An ovum, or human egg cell, is the female sex cell that joins with a sperm cell to begin a new human. It is about 0.12 millimeters (120 micrometers) across, a little wider than a human hair. That is the diameter of the cell itself, not counting the clear shell around it. A micrometer is a thousandth of a millimeter.

Size

By the time it leaves the ovary, a human egg cell has reached its full diameter of about 110–120 micrometers. A 2022 study measured 1,563 egg cells under a microscope and found an average of 115 micrometers. Egg cells that finished maturing in the laboratory averaged 123 micrometers, above that range because larger egg cells are more likely to complete maturation.

Around the cell is a clear shell called the zona pellucida, about 17 micrometers thick. A 2011 study of 314 mature egg cells measured the cell at about 114 micrometers across; including the shell and a narrow, fluid-filled gap between the cell and the shell, the whole egg measured about 164 micrometers. Egg cells from older women tended to be slightly smaller, with thinner shells.

Strictly, the cell released from the ovary is called a secondary oocyte. It becomes an ovum only when a sperm enters it and it completes its last division. In everyday use, both are called egg cells.

Size comparisons

A typical human hair is about 0.1 millimeters wide, so the egg cell is about 1.2 times as wide as a hair.
About 16 red blood cells, each about 7.5 micrometers across, would fit side by side across the egg cell.
About eight egg cells side by side would span 1 millimeter, the smallest mark on a metric ruler.
A paramecium of a large species, about 0.2 millimeters long, is about 1.7 times the egg cell's width.

Why the egg cell is large

An egg cell forms through an uneven division. One of the two new cells keeps most of the cytoplasm, the material that fills a cell around its nucleus, and becomes the egg. The much smaller cell, called a polar body, breaks down.

The egg's large supply of cytoplasm feeds the embryo from fertilization until it settles into the wall of the uterus. A sperm cell contributes only its DNA. The early embryo's cytoplasm comes from the egg, and so do all the organelles in it, the small working parts of a cell such as mitochondria.

How an egg cell grows

A baby girl is born with one to two million immature egg cells in her ovaries, each wrapped in a layer of support cells to form a follicle. About 400,000 remain at puberty. From then on, a new follicle usually releases its egg cell about every 28 days.

A resting egg cell is about 35 micrometers across. When its follicle starts to grow, the egg cell grows too, passing 100 micrometers by the time the follicle begins to fill with fluid. Growing from a resting follicle to one ready to release its egg takes up to half a year. By the end, the egg cell is about 3.4 times as wide as when it started and holds about 40 times the volume.

Sources

Oocyte diameter predicts the maturation rate of human immature oocytes collected ex vivo. Pors et al., Journal of Assisted Reproduction and Genetics, 2022.
Correlation of oocyte morphometry parameters with woman's age. Valeri et al., Journal of Assisted Reproduction and Genetics, 2011.
27.2 Anatomy and Physiology of the Ovarian Reproductive System. In Anatomy and Physiology, 2nd edition. OpenStax, 2022.
Just How Small Is "Nano"?. National Nanotechnology Coordination Office.
18.3 Erythrocytes. In Anatomy and Physiology, 2nd edition. OpenStax, 2022.
Integrative Neuroscience of Paramecium, a "Swimming Neuron". Brette, eNeuro, 2021.

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