore is haploid (n).
• It enlarges considerably, absorbing water and nutrients from the nucellus.
• Its callose wall is dissolved and replaced by a cellulosic wall.
• It now undergoes megagametogenesis — three successive mitotic divisions to form the embryo sac (female gametophyte).

Summary of Megasporogenesis

Types of Embryo Sac Development (Post-Megasporogenesis)

Significance of Megasporogenesis

1. Maintains ploidy — meiosis ensures that the egg cell is haploid; after fertilization, the diploid state is restored.
2. Genetic diversity — crossing over during Meiosis I generates genetic variation in offspring.
3. Produces female gametophyte — the functional megaspore is the precursor of the embryo sac, which contains the egg cell for fertilization.
4. Links sporophyte and gametophyte generations — it is the transition point from the diploid sporophyte to the haploid gametophyte in the plant life cycle.

1 MMC (2n) → Meiosis → 4 megaspores (n) → 3 degenerate → 1 functional megaspore (n) → Mitosis (×3) → Embryo sac (7-celled, 8-nucleate)