Structure of Human Ovum

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:

  1. Inner thin vitelline membrane
  2. Middle zona pellucida
  3. 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

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:

Female Reproductive System

Difference between Sperm and Ovum

Difference between Spermatogenesis and Oogenesis

Keep exploring BYJU’S Biology to learn more such exciting topics.

Frequently Asked Questions

Q1

What is gametogenesis?

Gametogenesis is a process by which reproductive cells or gametes are produced. Oogenesis and spermatogenesis are the processes of formation of female and male gametes respectively. Oogenesis happens in the outermost layers of the ovary and is also known as the differentiation process of the ovum. This process starts in the early foetus. Spermatogenesis is a continuous process that happens in the testes and results in the formation of motile gametes.
Q2

What happens during fertilisation and implantation?

During fertilisation, the sperm penetrates the layers of the ovum which is facilitated by proteolytic enzymes and hyaluronidase found in the acrosome of sperm. With the help of these enzymes, the successful sperm scatter through the zona pellucida and inactivate the other sperms that try to enter the ovum. After fertilisation, the ovum is known as a zygote. The process by which the fertilised zygote implants in the endometrium of the uterus is called implantation.
Q3

What happens to the corpus luteum if the ovum is fertilised?

If the ovum is fertilised and pregnancy occurs, the corpus luteum continues to increase in size. It is then modified into the corpus luteum of pregnancy or corpus luteum graviditatis. Also, it persists in the ovary for around 3 to 4 months. During this period, it secretes a small quantity of oestrogen and a large amount of progesterone which is essential for the maintenance of pregnancy. After this period, the placenta starts producing these hormones and thus the corpus luteum degenerates.