Egg cell

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Egg cell (ovum)

An egg cell is the female reproductive cell used in sexual reproduction. It is non-motile (it doesn’t swim) and, when fertilized by a sperm cell, forms a zygote that grows into a new organism. In animals, egg cells are called ova and are produced in the ovaries. Human ova are very large for cells, about 120 micrometers across, so they can often be seen with the naked eye.

Fertilization and offspring
When an egg and a sperm join, their nuclei combine to create a diploid zygote. This zygote then divides and grows into an embryo. In humans and many other animals, the egg is fertilized inside the female body.

Varieties and terminology
- Ooplasm is the cytoplasm around the nucleus in the egg and contains nutrients for early development.
- Oogenesis is the process that makes mature eggs in females.
- Oogamy refers to reproduction involving a large, non-motile female egg and a smaller, usually motile male sperm.

Eggs in different organisms
- In many plants, the egg is part of the female gametophyte inside an ovule. After fertilization, the ovule becomes a seed, and the surrounding ovary may become a fruit.
- In some algae, fungi, and bryophytes, the egg is called an oosphere.

Other notes
There is ongoing research about whether mammals can produce new egg cells after birth. Historically, it was believed that females have a fixed number of eggs from birth, but some studies suggest there may be stem cells in the ovaries that could form new eggs, though this is still uncertain.