Difference Between Fertilized and Unfertilized Eggs

Main Difference – Fertilized vs Unfertilized Egg

Sperms and eggs are the gametes produced by male and female animals respectively in sexual reproduction. Fertilization is the process of fusion of the two types of gametes. Fertilized egg and unfertilized egg are two stages of the egg. The main difference between fertilized egg and unfertilized egg is that fertilized egg is an egg that is fused with a sperm of the same species whereas unfertilized egg is an egg that has not fused with the sperm. Fertilized egg develops into a diploid embryo. Some unfertilized eggs develop into haploid organisms.

Key Areas Covered

1. What is Fertilized Egg
      – Definition, Facts, Role
2. What is Unfertilized Egg
     – Definition, Facts, Role
3. What are the Similarities Between Fertilized and Unfertilized Egg
     – Outline of Common Features
4. What is the Difference Between Fertilized and Unfertilized Egg
     – Comparison of Key Differences

Key Terms: Embryo, Fertilization, Fertilized Egg, Sexual Reproduction, Sperm, Unfertilized Egg

What is Fertilized Egg

Fertilized egg refers to the egg that is fused with a sperm of the same species. It is formed in a process known as fertilization. During fertilization, the fusion of the spermatozoal nucleus with the egg nucleus occurs, forming the primary nucleus of the embryo. In fact, it is a type of a union of the genetic material of two different gametes. Both egg and sperm carry a half number of chromosomes of a particular organism. The fusion of the membranes of two gametes is the first event of fertilization, forming a channel that allows the passage of genetic material from the sperm to the egg. Generally, when one sperm enters into the egg, the membrane of the egg becomes hard in order to prevent a second penetration by a sperm. After fertilization, the fertilized egg is activated to undergo cell division. The early stage of the development of the embryo is known as the blastocyst. Fertilization of the egg is shown in figure 1.

Figure 1: Fertilization of the Egg

The activation of the egg to form an embryo does not necessarily require the intervention of a sperm. It can be accomplished by the intervention of chemical or physical agents to form a haploid embryo. This can be identified in invertebrates such as bees, aphids, and rotifers. Fertilization can occur in two ways: internal fertilization and external fertilization. Internal fertilization occurs inside the female’s body while external fertilization occurs in the external environment.

What is Unfertilized Egg

Unfertilized egg refers to an egg that has not fused with a sperm. In mammals, it is known as the ovum. It gives rise to the embryo after fertilization. The unfertilized egg exists in the secondary oocyte stage. It is arrested at the metaphase 2 of the meiosis 2 and resumes meiosis 2 after fertilization by a sperm. The outer layer of the unfertilized egg is enclosed by a follicle cell layer. This cell layer is also known as corona radiata. The nucleus of the unfertilized egg is enclosed by the cell plasma, which is also called the yolk. The yolk contains essential nutrients for the growth of the egg. A human egg cell is shown in figure 2.

Figure 2: Unfertilized Egg

The completion of meiosis 2 produces one ootid and another polar body that contains excess DNA produced by the fertilization. The ootid matures into the ovum. The maturation of the ootid occurs under the influence of the follicle stimulating hormone (FSH).

Similarities Between Fertilized and Unfertilized Egg

• Fertilized and unfertilized eggs are two stages of the female gamete.
• Both fertilized and unfertilized eggs develop into organisms.

Difference Between Fertilized and Unfertilized Egg

Definition

Fertilized Egg: Fertilized egg refers to an egg that is fused with a sperm of the same species.

Unfertilized Egg: Unfertilized egg refers to an egg that is not fused with a sperm.

Develop into

Fertilized Egg: Fertilized egg develops into a diploid embryo.

Unfertilized Egg: Some unfertilized eggs develop into haploid organisms.

Ploidy

Fertilized Egg: Fertilized egg is diploid.

Unfertilized Egg: Unfertilized egg is haploid.

Cell Division

Fertilized Egg: Fertilized egg actively divides by mitosis.

Unfertilized Egg: Unfertilized egg is arrested at the metaphase 2 of the meiosis 2.

Conclusion

Fertilized and unfertilized eggs are two stages of the female gamete that occur during the sexual reproduction of animals. Fertilized egg is the egg that fuses with a sperm whereas unfertilized egg is not fused with a sperm. Therefore, the main difference between fertilized and unfertilized egg is the stage of fertilization.

Reference:

1. Monroy, Alberto. “Fertilization.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 22 Nov. 2017, Available here.
2. “Ovum.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 22 Nov. 2010, Available here.

Image Courtesy:

1. “2901 Sperm Fertilization” By OpenStax College – Anatomy & Physiology, Connexions Web site, Jun 19, 2013 (CC BY 3.0) via Commons Wikimedia
2. “Gray3” By Henry Vandyke Carter – Henry Gray (1918) Anatomy of the Human Body -Bartleby.com: Gray’s Anatomy, Plate 3 (Public Domain) via Commons Wikimedia