Oocytes are the female reproductive cells, also known as egg cells, that are essential for sexual reproduction. They are produced in the ovaries and undergo a process called oogenesis, which begins before birth and continues until menopause. Each oocyte is contained within a structure called a follicle, and typically, only one oocyte is released during each menstrual cycle. When an oocyte is fertilized by a sperm cell, it can develop into an embryo, leading to pregnancy. Oocytes are crucial for the formation of zygotes, which are the initial stages of development in humans and other animals. The quality and quantity of oocytes can significantly impact fertility.