“From Megasporangium to Embryo Sac: The Journey of the Female Gametophyte in Angiosperms”

Megasporangium: Structure and development of Ovule

What is megasporangium?

In angiosperms, the ovule itself functions megasporangium.

It is the structure that produces megaspores, which develop into the female gametophyte(embryo sac). The ovule is located inside the ovary of the flower.

🌱 Structure of a typical ovule

A typical angiosperm ovule consists of:

1️⃣ Funiculus

Stalk that attaches ovule to placenta.

2️⃣ Hilum

Junction between funiculus and ovule body.

3️⃣ Integuments

Protective layers surrounding nucellus.

Usually two (bitegmic).

Leave a small opening called micropyle.

4️⃣ Micropyle

Opening through which pollen tube enters.

5️⃣ Nucellus

Central mass of parenchymatous tissue.

Functions as megasporangium.

6️⃣ Chalaza

Region opposite to micropyle where integuments meet.

🌺 Development of Ovule

Ovule develops from a small mass of meristematic tissue on placenta.

Steps:

1.A hypodermal archesporial cell differentiates in nucellus.

2.It divides periclinally into:Primary parietal cell Primary sporogenous cell

3.Sporogenous cell becomes Megaspore Mother Cell (MMC).

4.Integuments develop from chalazal region.

5.Ovule becomes anatropous in most angiosperms (inverted during development).

Megasporogenesis

 Definition

Formation of megaspores from Megaspore Mother Cell (MMC) through meiosis.

 Process

1.MMC (2n) undergoes meiosis.

2.Forms a linear tetrad of four haploid megaspores (n).

3.Usually, three degenerate.

4.One functional megaspore remains (monosporic type).

5.This functional megaspore develops into the embryo sac.

Structure of Mature Embryo Sac (Ultra-structure Overview)

🔹 Micropylar End:

1 Egg cell

2 Synergids (with filiform apparatus)

🔹 Central Cell:

2 Polar nuclei (fuse → secondary nucleus)

🔹 Chalazal End:

3 Antipodal cells

Types of Embryo Sac Based on Development

Classification is based on number of megaspores participating in embryo sac formation.

1️⃣ Monosporic Type

Develops from one megaspore.

Most common type.

Example:

Liliaceae

Solanaceae

Example: Polygonum type

✔ 8-nucleate, 7-celled

2️⃣ Bisporic Type

Develops from two megaspore nuclei.

One meiotic division incomplete.

Example:

Allium (Allium type)

Functional megaspore contains 2 nuclei.

3️⃣Tetrasporic Type

Develops from all four megaspore nuclei.

No cytokinesis after meiosis.

Example:

Fritillaria (Fritillaria type)

All four nuclei participate.