The ovarian follicle has two major functions. It provides for the maturation and release of a fertilizable oocyte. It also forms the corpus luteum, which promotes and maintains implantation of the embryo. For these processes to occur, the follicle has to grow, ovulate, and luteinize.

An ovarian follicle is a roughly spheroid cellular aggregation set found in the ovaries. It secretes hormones that influence stages of the menstrual cycle. Women begin puberty with about , follicles, [1] each with the potential to release an egg cell ovum at ovulation for fertilization. Ovarian follicles are the basic units of female reproductive biology. Each of them contains a single oocyte immature ovum or egg cell. These structures are periodically initiated to grow and develop, culminating in ovulation of usually a single competent oocyte in humans.

The image shows part of a section of the ovary. Follicles consisting of an oocyte surrounded by layers of follicle cells develop in the cortex of the ovary. The arrow bar shows the diameter of one mature or Graafian follicle.

Typically, the human ovaries produce a single dominant follicle that results in a single ovulation each menstrual cycle Fig. In any given cycle, the dominant follicle must complete all the steps in folliculogenesis in a timely manner. In this capacity, it survives the negative events that operate to destroy the other follicles by atresia. Recognition that only a few follicles become dominant beautifully demonstrates the fundamental principle that folliculogenesis in mammals is a highly selective process. This chapter considers what is known about the process underlying the expression of the structural and functional organization of developing follicles and how they are controlled.