Structure of the Egg and Its Function in Reproduction
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 in Biology Definition Structure and Role
1. What is an egg in biology?
An egg in biology is a female reproductive cell, also called an ovum, that can develop into a new organism after fertilization. In animals, the egg contains the haploid nucleus, cytoplasm, and stored nutrients needed for early development. In birds and reptiles, the term egg also refers to the entire protective structure containing the embryo and food supply.
2. What are the main parts of a bird’s egg?
A bird’s egg consists of protective layers and nutritive components that support embryo development. The main parts include:
- Shell – Hard outer covering made of calcium carbonate
- Shell membranes – Protective inner layers beneath the shell
- Albumen – Egg white that provides water and protein
- Yolk – Nutrient-rich food source
- Germinal disc – Region where the embryo develops after fertilization
These structures together ensure protection, nutrition, and gas exchange.
3. What is the function of the egg yolk?
The egg yolk functions as the main nutrient source for the developing embryo. It contains:
- Lipids (fats) for energy
- Proteins for growth
- Vitamins and minerals
In oviparous animals like birds and reptiles, the yolk sustains the embryo until hatching.
4. How does fertilization occur in an egg?
Fertilization occurs when a sperm cell fuses with the ovum to form a zygote. The process involves:
- Sperm penetrating the egg’s outer layers
- Fusion of male and female nuclei
- Restoration of the diploid chromosome number
This marks the beginning of embryonic development.
5. What is the difference between oviparous and viviparous animals?
The key difference is that oviparous animals lay eggs outside the body, while viviparous animals give birth to live young. In oviparous species like birds, embryo development occurs inside an external egg. In viviparous species like humans, the embryo develops inside the uterus and receives nutrients directly from the mother.
6. Why do eggs have a hard shell?
Eggs have a hard shell to protect the developing embryo and prevent water loss. The calcium carbonate shell:
- Provides mechanical protection
- Reduces dehydration
- Contains pores for gas exchange
This adaptation is especially important for terrestrial reproduction.
7. What is the structure of a human egg cell?
A human egg cell is a large, spherical haploid cell specialized for reproduction. Its main structures include:
- Nucleus – Contains 23 chromosomes
- Cytoplasm – Rich in organelles and nutrients
- Zona pellucida – Protective glycoprotein layer
- Cell membrane – Regulates entry of sperm
It is the largest cell in the human body.
8. What is the role of albumen in an egg?
The albumen, or egg white, protects and nourishes the embryo. It:
- Provides water and proteins
- Acts as a shock absorber
- Contains antimicrobial proteins like lysozyme
This helps maintain a safe environment for embryonic development.
9. How does an embryo develop inside an egg?
An embryo develops inside an egg through a series of cell divisions and differentiation processes. The steps include:
- Formation of the zygote after fertilization
- Repeated mitotic divisions
- Formation of embryonic layers (ectoderm, mesoderm, endoderm)
- Organ formation and growth
The embryo uses yolk nutrients until it hatches.
10. What is the difference between a fertilized and unfertilized egg?
A fertilized egg contains a zygote formed by fusion of sperm and ovum, while an unfertilized egg does not develop into an embryo. In birds, a fertilized egg can develop into a chick under proper incubation, whereas an unfertilized egg remains a single reproductive cell.
