Egg cell
Studies performed on humans, dogs, and cats in the 1870s suggested production stops shortly after birth. A review by zoologist Solomon Zuckerman covered reports from 1900 to 1950. His work cemented the belief that females have a finite number of oocytes formed before they are born. This dogma held for over a century until recent challenges emerged. Several studies since 2004 suggest ovarian stem cells exist within the mammalian ovary. Whether mature mammals can actually create new egg cells remains uncertain. Researchers continue investigating if these stem cells generate functional gametes or remain dormant.
The human ovum measures approximately one millimeter in diameter. It is one of the largest cells in the human body yet visible to the naked eye without magnification. Primitive germ cells embedded in ovarian substance grow into these large ova. Fertilization occurs inside the female body in all mammals. Maternal DNA recombines approximately 42 times on average during this process. Paternal DNA recombines approximately 27 times on average instead. These differing rates shape genetic diversity between parents and offspring. The zygote forms when sperm and egg fuse together rapidly growing into a new organism.
Ooplasm acts like yolk at the center of the ovum containing its nucleus. The nucleus named the germinal vesicle holds the nucleolus called the germinal disc. Cytoplasm includes spongioplasm and hyaloplasm often termed formative yolk. Nutritive yolk or deutoplasm consists of rounded granules of fatty substances imbedded in cytoplasm. Mammalian ova contain only a tiny amount of nutritive yolk for early embryonic nourishment. Bird eggs supply enough nutriment throughout the whole period of incubation. This difference reflects how mammalian embryos receive nutrients after implantation versus external development.
Oviparous animals include birds, most fish, amphibians, and reptiles. Their ova develop protective layers passing through the oviduct to the outside body. Fertilization happens inside female bodies as in birds or outside as in many fish. Embryos hatch from eggs outside the mother's body after developing within them. Ovoviviparity represents an intermediate form where embryos develop within eggs but hatch inside mothers. Some fish, reptiles, and many invertebrates use this technique before birth occurs. Mitosis allows egg cells to reproduce via cytoplasm and mitochondria forming blastocysts.
Nearly all land plants alternate between diploid and haploid generations known as gametophytes. Female gametophytes produce structures called archegonia containing egg cells formed via mitosis. A typical bryophyte archegonium has a long neck with a wider base holding the egg cell. The neck opens upon maturation allowing sperm cells to swim into the archegonium for fertilization. Seed plants contain ovules housing female gametophytes that produce single egg cells. Pollen tubes deliver sperm nuclei fusing with egg nuclei during pollination events. Flowering plant embryo sacs reduce to just eight cells inside the ovule structure.
Common questions
When did William Harvey live and what was his theory about animal reproduction?
William Harvey lived from 1578 to 1657 and proposed that every living animal comes from an egg. This doctrine rejected the idea of spontaneous generation and preformationism.
Who discovered the mammalian ovum and when was this discovery made?
Karl Ernst von Baer discovered the mammalian ovum in 1827. This discovery proved Harvey correct about mammals but required a microscope to see the tiny cell.
What year did Oskar Hertwig observe sperm fusing with ova of a starfish?
Oskar Hertwig observed sperm fusing with ova of a starfish in 1876. The fusion created a diploid zygote that grew into a new organism.
How large is the human ovum and can it be seen without magnification?
The human ovum measures approximately one millimeter in diameter. It is one of the largest cells in the human body yet visible to the naked eye without magnification.
Which animals are classified as oviparous and how do their eggs develop?
Oviparous animals include birds, most fish, amphibians, and reptiles. Their ova develop protective layers passing through the oviduct to the outside body.