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Ova – Female Reproductive Egg

May 22, 2022 4 Mins Read
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Ova, singular ovum, is also called an egg cell formed in the female reproductive organ; the ovary develops into offspring when fertilised by sperm. 

A white blob with a tail next to a blue circle,

An ovum is present inside the ovary of the female reproductive system. The ovum starts as an immature egg cell. The maturation process from a primary egg cell into a matured ovum is called oogenesis. A follicle surrounds the ovum, which then matures inside the follicle that stays dormant. Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH) regulate the growth and release of the ovum from the follicle. Ovulation is the process of releasing an egg (ovum) from the follicle. After releasing the ovum (egg), it travels down through the fallopian tube into the uterus. After conception, when the sperm meets the egg and fuses, the egg fertilises to form a zygote. The zygote then travels into the uterus, where after a few weeks, the zygote adheres to the uterine wall. This zygote matures into an embryo which grows into a fetus, finally developing into a fully grown offspring. 

Structure of an Ovum

An ovum is a haploid female gamete. The ovum consists of corona radiata, perivitelline space, vitelline membrane, polar body, zona pellucida, and nucleus. Corona Radiata is multiple layers of follicle cells attached to the ovum before releasing from the follicle. The head of sperm consists of acrosome that contains enzymes that break down the matrix that binds the corona radiata cells together. After breaking through the corona radiata, the sperm enters into zona pellucida, a glycoprotein matrix. Zona pellucida is responsible for transmitting signals between the ovum and the follicle cells. The enzymes present in acrosome disintegrate the glycoprotein matrix in zona pellucida to enter into the perivitelline space. 

A circle containing a black dot surrounded by small black dots

Perivitelline space contains a hyaluronan matrix, cortical granules, and a polar body; interacting with the sperm releases cortical granules. When fertilised by the sperm, the cortical granules modify the extracellular conditions to block the other sperms from reaching the egg cell. The polar body is a byproduct of meiotic division. During meiosis, the egg cell does not divide equally, resulting in a small DNA and cytoplasm formation into a polar body. The polar body functions to aid the egg cell in its development after fertilisation. The vitelline membrane envelopes the main nucleus and the membrane called ooplasm (cytoplasm in the ovum). The role of the vitelline membrane is to maintain the shape of the ovum and protects its contents, such as the nucleus. The nucleus is a haploid containing 23 chromosomes. 

Formation of Ovum – Oogenesis

Oogenesis is transforming from an immature ovum into a matured ovum. The follicles in the ovary are formed by stimulating follicle-stimulating hormone (FSH) secreted by the pituitary gland. The immature ovum is called an oocyte, which matures inside the follicle. When the fetus is 8 – 20 weeks old, the ova in the ovaries starts to grow and multiply by mitosis to develop later. After birth, the primary oocyte undergoes the first meiotic division to enter a dormant stage. Once the female baby reaches its sexual reproductive maturity stage, the egg cells are released, beginning the menstruation phase. These immature cells are called primary ova that remain dormant until released during ovulation. The egg cell then undergoes meiotic division to produce haploid egg cells. One of the cells is larger than the other; the large cell is the ovum that reaches maturation and is released into the fallopian tubes.

A primordial follicle is an independent hormonal phase (FSH and LH) that grows into primary and secondary follicles. The primordial follicles contain primordial sex cells (a primary oocyte) called oogonia formed by mitotic division surrounded by follicular cells. In contrast, the smaller cell is the polar body that assists the larger cell in developing and maturing. 

Flowchart of formation of ovum

The secondary follicle develops into the preantral follicle stage that the FSH stimulates to grow into the antral follicle stage, containing space between the developing cells surrounding the ovum. The antral follicle reaches into Graafian follicle stage due to the FSH and LH stimulation. At the antral follicle stage, the primary oocyte undergoes meiotic division to form a secondary oocyte (larger cell) and a polar body (smaller cell), called a mature Graafian follicle. This matured Graafian follicle reaches the inner surface of the ovary to break open to release the ovum. The LH hormone induces ovulation by releasing the ovum from the Graafian follicle. The released ovum (secondary oocyte) is surrounded by follicular cells. Meanwhile, under the influence of LH, the Graafian follicle closes itself up to transform into corpus luteum (CL).

Factors affecting ovulation

Any issues leading to affecting ovulation can negatively impact fertility. This infertility could lead to the inability to produce offspring. Some common factors that affect the ovulation are: 

  • Age, when a female is born, the number of eggs produced in the ovary disintegrates as they keep growing. Therefore, when the female reaches the mid-30s, the available number of eggs and their quality is reduced. Hence, after the mid-30s, the chances for a woman to get pregnant are less likely.
  • Weight is another major factor that influences ovulation. It has been observed that most overweight women have an irregular menstrual cycle (ovulation) compared to women with a regular cycle, reducing the likelihood of getting pregnant. 
  • Smoking women have an increased likelihood of a reduced chance of becoming pregnant by decreasing the eggs in the ovary, additionally damaging the walls of the fallopian tube. The drugs can also damage the fallopian tube and affect ovulation functions. Alcohol and other drugs also lead to delays in fetal development and may cause defects at birth. 
By :- Asha Rachel Thomas

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