Structure of Ovum :
1. Ovum is round, haploid, non-motile female gamete. It is actually secondary oocyte when it is discharged by the ovary during ovulation.
2. Ovum is the largest cell of the body measuring about `0 .1 mm or 100 mu` in diameter. It is called microlecithal because it is almost free of yolk.
3. It has abundant cytoplasm called ooplasm which contains a large eccentric and prominent nucleus at the centre.
4. The plasma membrane of ovum is called vitellin membrane after fertilisation. Centrioles are absent in the ovum. Ovum shows polarity having two poles.
Structure of Ovum (unfertilized)
5. The side which shows the presence of polar body and nucleus is called animal pole while the opposite side is termed as vegetal pole.
6. The ovum is enclosed by two additional coats, viz, zona pellucida and corona radiata.
7. Zona pellucida is inner thin, transparent and non-cellular layer while corona radiata is outer, thicker and cellular layer. The zona pellucida is secreted by ovum itself. Corona radiata is formed of radially elongated follicular cells which are glued together by hyaluronic acid.
8. There is a narrow perivitelline space between the vitelline membrane and zona pellucida. In vitro fertilization can be used to overcome this problem. Other techniques are zygote intra-fallopian transfer and gamete intra-fallopian transfer.
