Early in the life of a female mammal, each ovary develops a number of immature eggs embedded in groups of other cells called follicles. In humans the total number of immature eggs is thought to be fixed early in life. As the animal (or person) becomes reproductively mature (the process called puberty in humans), eggs will periodically mature and be released from the ovary (a process called ovulation) so that they will be available for fertilization by sperm. A fertilized egg resulting from union with a sperm becomes a zygote and then an embryo as it develops.
Animal and human ovaries also produce various steroid and peptide hormones. Estrogen and progesterone are the most important of these in mammals. These hormones induce and maintain the physical changes of puberty and the secondary sex characteristics. They support maturation of the uterineendometrium in preparation of implantation of a fertilized egg. They provide signals to the hypothalamus and pituitary that help maintain the menstrual cycle. Estrogen plays an important role in maintaining subcutaneous fat, bone strength, and some aspects of brain function.
In humans, an egg launched from an ovary has to traverse a slight space before entering the fallopian tube and moving gradually down to the uterus where it implants itself into the lining and develops as the pregnancy continues. If the egg settles into the fallopian tube instead of the uterus an ectopic pregnancy will result. If the egg fails to release from the follicle in the ovary an ovarian cyst may form. Small ovarian cysts are common in helathy women but large cysts can be an advanced manifestation of polycystic ovary syndrome.
