The ovum or egg cell is a non-motile female gamete that fuses with the motile sperm during fertilisation and results in the formation of the zygote. Here, let’s discuss more about the structure of the human egg cell or ovum in detail.
Table of Contents
What is a Human Egg Cell?
Gonads are the primary reproductive organs which produce the gametes. A pair of testes produce sperms in males and a pair of ovaries produce ovum in females. An egg cell or ovum is a female reproductive cell that fuses with sperm in the process of fertilisation. This ovum develops into its mature form via a process called oogenesis.
In viviparous animals, this ovum is fertilised inside the body of the females. Whereas, the embryo development takes place in the uterus.
Ovum – Structure
The ovum is one of the largest cells that measures approximately 120 µm in diameter. The ovum has a large, centrally located nucleus which is covered by cytoplasm. This oocyte nucleus and nucleolus are termed germinal vesicle and germinal disc respectively. Likewise, the cytoplasm (yolk) of an ovum is termed ooplasm. It has less amount of yolk (in humans) and hence it is alecithal. This ooplasm is enclosed by a peripheral layer called the cortex which has many microvilli. These microvilli are tubular projections of the plasmalemma that aids in the transportation of substances in and out of the cytoplasm.
The human ovum is typically covered by 3 layers:
- Inner thin vitelline membrane
- Middle zona pellucida
- Outer corona radiata
The narrow space present between the vitelline membrane and zona pellucida is called the perivitelline space. The zona pellucida is often seen as a thick girdle that is surrounded by the corona radiata.
See more:Fertilisation and Implantation
For fertilisation to occur, the hyaluronidase in the acrosome of the sperm has to scatter the outer corona radiata from the middle zona pellucida of an ovulated oocyte. This invariably allows contact between the oocyte’s core and the sperm.
Sectional View of Ovary Showing Ovum
Functions – Fertilisation
As mentioned earlier, the ovum is a vital component in the fertilisation process. During ovulation, the ovum enters the fallopian tube via the fimbriated end. The ovum’s entry is enabled by the movement of cilia found on the inner side of the fimbriated end. Now, the ovum from a matured follicle is released by ovulation. It is present in the ovary in its primary oocyte stage with 23 pairs of chromosomes (diploid). The meiotic division occurs in the ovum just before the ovulation process. Thus, the division of the primary oocyte into a first polar body and a secondary oocyte happens. The first polar body is then expelled out and the secondary oocyte, which contains only 23 chromosomes (haploid), remains. The remaining 23 chromosomes are expelled along with the first polar body.
Soon after fertilisation, the female gamete, which is in its secondary oocyte stage, cleaves into a second polar body and a mature ovum. Later, the second polar body is also expelled. Now the nucleus of the matured ovum sets off as a female pronucleus with one sex chromosome (X) and 22 autosomes.
Also Check:
Keep exploring BYJU’S Biology to learn more such exciting topics.
