Embryo Sac Development
கருப்பை வளர்ச்சி
Most students confuse the number of nuclei with the number of cells in an embryo sac. Remember: 8 nuclei, but only 7 cells. This is the most common point of failure in NEET biology papers.
The idea
The functional megaspore undergoes three successive free nuclear mitotic divisions. This results in 8 nuclei. Subsequently, cell walls form to organize the 7-celled structure. It comprises an egg apparatus (2 synergids, 1 egg cell), 3 antipodals, and a large central cell containing 2 polar nuclei.
Why it matters
This specific organization ensures the presence of the egg cell for fertilization and the central cell for endosperm formation.
NEET lens
NEET expects you to identify the ploidy of each cell: synergids and antipodals are haploid (n), while the central cell is diploid (2n) before fusion. Focus on the 'free nuclear' aspect; cell walls do not form until after the 8-nucleate stage. Expect a diagram-based question asking to label the filiform apparatus.
Clinical significance
In plants, failure of this development leads to total reproductive sterility. In human embryology, similar precise mitotic coordination is required for the formation of the blastocyst. Any error in nuclear division timing results in aneuploidy or developmental arrest.
Introduction
The embryo sac (female gametophyte) develops from the megaspore through megagametogenesis — a series of mitotic divisions producing a 7-celled, 8-nucleate structure that receives pollen tube contents during fertilization.
Core concept
Development (Polygonum type — most common): Functional megaspore (haploid, n) → 3 mitotic divisions (without cell wall formation) → 8 nuclei. Organization: 3 antipodal cells (at chalazal end, degenerate). 2 synergid cells flanking egg cell (at micropylar end, with filiform apparatus). 1 egg cell (female gamete, n). 2 polar nuclei fusing → central cell (secondary nucleus, 2n). Final structure: 7 cells (3 antipodal + 2 synergid + 1 egg + 1 central cell) with 8 nuclei (antipodals = 3n, synergids = 2n, egg = 1n, central cell = 2n). Filiform apparatus: ingrowths in synergid cell walls → guides pollen tube entry.
Go deeper
Function of embryo sac cells: Egg cell → embryo (after fertilization). Synergids → guide pollen tube (chemical attractants), degenerate after tube entry. Central cell → endosperm (after triple fusion). Antipodals → nutrient absorption from nucellus (degenerate after fertilization).
From Class 10
The female gametophyte in flowering plants develops inside the ovule. This structure — the embryo sac — is where fertilization occurs.