Egg cells

Egg Cell

The egg cell, also known as the ovum (plural: ova), is the female gamete in sexually reproducing organisms. It is a crucial component of the reproductive system and plays a vital role in the process of fertilization and embryonic development.

Structure

The egg cell is typically a large, spherical cell. In humans, it is one of the largest cells in the body, with a diameter of approximately 100 micrometers. The egg cell consists of several key components:

• Nucleus: Contains the genetic material (DNA) that will combine with the sperm's DNA during fertilization.
• Cytoplasm: A jelly-like substance that contains nutrients and organelles necessary for the early stages of embryonic development.
• Zona Pellucida: A glycoprotein layer surrounding the plasma membrane of the egg cell, playing a critical role in sperm binding and prevention of polyspermy.
• Corona Radiata: A layer of follicular cells that surround the zona pellucida, providing support and nourishment to the egg cell.

Development

Egg cells are produced through a process called oogenesis, which occurs in the ovaries. This process begins before birth and continues until menopause. Oogenesis involves several stages:

1. Oogonia: The primordial germ cells that divide mitotically to produce primary oocytes.
2. Primary Oocytes: These cells begin meiosis but are arrested in prophase I until puberty.
3. Secondary Oocytes: At puberty, hormonal changes trigger the completion of meiosis I, resulting in a secondary oocyte and a polar body.
4. Ovulation: The secondary oocyte is released from the ovary and enters the fallopian tube.
5. Fertilization: If a sperm cell penetrates the secondary oocyte, meiosis II is completed, forming a mature ovum and another polar body.

Function

The primary function of the egg cell is to provide half of the genetic material required for the formation of a new organism. Upon fertilization by a sperm cell, the egg cell contributes its haploid set of chromosomes to the resulting zygote. Additionally, the egg cell provides the initial cytoplasmic environment and nutrients necessary for the early stages of embryonic development.

Fertilization

Fertilization typically occurs in the fallopian tube. The process involves several steps:

• Sperm Binding: The sperm binds to the zona pellucida of the egg cell.
• Acrosome Reaction: Enzymes from the sperm's acrosome digest the zona pellucida, allowing the sperm to penetrate the egg.
• Fusion: The sperm and egg membranes fuse, allowing the sperm nucleus to enter the egg.
• Cortical Reaction: Prevents additional sperm from entering the egg, ensuring monospermy.
• Nuclear Fusion: The sperm and egg nuclei fuse to form a diploid zygote.

Clinical Significance

Egg cells are central to various assisted reproductive technologies, such as in vitro fertilization (IVF). Understanding the biology of egg cells is crucial for addressing issues related to infertility and developmental disorders.

See Also

• Sperm cell
• Fertilization
• Embryogenesis
• Reproductive system

References

• Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular Biology of the Cell. Garland Science.
• Gilbert, S. F. (2000). Developmental Biology. Sinauer Associates.