The human egg cell explained

What is the structure of an egg?
Above you will see a diagram that labels the main parts of the human egg, together with an illustration of a real human egg. This page explains what women donate when they become egg donors.

The nucleus is the heart of the egg cell; it contains most of the genetic material in the form of chromosomes. This is where the genes are situated. An egg, like a sperm, contains half the number of chromosomes as a normal cell, i.e. 23 each. So once an egg and sperm combine during fertilisation the resulting embryo will have the normal 46 chromosomes in total.

The cytoplasm is a gel-like substance that holds all the cell’s other internal structures, called organelles. It is in the cytoplasm that all the cell’s activities take place to keep it alive and functioning properly. Amongst the more important organelles are structures called mitochondria, which supply most of the energy for the cell.

Zona Pellucida
The zona pellucida (or egg wall) is an outer membrane of the egg. This structure helps the sperm to enter the egg through its hard outer layers. The egg wall hardens with age – the reason that an egg does not fertilise. ‘Assisted hatching’ is a process whereby small openings are created using various techniques (mechanical, chemical or laser) on the egg wall, allowing the developing cluster of cells to ‘hatch’. Without this opening, they would not be able to break out of their tough shell and implantation of a pregnancy would not occur.

Corona Radiata
The corona radiata surrounds an egg and consists of two or three layers of cells from the follicle. They are attached to the zona pellucida – the outer protective layer of the egg – and their main purpose is to supply vital proteins to the cell.

How big is a human egg?
The human egg, or ovum, is one of the largest cells in the human body. That said, it is still very small and measures approximately 0.12 mm in diameter. You would need 9 eggs to reach a millimeter in length, and if you laid 100 of them side by side they would sit on a line just 12 mm (1.2cm) long.

How are eggs produced?
Eggs are produced in the ovaries, which are normally situated towards the back of a woman’s abdomen below the kidneys. The eggs develop from tiny cells inside the ovaries, going through various stages of development – known as oogenesis – until they are released once a month during ovulation. Usually each ovary takes turns releasing eggs every month; however, if one ovary is absent or dysfunctional then the other ovary continues to provide eggs to be released.

How many eggs are there in an ovary?
A woman is born with approximately 500,000 potential eggs, or follicles, in each ovary. From birth onwards she will not produce any more; in fact the number of eggs will steadily decline over her lifetime and be absorbed back into the body in a process known as atresia.

By the time a woman reaches puberty, the million original follicles will have reduced to roughly 300,000; they will continue to decline right through until the menopause. With each menstrual cycle a dominant follicle will recruit a potentially mature egg, which is then released into the fallopian tube during ovulation.

Given an average span of 40 years between puberty and menopause, with one egg being released per month, only 400-500 eggs in total will actually be released. By the time a woman reaches the menopause, few or no follicles remain. Any that do are unlikely to mature and become viable eggs because of the hormonal changes that happen during this time.

About 1% of women will experience premature menopause (or premature ovarian failure), meaning that they will run out of eggs well before the normal age of menopause, sometimes when they are still teenagers. This is one of the reasons why otherwise healthy women need egg donation.

How does an egg develop?
At the beginning of each menstrual cycle, a group of 10-20 primary follicles begin to develop under the influence of Follicle Stimulating Hormone (FSH). By around Day 9 of the cycle, only one healthy follicle normally remains, with the rest having degenerated. On approximately Day 14 of the cycle, a surge of Luteinising Hormone (LH) occurs, which causes the mature follicle to ovulate approximately 24 – 36 hours later.

What’s different in an egg donation cycle?
To donate eggs, a donor is stimulated with a synthetic version of the naturally produced hormone Follicle Stimulating, in order to encourage the growth of the whole group of 10-20 follicles. This encourages all of the eggs to develop to the same stage of maturity as the one egg that would normally be released. Rather than let nature take it’s course, ovulation is triggered by medication and the eggs are surgically removed 36 hours later and placed in a dish in an incubator ready for fertilisation.

What is egg quality?
Egg quality means how capable an egg is of being fertilised and going through the developmental stages to form a viable embryo. This is largely determined by two factors: the number of chromosomes present within the egg, and the energy supply of the egg. As both of these factors tend to reduce over time, age is one of the biggest factors affecting egg quality in a woman, with the quality gradually declining as she gets older. This is the main reason that egg donors need to be below 35 years – the age when the egg quality begins to reduce.

Other factors that affect egg quality are lifestyle issues such as smoking, drinking, drugs (medical or otherwise) and general health.

Find out more about donating eggs
Click here to find out more about becoming an egg donor with Altrui