The primary oocytes are contained within the ovaries, which release one (or occasionally a few) oocytes each month during the ovarian cycle. This release is coordinated via folliculogenesis, the formation of the ovarian follicle. The follicle is the ovarian structure that houses the oocyte, surrounded by one or more layers of cells, inside the ovary. Ovulation is the release of the egg from the ovary.
Folliculogenesis begins with the primordial follicle, an immature follicle that surrounds the primary oocyte. This stage is considered the resting stage because the primordial follicles will remain in this stage until triggered to move on to the next stage during the ovarian cycle. However, at any given moment groups of follicles exist at all the stages of folliculogenesis. The process occurs continuously after puberty.
After the resting stage, the granulosa cells thicken within the primary follicle. A granulosa cell is a cell that makes up the follicle envelope. The follicle envelope is a layer inside which the oocyte develops and grows larger. A protein capsule called the zona pellucida is the noncellular layer that immediately surrounds the plasma membrane of an oocyte. This layer helps with fertilization and is a strong protective coating for the oocyte. The zona pellucida remains with the oocyte after it is released from the ovary.
The follicle enters the secondary stage, when the granulosa cells differentiate into two layers of theca folliculi, a layer of cells that supply blood and nutrients to the follicle. The theca interna is the inner layer that surrounds the oocyte, while the theca externa is the outer layer that lies at the edge of the follicle. Prior to ovulation, the follicle contains the primary oocyte arrested in prophase of meiosis I. During the late preovulatory phase, the primary oocyte continues meiosis to become the secondary oocyte. This secondary oocyte will be halted in metaphase II and represents the stage of the released oocyte at ovulation. The secondary oocyte only completes meiosis II (to become an ootid and mature ovum) upon fertilization.
In the third stage, also called the antral stage, the antrum forms. The antrum is a fluid-filled cavity within the follicle. At this stage, many follicles die during a process called atresia. During folliculogenesis, atresia is the degeneration and reabsorption of ovarian follicles that are not ultimately ovulated. In response to a rise in follicle-stimulating hormone (FSH), surviving antral follicles secrete the hormones estrogen and inhibin, which have a negative feedback effect on FSH. Those follicles with fewer FSH receptors are unable to continue development. Eventually only one follicle, the dominant follicle, remains. It is prepared for ovulation, which is the release of the egg from the ovary into the fallopian tube. The antrum continues to grow to its maximum size within a tertiary follicle during this stage. The dominant follicle will grow up to 20 mm in diameter and become the preovulatory follicle.Finally, the preovulatory follicle, also called the Graafian follicle, forms. It is characterized by a large antrum and distinct layers of differentiated theca. The oocyte within this follicle is ready to be released. During ovulation, the granulosa cells of the follicle merge with the membrane cells of the ovary and form an opening. The oocyte is passed through this opening into the peritoneal cavity, where it is swept into the fallopian tube. The ruptured follicle quickly forms the corpus luteum, a clump of cells that release hormones (estrogen and progesterone) that prepare the uterus for implantation of the zygote. The formation of the corpus luteum is the end of folliculogenesis because the structure is no longer a follicle.