Ovum
|
Sperm cells attempting to fertilize an ovum |
An
ovum (plural
ova) is a
haploid female reproductive cell or
gamete. The word is derived from
Latin, meaning
egg or
egg cell. Both
animals and
embryophytes have ova. The term
ovule is used for the young ovum of an animal. In some
plants, such as
algae, it is also called
oosphere.
=Material contribution to offspring=The egg is the sole provider of such
endosymbiotic organelles, including mitochondria and chloroplasts within the cytoplasm. These cannot be produced with nuclear DNA alone and must be manufactured from DNA within existing organelles of their type (such as
mitochondrial DNA) — this is important in
mitochondrial genetics and can be used to trace maternal and paternal
ancestry, especially as plants contain chloroplasts as well. Sperm are often too small to contribute anything physical except DNA and its own mitochondria gets destroyed by the egg.
In higher animals, ova are produced by female
gonads (sexual glands) called
ovaries and all of them are present at birth in
mammals, and mature via
oogenesis.
Human and mammal ova
|
A sperm cell fusing with an ovum |
In the
viviparous animals (which include
humans and all other placental
mammals), the ovum is fertilized inside the female body, and the embryo then develops inside the
uterus until it is born. It receives nutrition directly from the mother.The ovum is the largest
cell in the human body, typically visible to the naked eye without the aid of a
microscope or other magnification device. The human ovum measures on average, 145
µm in diameter.
In the
oviparous animals (all
birds, most
fishes,
amphibians and
reptiles) 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 (as in birds), or outside (as in many fishes). 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 (biology) for a discussion of eggs of oviparous animals.
The egg cell's
cytoplasm and
mitochondria (and
chloroplasts in plants) are the sole means of the egg being able to reproduce by
mitosis and eventually form a
blastocyst after fertilisation.
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.
*
Egg (biology)*
Fertilisation*
Insemination*
Menstrual cycle*
Ovulation *
Pollination*
Pregnancy*
Respiratory burst*
Sperm*
Zygote*
The Ovarian Kaleidoscope Database description of 1800 genes involved in ovarian functions
