Embryo sac: The embryo sac, also known as the female gametophyte, is an oval structure found in the ovule of flowering plants. When the haploid megaspore nucleus divides, an embryo sac is said to form. It has two haploid nuclei and six haploid cells with no cell walls. The two haploid, polar nuclei can sometimes fuse to form a single endosperm mother cell.

Classification of Embryo Sac

Based on the number of megaspores, embryo sacs can be divided into three types:

• Monosporic: Meiosis of the diploid megaspore mother cell in the nucellus generates four haploid megaspores in the monosporic or Polygonum-type embryo sac. Three of the megaspores, generally those at the nucellus’ micropylar end, die as a result of programmed cell death, leaving only one functioning megaspore.
• Bisporic: Due to the absence of cytokinesis and cell plate development after the second meiotic division in bisporic embryo sacs, meiosis creates only two megaspores, each containing two haploid nuclei. The megaspore closest to the micropyle then dies, leaving just a single functioning megaspore with two haploid nuclei.
• Tetrasporic: These embryo sacs have a single four-nucleate megaspore because cell plates fail to develop during both meiotic divisions.

Formation of Embryo Sac

The embryo sac formation occurs in two stages:

Stage 1:

• In the first stage, megasporogenesis occurs, where the haploid megaspore tetrad forms due to the meiosis of a single diploid mother cell.
• Out of this, three disintegrates, and one sustains, which later develops into the embryo sac.

Stage 2:

• The second stage is the megagametogenesis, where the embryo sac is formed due to mitosis of the functional haploid megaspore to produce 8-nucleate, 7-celled gametophyte.
• The polar nuclei move to the centre out of all the eight nuclei, to combine and generate a single diploid cell at that location (centre).
• This single diploid cell then fuses with the sperm to generate the triploid endosperm.
• The other three nuclei grow into antipodal cells while two modify into synergid cells which degenerate gradually.

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By Team Learning Mantras