# Ovarian Follicle --- An **ovarian follicle** is tiny little sac in the [[ovaries]] that protect a single [[oocyte]]. - granular cells + thecal cells - protect the oocytes from immune cells - **primordial follicle** has **primary oocytes** - you're born with as many follicles as you have - secretes estrogen as it matures - stimulates changes in uterine lining - **primary follicles** is first stage of maturation - **secondary follicles** **mature follicle** has **secondary oocyte** - ovulation: when the 2dry oocyte is yote out of the ovary - becomes a [[corpus luteum]] after ovulation ![[ovarian follicles.png]] When a [[female]] is born, their [[ovaries]] contain ~1.5 million **follicle cells**, which begin at a stage called **primordial follicles**. No new follicle cells ever develop, these are all they get. All of the primordial follicles stay this way until the child reaches [[puberty]], when the [[ovarian cycle]] begins. There are two main types of cells in the follicle, granular cells + thecal cells ## Development: From Primordial to Mature Starting at puberty, [[luteinizing hormone]] or LH from the pituitary gland trigger some follicles to develop—each [[ovarian cycle]] triggers more follicles. At first, all follicles are **primordial follicles**, which house [[oocyte|primary oocytes]], which are "paused" or arrested in the [[meiosis i|prophase I]] of meiosis. Primordial develop into **primary follicles** at the beginning of the [[ovarian cycle]], and then become **secondary follicles**, and then **antral follicles**. Antral follicles are identifiable by the visible "empty pocket" called the **antrium**. ![[antral follicle.png]] (Note: there is a small typo in the above image—it should read theca**l**) which house **secondary oocytes**, which are then ovulated out of the ovary. The follicle itself remains behind and becomes a structure called the [[corpus luteum]] (or "yellow body"), which plays an important role in early pregnancy. ___