The human egg is one of the few cells you can see without a microscope
At roughly 120 micrometres across, the human egg cell is among the largest cells in the body, just visible to the naked eye. For more than a century biologists taught that females are born with every egg they will ever have. Since 2004, studies hinting at ovarian stem cells have reopened that question.
An egg cell, or ovum, is the female gamete in organisms that reproduce with a large female sex cell and a smaller male one. The term applies when the egg itself cannot move. When a mobile sperm fuses with it, the result is a zygote carrying two sets of chromosomes, which quickly begins growing into a new organism. In some algae, fungi and mosses the equivalent immobile cell is called an oosphere.
The idea that mammals come from eggs was once a bold guess. William Harvey, who lived from 1578 to 1657, is linked to the slogan every living thing comes from an egg, rejecting spontaneous generation and the notion of miniature preformed bodies. Proof arrived slowly: Karl Ernst von Baer identified the mammalian ovum in 1827, and Oskar Hertwig watched sperm fuse with starfish eggs in 1876.
Research on humans, dogs and cats in the 1870s suggested egg production halts around birth, and the zoologist Solomon Zuckerman's review of reports from 1900 to 1950 hardened this into dogma. Several studies since 2004 suggest mammalian ovaries may contain stem cells, but whether adult mammals truly make new eggs remains unresolved. Another curiosity: during the cell divisions that produce sex cells, maternal DNA recombines about 42 times on average, compared with about 27 for paternal DNA.
Inside, the ovum's central substance, the ooplasm, holds the nucleus and a nutritive yolk of fatty and protein granules. Mammalian eggs carry only a tiny amount, enough for the earliest stages, because fertilisation happens inside the mother's body. Bird eggs pack enough to feed the chick through incubation. Egg-laying animals, including all birds and most fish, amphibians and reptiles, wrap ova in protective layers, and some fish and reptiles take a middle path, hatching eggs inside the mother just before birth.
Source: Egg cell
