The egg cell, or ovum, is the female reproductive cell, or gamete, in most anisogamous organisms. The term is used when the female gamete is not capable of movement. If the male gamete is capable of movement, the type of sexual reproduction is also classified as oogamous. When egg and sperm fuse during fertilisation, a diploid cell is formed, which rapidly grows into a new organism.
History
While the non-mammalian animal egg was obvious, the doctrine ex ovo omne vivum, associated with William Harvey, was a rejection of spontaneous generation and preformationism as well as a bold assumption that mammals also reproduced via eggs. Karl Ernst von Baer discovered the mammalian ovum in 1827. The fusion of spermatozoa with ova was observed by Oskar Hertwig in 1876.
Animals
In animals, egg cells are also known as ova. The term ovule in animals is used for the young ovum of an animal. In vertebrates, ova are produced by female gonads called ovaries. A number of ova are present at birth in mammals and mature via oogenesis. White et al. disproved the longstanding dogma that all of the ova are produced before birth. The team from the Vincent Center for Reproductive Biology, Massachusetts, Boston showed that oocyte formation takes place in ovaries of reproductive-age women. This report challenged a fundamental belief, held since the 1950s, that female mammals are born with a finite supply of eggs that is depleted throughout life and exhausted at menopause.
Mammals including humans
In all mammals the ovum is fertilized inside the female body. The human ova grow from primitive germ cells that are embedded in the substance of the ovaries. Each of them divides repeatedly to give secretions of the uterine glands, ultimately forming a blastocyst. The ovum is one of the largest cells in the human body, typically visible to the naked eye without the aid of a microscope or other magnification device. The human ovum measures approximately in diameter.
Ooplasm
Ooplasm is the yolk of the ovum, a cell substance at its center, which contains its nucleus, named the germinal vesicle, and the nucleolus, called the germinal spot. The ooplasm consists of the cytoplasm of the ordinary animal cell with its spongioplasm and hyaloplasm, often called the formative yolk; and the nutritive yolk or deutoplasm, made of rounded granules of fatty and albuminoid substances imbedded in the cytoplasm. Mammalian ova contain only a tiny amount of the nutritive yolk, for nourishing the embryo in the early stages of its development only. In contrast, bird eggs contain enough to supply the chick with nutriment throughout the whole period of incubation.
In the oviparous animals the ova develop protective layers and pass through the oviduct to the outside of the body. They are fertilized by male sperm either inside the female body, or outside. After fertilization, an embryo develops, nourished by nutrients contained in the egg. It then hatches from the egg, outside the mother's body. See egg for a discussion of eggs of oviparous animals. The egg cell's cytoplasm and mitochondria are the sole means the egg can reproduce by mitosis and eventually form a blastocyst after fertilization.
Ovoviviparity
There is an intermediate form, the ovoviviparous animals: the embryo develops within and is nourished by an egg as in the oviparous case, but then it hatches inside the mother's body shortly before birth, or just after the egg leaves the mother's body. Some fish, reptiles and many invertebrates use this technique.
Plants
Nearly all land plants have alternating diploid and haploid generations. Gametes are produced by the gametophyte, which is the haploid generation. The female gametophyte produces structures called archegonia, and the egg cells form within them via mitosis. The typical bryophyte archegonium consists of a long neck with a wider base containing the egg cell. Upon maturation, the neck opens to allow sperm cells to swim into the archegonium and fertilize the egg. The resulting zygote then gives rise to an embryo, which will grow into a new diploid individual. In seed plants, a structure called ovule, which contains the female gametophyte. The gametophyte produces an egg cell. After fertilization, the ovule develops into a seed containing the embryo. In flowering plants, the female gametophyte has been reduced to just eight cells inside the ovule. The gametophyte cell closest to the micropyle opening of the ovule develops into the egg cell. Upon pollination, a pollen tube delivers sperm into the gametophyte and one sperm nucleus fuses with the egg nucleus. The resulting zygote develops into an embryo inside the ovule. The ovule, in turn, develops into a seed and in many cases, the plant ovary develops into a fruit to facilitate the dispersal of the seeds. Upon germination, the embryo grows into a seedling. . The Polycomb gene FIE is expressed in unfertilized egg cells of the mossPhyscomitrella patens and expression ceases after fertilization in the developing diploid sporophyte. In situGUS staining of two female sex organs of a transgenic plant expressing a translational fusion of FIE-uidA under control of the native FIE promoter In the moss Physcomitrella patens'', the Polycomb protein FIE is expressed in the unfertilised egg cell as the blue colour after GUS staining reveals. Soon after fertilisation the FIE gene is inactivated in the young embryo.
Other organisms
In algae, the egg cell is often called oosphere. Drosophila oocytes develop in individual egg chambers that are supported by nurse cells and surrounded by somatic follicle cells. The nurse cells are large polyploid cells that synthesize and transfer RNA, proteins, and organelles to the oocytes. This transfer is followed by the programmed cell death of the nurse cells. During oogenesis, 15 nurse cells die for every oocyte that is produced. In addition to this developmentally regulated cell death, egg cells may also undergo apoptosis in response to starvation and other insults.
