The ovaries are the primary female reproductive organs. They sit on either side of the uterus and produce gametes: oocytes, or egg cells. They also secrete estrogen and progesterone, hormones important to reproduction. The ovarian cycle is the process by which one of the ovaries releases an egg into one of the uterine tubes each month. It has three phases: the follicular phase, ovulation, and the luteal phase. Let's go through each one. Before the follicular phase starts, we need to quickly go over what follicles and oocytes are and how they relate to one another. Primary oocytes form during fetal development. They begin meiosis but stop at prophase 1. Primary oocytes are contained within primordial follicles. Once female puberty begins, primordial follicles start maturing into primary follicles with the help of follicle stimulating hormone (FSH). Each month, around 20 primordial follicles mature into primary follicles. One of these primary follicles will mature further and become a secondary follicle. This secondary follicle contains a secondary oocyte, a 23-chromosome cell that has completed meiosis 1 at last. A few days before ovulation, the secondary follicle develops into a vesicular follicle. The vesicular follicle contains a fluid filled space called the antrum that surrounds the secondary oocyte. The antrum is full of hormones, enzymes, electrolytes, and anticoagulants. When it's time for ovulation, the vesicular follicle ruptures, releasing the contents of the antrum as well as the secondary oocyte. The oocyte then begins its journey down the uterine tube. Within the uterine tube, the oocyte can be fertilized by sperm. If no sperm are present, the oocyte will continue on until it reaches the uterus and it will leave the body during the menstrual period. If the oocyte does become fertilized, it finally gets to complete meiosis 2. Then, the fertilized cell develops into a zygote, which continues dividing until it's a little ball of cells that burrows into the wall of the uterus. Were you wondering what happened to the follicle after it released the oocyte? What's left of it forms the corpus luteum after ovulation. The corpus luteum is a glandular structure that releases estrogen and progesterone. If the egg wasn't fertilized, luteolysis, the breakdown of the corpus luteum, will begin after around 10 days. If the egg was fertilized, the corpus luteum will produce progesterone for around 12 weeks. After that, the placenta will take over progesterone duty and the corpus luteum can break down. Luteolysis leaves behind a scar-like structure called the corpus albicans. Visible Body Suite has all the 3D models and animations you need to understand the anatomy and physiology of the reproductive system. Learn more at visiblebody.com/vbsuite