Megasporangium, Megasporogenesis, and Female Gametophyte

1. Megasporangium (Nucellus)

Definition and Location

Structure of the Ovule

Ovule Components:
├── Funiculus — stalk attaching ovule to placenta
├── Hilum — point of funiculus attachment
├── Integuments — protective outer layers (1 or 2)
│   ├── Outer integument
│   └── Inner integument
├── Micropyle — small pore at integument tip (for pollen tube entry)
├── Nucellus (= Megasporangium) — central tissue mass
│   └── Contains the megaspore mother cell (MMC)
└── Chalaza — basal region where integuments merge with nucellus

Types of Ovules Based on Orientation

Types of Nucellus

• Tenuinucellate ovule: Nucellus consists of only 1–2 layers of cells; the MMC is subepidermal or epidermal. Common in advanced angiosperms.
• Crassinucellate ovule: Nucellus is multi-layered and thick; the MMC is deeply embedded. Common in primitive angiosperms and gymnosperms.

2. Megasporogenesis

Step-by-Step Process

Step 1 — Differentiation of the Megaspore Mother Cell (MMC)

• Also called the megasporocyte or archesporial cell
• One cell in the nucellus (usually hypodermal/subepidermal) becomes enlarged, dense cytoplasm, prominent nucleus → this is the MMC
• The MMC is diploid (2n)
• In tenuinucellate ovules, the MMC may be directly the archesporial cell; in crassinucellate, the archesporial cell first divides to give a primary parietal cell and the MMC

Step 2 — Meiosis I (Reductional Division)

• The MMC undergoes meiosis I (first meiotic division)
• Result: 2 haploid cells (dyad)
• A cell plate forms between them, creating a dyad of haploid cells

Step 3 — Meiosis II (Equational Division)

• Both cells of the dyad undergo meiosis II
• Result: 4 haploid cells arranged in a linear tetrad (usually) → called the megaspore tetrad

MMC (2n)
    ↓ Meiosis I
Dyad (n + n)
    ↓ Meiosis II
Linear Tetrad of 4 Megaspores (n)
    [Chalazal end] [  ] [  ] [Micropylar end]
         ↑
    Functional megaspore (survives)

Step 4 — Degeneration of Three Megaspores

• Of the 4 megaspores, 3 degenerate (typically the 3 micropylar megaspores)
• The chalazal-most megaspore (farthest from the micropyle) is the functional megaspore
• This is the monosporic pattern (Polygonum type — most common in angiosperms)

Why the chalazal megaspore survives:

• It is closest to the nutrient-rich chalazal tissue
• It receives vascular supply and metabolic support
• The micropylar megaspores are cut off from this supply and degrade

Arrangement Variants of Megaspore Tetrad

3. Female Gametophyte (Embryo Sac)

Types of Embryo Sac Development

The Polygonum Type (Most Important — Monosporic, 8-Nucleate)

Development (3 successive mitotic divisions of functional megaspore)

• Functional megaspore (n) → undergoes mitosis → 2 nuclei
• No cell wall forms; nuclei migrate to opposite poles
• A large central vacuole develops

• Each nucleus divides again → 4 nuclei (2 at each pole)

• Each pair divides again → 8 nuclei (4 at each pole)
• One nucleus from each pole migrates to the center → these become the polar nuclei

Final Structure of the Mature Embryo Sac

MICROPYLAR END
┌─────────────────────────────┐
│   EGG APPARATUS (3 cells)   │
│   ┌──────┐  ┌──────────┐   │
│   │Synerg│  │  Egg Cell│   │  ← The actual female gamete (n)
│   │id 1  │  │  (Oosphere) │  │
│   └──────┘  └──────────┘   │
│      Synergid 2             │
│                             │
│     CENTRAL CELL            │
│   ┌─────────────────────┐   │
│   │  Polar Nucleus 1    │   │  ← (n)
│   │  Polar Nucleus 2    │   │  ← (n) → together form the secondary nucleus (2n)
│   └─────────────────────┘   │
│                             │
│   ANTIPODAL CELLS (3 cells) │
│   [ A1 ]  [ A2 ]  [ A3 ]   │  ← (n) each; degenerate after fertilization
└─────────────────────────────┘
CHALAZAL END

Cell-by-Cell Description

Synergid Structure — Filiform Apparatus

• Increases surface area for absorption and secretion
• Secretes pollen tube attractants (e.g., LURE peptides in Torenia)
• One synergid degenerates before or during pollen tube entry to receive the pollen tube

Double Fertilization in Angiosperms

Pollen tube delivers 2 sperm nuclei into embryo sac:

Sperm 1 + Egg cell (n)           → Zygote (2n)        → Embryo
Sperm 2 + Polar nuclei (n + n)   → Primary endosperm  → Endosperm (3n)
                                    nucleus (3n)

4. Comparison: Monosporic vs. Bisporic vs. Tetrasporic Embryo Sacs

5. Summary Flow Diagram

OVULE (megasporangium = nucellus inside)
         │
         ▼
Megaspore Mother Cell (MMC) — diploid (2n)
         │
         ▼ Meiosis I & II
4 Megaspores (linear tetrad) — all haploid (n)
         │
         ▼ 3 degenerate (micropylar)
Functional Megaspore (chalazal) — (n)
         │
         ▼ 3 mitotic divisions (Polygonum type)
8 Nuclei → cellularization
         │
         ▼
Mature Embryo Sac:
  • 3 Antipodal cells (chalazal)
  • 2 Polar nuclei → Central cell
  • 1 Egg cell        ┐ Egg apparatus
  • 2 Synergids       ┘ (micropylar)
         │
         ▼ Double fertilization
  Embryo (2n) + Endosperm (3n)

Key Points to Remember

1. Megasporangium = Nucellus in angiosperms — the site where megasporogenesis occurs
2. Megasporogenesis = meiosis of MMC → 4 megaspores; 3 degenerate, 1 (chalazal) is functional
3. Female gametophyte = Embryo sac — develops from functional megaspore by 3 mitotic divisions (Polygonum type)
4. The mature embryo sac has 7 cells, 8 nuclei (Polygonum type)
5. Synergids have the filiform apparatus and guide the pollen tube
6. Double fertilization is unique to angiosperms: egg + sperm → embryo; polar nuclei + sperm → endosperm
7. The antipodal cells have no direct role in fertilization; they help in nutrition
8. In gymnosperms, the female gametophyte is much larger and multicellular (archegonia-bearing), less reduced than in angiosperms