Ovum
The ovum, or egg cell, is the female gamete that is involved in sexual reproduction. It is a non-motile, haploid cell that is specialized for fertilization. In humans, the ovum is released from the ovary at the secondary oocyte stage, where the second maturation division (Meiosis II) is yet to be completed. The ovum is spherical and non-motile. It is one of the largest cells in the human body with a size of approximately 0.1 mm (100 microns). The ovum is microlecithal (contains a negligible amount of yolk) and is also called alecithal. The cytoplasm of the ovum is known as ooplasm. It contains essential organelles and nutrients necessary for early embryonic development. The cytoplasm is rich in RNA, DNA, ATP, and enzymes, which help in early growth post-fertilization. The germinal vesicle is the nucleus of the egg, which contains a prominent nucleolus. Egg Membranes: The ovum is surrounded by multiple membranes that protect and support it. Plasma membrane (Vitelline membrane): The innermost, thin, transparent membrane surrounding the ooplasm. Zona pellucida: A thick, non-cellular, glycoprotein membrane secreted by the ovum itself. It plays a crucial role in species-specific sperm binding. Corona radiata: The outermost layer of radially elongated follicular cells, which are derived from the Graafian follicle. It provides protection and aids in sperm attraction. Polarity of Ovum: The ovum shows polarity, with two distinct poles: Animal pole – The region where the nucleus and first polar body are located. Vegetal pole – The opposite side, which stores yolk (though minimal in humans). Perivitelline Space: It is a narrow space present between the zona pellucida and the plasma membrane. The first polar body is found in this space. It plays a role in sperm entry and blocks polyspermy (entry of multiple sperms).
Cortical Granules: The ooplasm contains cortical granules, which are small vesicles that release enzymes after sperm entry to prevent polyspermy.
Release of the Ovum The ovum is discharged from the Graafian follicle during ovulation, along with one polar body. It remains arrested in metaphase-II until a sperm enters for fertilization. Upon fertilization, the ovum completes Meiosis II and transforms into a mature female gamete, forming a zygote.