Egg cells, or ova, are the female reproductive cells in many organisms, including humans. They are produced in the ovaries and are essential for sexual reproduction. Each egg cell contains half the genetic material needed to form a new individual, combining with a sperm cell during fertilization to create a zygote. Egg cells are typically larger than sperm cells and are surrounded by protective layers. In humans, a female is born with all the egg cells she will ever have, and they mature over time, with one usually being released during each menstrual cycle.