An ovum is a female gamete, a specialized cell used in sexual reproduction. The ovum contains a set of haploid DNA, half the DNA needed to code for a whole organism. When it encounters a male gamete, the ovum becomes fertilized, turning into a zygote which will in turn develop into an embryo, a fetus, and eventually an adult organism. Ova, as they are known in the plural, may also sometimes be referred to as oocytes.
Technically, the term “ovum” is reserved for a fully matured female gamete which is ready for fertilization. In practice, however, people may use this term for egg cells at various stages of maturity. Sometimes qualifiers are added, such as “immature ovum,” to indicate which kind of gamete is under discussion. It can be important to be precise in conversations about gametes at varying stages of development, to make sure that everyone involved understands what is under discussion.
Formation of ova happens inside the ovaries in animals, and inside similar structures in plants. Some organisms use a structure called an archegonium. Once the ovum is fully developed, it survives for a relatively limited time. If it is not fertilized, the ovum will not survive, and the organism will have to make more in the hopes that a chance at fertilization will present itself.
Many organisms supply their ova with nutrition which is designed to help the embryo during early stages of development. This makes ova valuable as a good source. Eggs from fowl, for example, are eaten by humans and many other organisms because they are high in protein and various nutrients. Likewise, many animals eat grains, which are also ova, supplied with nutrients which are designed to assist with development and germination.
A great deal of research has been done on the human ovum, since it is such a critical part of reproduction. People who are having difficulty conceiving may in fact have ova harvested for fertilization outside the body, with the doctor introducing the fertilized embryos and encouraging them to implant. Studying ova has provided researchers with a great deal of information about how humans reproduce and how ova develop in the body, from start to finish.
In humans, ova are produced on a regular cycle, accompanied with consistent changes in hormone levels which are designed to prepare the body for fertilization of an ovum and subsequent implantation and pregnancy. The body will continue to repeat these cycles until pregnancy occurs or menopause sets in.