Why Are Eggs Essential in Biology?
Fertilization and implantation are the foremost critical events within the reproduction process. In this process, both egg and sperm are fused together to make a zygote. Later, it gets implanted into the uterus and therefore the development of an organism.
Fertilization in Humans
When the fusion of male and female gametes occurs, it facilitates the development of a new organism. This process is known as fertilization in humans.
Fertilization is that natural life process, which is administered by the fusion of gametes from female and male counterparts, which ends up with the formation of a zygote. In humans, within the fallopian tube, the process of fertilization takes place.
During this process, semen comprising thousands of sperms are inseminated into the vagina of the female during coitus. An utmost important component for reproduction viz. sperm moves towards the uterus and enters the opening of the fallopian tube at the time of ejaculation from the male reproductive organ, penis. In this procedure, only a few sperms succeed in reaching the opening of the fallopian tube.
The secondary oocyte is released from the mature Grafian follicle of the ovary and enters into the fallopian tube, where it is fertilized within 24 hours, after which it is released from the ovary.
Though surrounded by several sperms, the oocyte is fertilized by one sperm. The sperm enters the secondary oocyte and completes the meiosis-II procedure. After this procedure, the secondary oocyte is known as the egg.
Both sperm and egg remain viable only for a limited time period. Sperm is alive for 48-72 hours inside a female genital system, whereas the egg remains viable until twenty-four hours before it's fertilised or released.
Steps Involved in Fertilization in Humans
In human beings, the fertilization process takes place in many stages that involve both chemical and physical events. Different stages of fertilization in humans are as mentioned below -
Acrosomal Reaction
The sperms go through the reaction called acrosomal and release certain chemicals known as sperm lysins that can be seen in acrosomes.
The plasma membrane of the secondary oocyte and the sperm are fused together so that the contents of the sperms can enter due to the acrosomal reactions. When the plasma membrane of the sperm binds with that of the secondary oocyte, the plasma membrane of the oocyte depolarizes. This prevents polyspermy.
Calcium ions have a very important role in this reaction of acrosomes. Optimum pH, temperature, and calcium and magnesium concentration are the main factors essential for acrosomal reactions.
Cortical Reaction
Soon after the fusion of the plasma membranes, the oocyte shows cortical reactions. Cortical granules are found under the plasma membrane of the oocyte, which fuses with the plasma membrane and releases cortical enzymes between the zona pellucida and plasma membrane. The zone is hardened by the cortical enzymes that prevent polyspermy.
Sperm Entry
A projection referred to as the cone of reception is made by the secondary oocyte at the site of sperm contact. This cone of reception receives the sperm.
Karyogamy
After the entry of the sperm, the suspended second meiotic division is completed by the secondary oocyte. This gives rise to a haploid ovum and a second cell.
The head of the sperm containing the nucleus detaches from the whole sperm and is called male pronucleus. The tail and the second polar body degenerates. The nucleus of the ovum is known as female pronuclei.
The nuclear membranes degenerate when there is a male and female pronuclei fusion. This fusion of the chromosome of male and female gametes is called karyogamy. After the fertilization procedure of the ovum, the resulting structure is called the zygote.
Activation of Eggs
The entry of sperm triggers the metabolism within the zygote. Consequently, protein synthesis and cellular respiration increase.
Implantation
Once fertilization happens, the cell starts to divide and multiply within 24 hours within the Fallopian tube. This detached multi-celled structure is called a zygote. When finally after 3-4 days it travels to the uterus, then it can be called an embryo.
After the embryo is developed, it undergoes various stages before getting attached to the endometrial layer of the uterus. This process of attachment is known as implantation.
Sex Determination
Fertilization is that the process during which a replacement cell is made when two gametes (sex cells) –sperm and ova fuse together. All genetic information is transferred from both the parents to their child and the gender of the child is determined which is known to be a completely unbiased procedure that takes place in the body on its own.
The sperm of a father determines the sex of the child. If the sperm carries a Y chromosome, then it is determined that the child born will be a boy and if the sperm carries an X chromosome, then it is determined that the child born will be a girl.
FAQs on Egg: Structure, Types, and Biological Role
1. What is an egg cell in a biological context?
An egg cell, technically called an ovum, is the female reproductive cell (gamete). Its main job is to fuse with a male gamete (sperm) during fertilisation. It contains half of the genetic information needed to create a new organism and provides the initial nutrients for development.
2. What are the different layers of a human egg?
A human egg is protected by several layers. From the outside in, they are:
- Corona Radiata: The outermost layer made of follicle cells that supported the egg in the ovary.
- Zona Pellucida: A thick, jelly-like layer beneath the corona radiata that a sperm must penetrate.
- Plasma Membrane: The cell's own outer boundary, which encloses the cytoplasm and nucleus.
3. What is the main difference between an egg cell and a zygote?
An egg cell (ovum) is an unfertilised female gamete with a single set of chromosomes (haploid). A zygote is the cell that forms immediately after the egg is successfully fertilised by a sperm. It contains a complete set of chromosomes (diploid), with half from the mother and half from the father.
4. Why is a human egg cell so much larger than a sperm cell?
The egg cell is significantly larger because it performs a different function. While the sperm is small and streamlined just to deliver genetic material, the egg contains a large amount of cytoplasm and nutrients. This material is essential to nourish and support the embryo during its first few days of development after fertilisation, before it implants in the uterus.
5. How does an egg prevent more than one sperm from fertilising it?
The egg has a clever mechanism to block multiple sperm, a condition called polyspermy. As soon as one sperm successfully fuses with the egg's plasma membrane, the egg undergoes a cortical reaction. It releases enzymes that instantly harden the zona pellucida (the jelly-like outer layer), making it impossible for any other sperm to get through.
6. What part does the egg play in determining the sex of a baby?
The egg cell does not determine the sex of a baby. Every egg cell always carries one X chromosome. The sex is determined by the sperm, which can carry either an X or a Y chromosome. If a sperm with an X chromosome fertilises the egg, the baby will be female (XX). If a sperm with a Y chromosome fertilises it, the baby will be male (XY).
7. What is the purpose of the polar bodies formed during the creation of an egg?
During meiosis (the cell division that creates eggs), the division of cytoplasm is unequal. One cell, the ovum, gets almost all of it. The other smaller cells are called polar bodies. Their purpose is simply to hold the discarded sets of chromosomes, ensuring the final egg cell is haploid (has half the DNA) but retains all the necessary cytoplasm to support an embryo.
