Each month, one of a woman’s ovaries releases a single egg cell that has the potential to be fertilized. The egg cell, or ovum, is the female gamete responsible for enabling reproduction.
It is formed through a process known as oogenesis and has the potential to develop into a new organism when successfully fertilized by a sperm cell. Egg cells are haploid, meaning they contain half the number of chromosomes found in somatic cells.
Anatomy and Structure of the Egg Cell
Medically referred to as the ovum, the egg cell consists of three primary external layers:
Corona radiata: This is the outermost layer made up of follicular cells. These cells provide nutrients and protection to the egg cell.
Zona pellucida: Located beneath the corona radiata, this thick, transparent, acellular layer is composed of glycoproteins. It supports the physical structure of the egg and plays a vital role in fertilization—facilitating sperm binding and preventing polyspermy (fertilization by more than one sperm).
Tanya Mincah tentang Promil?
Vitelline membrane: The innermost thin, transparent membrane that provides an additional layer of protection.
Inside the ovum are:
Cytoplasm: A gel-like substance housing organelles, where all essential metabolic activities occur to keep the cell alive. Within the cytoplasm is the mitochondria, the powerhouse of the cell, supplying most of its energy.
Nucleus: Known as the control center, it houses the genetic material (DNA) essential for embryonic development. Like sperm cells, the egg contains 23 chromosomes. After fertilization, the resulting embryo will have a total of 46 chromosomes.
Size and Quantity of Egg Cells in Women
The egg cell is one of the largest cells in the human body, while the sperm is among the smallest. A human ovum ranges from 0.13–0.2 mm in diameter—visible to the naked eye. In contrast, sperm cells are 3–4 times smaller, measuring approximately 0.05–0.06 mm in length, with a head width of about 0.005 mm.
Girls are born with a fixed number of immature eggs in their ovaries. This number decreases with age:
At birth: approximately 2 million eggs
At puberty: around 400,000
At age 37: about 25,000
By menopause (around age 51): approximately 1,000 immature, non-viable eggs
With each menstrual cycle, one immature egg matures and is released during ovulation. Eggs that are not ovulated are reabsorbed by the body. As women age, both the quantity and quality of eggs decline. These two factors largely determine a woman’s fertility.
How Egg Cells Develop
Egg cells develop from small germ cells in the ovaries through a complex process called oogenesis. Each month, the ovaries take turns releasing a mature egg. If one ovary is absent or nonfunctional, the healthy ovary will continue ovulating.
This process is linked to folliculogenesis—the maturation of the ovarian follicle that houses the developing egg. At the start of each menstrual cycle, 10–20 primary follicles begin to develop, stimulated by follicle-stimulating hormone (FSH), secreted by the brain’s pituitary gland.
By day 9 of the cycle, only one dominant follicle continues to grow while the others degenerate. Around day 14, a surge of luteinizing hormone (LH) triggers the release of the mature egg within 24–36 hours—a process called ovulation. The released egg then travels through the fallopian tube, awaiting fertilization.
Some eggs will mature with age, others remain dormant for years, and some never develop at all.
Functions of the Egg Cell
The primary role of the egg cell is to contribute half of the genetic material upon fertilization, forming a zygote, which will later develop into an embryo and eventually a complete human organism.
Other functions include:
Nutrient supply: The cytoplasm contains yolk-like components that provide essential nutrients to the developing embryo.
Sperm attraction: The egg releases chemical signals to attract sperm and facilitate sperm entry during fertilization.
Activation of embryogenesis: Once fertilized, the egg alters its membrane potential and internal biochemistry to initiate zygote division.
Prevention of polyspermy: Mechanisms are triggered to ensure only one sperm fertilizes the egg.
How to Assess Egg Function and Quality
Eggs that function properly are of high quality, but unfortunately, there is no direct test to measure egg quality. The best method currently available is evaluating ovarian reserve.
This is done through ovarian reserve testing, typically used to assess a woman’s fertility potential. It compares a woman’s results to others in the same age group.
The test includes:
Hormone blood tests, including FSH and Anti-Müllerian Hormone (AMH)
Transvaginal ultrasound to count the number of antral follicles (AFC)
These findings help predict how well a woman might respond to fertility treatments like in vitro fertilization (IVF).
The only definitive way to assess egg quality is through fertilization with a sperm cell and subsequent genetic testing of the resulting embryo.
Age: A Major Determinant of Egg Quality and Function
Numerous studies confirm that age consistently affects egg quality. Consider these statistics:
According to the American Society for Reproductive Medicine, women under 30 have a 25% chance of conceiving naturally each cycle. This decreases to 20% at age 30, and just 5% by age 40.
IVF success rates also decline with age, especially when using a woman’s own eggs. By age 44, success rates drop to below 5%.
Conversely, using younger donor eggs or eggs that were frozen before age 35 can yield success rates of 50–51%.
Egg quality declines over time due to cumulative exposure to factors like environmental toxins, oxidative stress, and illness—all of which may damage DNA. As women age, their eggs are more likely to have chromosomal abnormalities, leading to difficulty conceiving, miscarriage, or birth defects.
Older women who become pregnant have a significantly higher risk of miscarriage or bearing children with genetic conditions like Down syndrome—from just 0.2% in their 20s to 5% in their 40s. This is why freezing eggs before age 35 increases the likelihood of a healthy pregnancy later in life.
Conclusion
The egg cell’s main function is to provide half the genetic material for a new embryo once fertilized by sperm. To ensure the formation of a healthy embryo, the egg must be of high quality. The facts above reinforce the critical role of egg age in determining female fertility. The older the egg, the lower its chances of successful fertilization and healthy pregnancy.
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Shebl O, Ebner T, Sir A, Schreier-Lechner E, Mayer RB, Tews G, Sommergruber M. Age-related distribution of basal serum AMH level in women of reproductive age and a presumably healthy cohort. Fertility and sterility. 2011 Feb 1;95(2):832-4.
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After graduating from the Faculty of Medicine, University of Indonesia, dr. Fiona served as a Non-Permanent Employee (PTT) doctor from the Indonesian Ministry of Health in a remote village in Luwuk-Banggai Regency, Central Sulawesi. This experience led her to continue her master's degree in International Health at Gadjah Mada University, Yogyakarta.
