Anatomy · Cells & tissues

Egg cell

The female gamete — the largest cell in the human body

Reproductive system · female germ cell

The female reproductive cell, or ovum: a large, round cell about 0.1 mm across, just visible to the naked eye as a speck. It is packed with cytoplasm and mitochondria that will sustain the early embryo, and it carries 23 chromosomes. It is wrapped in a clear glycoprotein coat, the zona pellucida, and surrounded by a halo of follicle cells, the corona radiata. Released from the ovary at ovulation, it survives for only about 12–24 hours unless it is fertilised.

How to spot them

▸Very large, round cell with a central nucleus in abundant granular cytoplasm
▸Clear glassy coat around it, the zona pellucida
▸Outside that, a ring of small follicle cells radiating outward, the corona radiata
▸Often shown with sperm crowding around its surface and one small polar body beside it
▸About 100 micrometres across, far larger than a sperm, whose head is only about 5 micrometres long

What to know

The largest cell in the human body, just visible without a microscope, about a tenth of a millimetre across

Egg cell — the ovum.

A clear, glassy layer of glycoproteins that sperm must recognise and penetrate

Egg cell — the zona pellucida.

A halo of follicle cells that stays with it after ovulation

Egg cell — the corona radiata.

It is released from the ovary arrested part-way through its second meiotic division, and finishes only if a sperm enters

Egg cell — a secondary oocyte, arrested in metaphase II.

Tiny by-products of division that hold discarded chromosomes and almost no cytoplasm

Egg cell — the polar bodies.

Granules under its surface release their contents after a sperm enters, hardening the coat to stop a second one

Egg cell — the cortical reaction prevents polyspermy.

The German-Estonian scientist Karl Ernst von Baer first observed the mammalian version in 1827, in a dog's ovary (1827)

Egg cell — von Baer's discovery of the mammalian ovum.

Key ideas

Unlike a sperm, the egg is stocked for two jobs: it provides half the chromosomes and also nearly all the cytoplasm, mitochondria and nutrients that the early embryo needs before implantation.
Fertilisation triggers a wave of calcium in the egg, completing meiosis, releasing the cortical granules and starting embryonic development.

Quick-fire facts

•Diameter about 100–120 micrometres; it is the largest human cell
•It is ovulated as a secondary oocyte (arrested in metaphase II) surrounded by the zona pellucida and corona radiata
•The egg survives for about 12–24 hours after ovulation
•A female is born with one to two million primary oocytes in her ovaries, falling to about 400,000 by puberty
•Only about 400–500 eggs are ovulated in a lifetime
•Mitochondrial DNA is inherited from the egg alone — the sperm's mitochondria are destroyed after fertilisation
•Fertilisation usually takes place in the ampulla of the uterine tube, not the uterus
•Karl Ernst von Baer (1792–1876) discovered the mammalian egg in 1827

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Sources

OpenStax — Anatomy and Physiology of the Ovarian Reproductive System (Anatomy and Physiology 2e, section 27.2)