Follicles are the functional units of the ovary. Each follicle is comprised of an egg surrounded by follicular cells. In addition to producing the eggs required for fertility, ovarian follicles are also responsible for producing the female hormones, oestrogen and progesterone. Over the course of 2-3 months, the earliest follicle-stage known as the primordial follicle, develops into a mature follicle, leading to the release of a mature egg at ovulation. In the case of PCOS, follicle development becomes arrested at an immature stage and no eggs are released resulting in infertility.