Egg cells, or ova, are the cells utilized by female organisms to reproduce offspring. In contrast, the reproductive cells used by males are referred to as sperm. In mammals, a new individual is formed when an egg from the mother and a sperm from the father come together and allow their genetic material to fuse.
Reproductive cells, or gametes, have half of the genetic information required to form a new individual; thus, the meeting of a sperm with an egg results in a full set of chromosomes. Mature mammalian egg cells are relatively large--0.0039 inches. in diameter--and contain many proteins and protein precursors. This is because when a sperm cell introduces its genetic information to that of the egg, the egg must respond quickly so that cell division may begin and a new organism may form.
Egg cells also contain many mitochondria which supply the energy required for cell replication and division. Mitochondrial deterioration occurs with age and is thought to contribute to the difficulties experienced by many women trying to conceive children in their later years.
Egg cells are found in a special place within the body called ovaries. A woman is born with all of the egg cells she will ever have, but they do not present themselves for fertilization until after puberty. This is when ovulation first occurs. During the menstrual cycle, eggs that are maturing and preparing for ovulation are encased in ovarian structures called follicles.
As these particular eggs mature, the size of the follicles that contain them and the level of estrogen in the woman's body increases. This hormonal change contributes to the bodily changes experienced by many women halfway through the menstrual cycle, such as increased libido and thinning of cervical mucous. Ovulation occurs when one follicle bursts open, releasing the egg inside of it to the folds of the woman's fallopian tube.
Once inside of the fallopian tube, an egg cell has about 48 hours to live. If it is not fertilized by a sperm within this time, it will die. The follicle that released the egg is now called a corpus luteum, and it will secrete a hormone called progesterone for about two weeks after ovulation. If the egg remains unfertilized the corpus luteum will deteriorate and stop secreting hormones. This leads to the shedding of the uterine lining and the onset of menstruation.
If the egg comes into contact with sperm while it descends the fallopian tubes on its way to the uterus, fertilization may occur. The egg is covered in a thick membrane which the sperm must penetrate. Once inside of the egg, a chemical reaction takes place to keep other sperm from gaining entry. Meanwhile, the successful sperm cell will lose its tail while its DNA packed head will fuse with the nucleus of the egg.
Uses in Biotechnology
Because egg cells are equipped with many energy producing mitochondria and an abundance of the cellular machinery required for protein synthesis, they have been used for decades by pharmaceutical companies for the purpose of drug development. Scientists simply have to introduce the genes or gene products that they are interested in studying to the egg cell and the cell will produce the proteins.
This useful feature of egg cells has also led to experimental cloning. The nucleus of the egg can be removed and replaced with the nucleus of a somatic (body) cell. This will prompt the egg to begin dividing as it would after fertilization, producing an embryo with the exact genetic combination of the substitute nucleus.