Follicles are packs of somatic cells that surround developing oocytes in the ovaries. Follicles develop through a process called folliculogenesis that occurs in tandem with oogenesis. At birth, women have all of their follicles; however, this number decreases as women age due to double stranded breaks in the DNA of primary oocytes contained within primordial follicles.

Folliculogenesis is the process of follicle development, which occurs simultaneously with oogenesis. The initial stage of follicle development is when dormant primordial follicles are formed prior to birth. Primordial follicles are composed of simple layers of cells. The exit from dormancy and initiation of further development is controlled by a complex interplay of hormones. During folliculogenesis, mitotic cell divisions and hormones promote development and increase the complexity of follicles.

Mature follicles contain two cell layers, theca cells and granulosa cells, which are formed during folliculogensis. Granulosa cells are present from the primordial follicle stage onwards and become surrounded by theca cells during the secondary follicle stage. After ovulation, theca cells secrete androgens and progesterone while granulosa cells secrete estrogen and progesterone.

Ovulation is the release of an oocyte from the follicle for fertilization. Upon release, the oocyte is a secondary oocyte arrested in the metaphase II stage of meiosis until fertilization.

The cumulus oophorus is a cluster of cells surrounding the oocyte after ovulation. The cumulus oophorus protects the oocyte and provides it with the energy substrates needed for further development.

As women age, the number of follicles and viable oocytes decrease; there is an inverse correlation between age and fertility. This is due to the decreased efficiency of the double stranded DNA break repair mechanism. The primordial follicles contain primary oocytes—formed by meiosis. They typically repair double stranded DNA breaks by homologous recombination. As women age, the efficiency of this repair mechanism declines and leads to a depletion of viable oocytes. 

When a sperm and egg fuse to form a zygote, the nucleus of the sperm enters the cytoplasm of the egg. As a result, the father's genome is passed onto the offspring, but no cellular organelles from the sperm are transferred. Any DNA contained in the mitochondria must come from the mother's egg, and could not have come from the cytoplasm of the sperm. The offspring will inherit all mitochondrial DNA from the mother.

This allows geneticists to trace mitochondrial lineages to find distant ancestors and track the evolution of species.

Following ovulation, the remaining follicle previously containing the egg is called the corpus luteum. This structure will release progesterone, and continue to do so if the egg is fertilized and a zygote is formed. If no pregnancy occurs, the corpus luteum will degrade into the corpus albicans.

The luteal surge is characterized by a sharp increase in estradiol (estrogen) levels, which then causes an increase in luteinizing hormone levels. This event causes ovulation to take place.

The release of the secondary oocyte from the follicle is called ovulation. During this process, a hole called the “stigma” is formed and it allows the secondary oocyte to leave the follicle surrounded by a layer of cells called the cumulus oophorus. After its release, the secondary oocyte enters the fallopian tube.