Each presumptive ovum (egg cell) within the ovary develops inside a sac-like group of cells which are called follicular_cells. The combination of oocyte and follicular cells is referred to as a "follicle".
Hundreds of thousands of oogonia form in the ovarian cortices during the development of the female fetus. Once formed, they are encapsulated by a single layer of follicular cells, and are then referred to as primordial_oocytes (one entire unit would be a primordial_follicle). After birth, these primordial follicles increase in size and enter the prophase stage of meiosis-I. The oocyte within the follicle is now called a primary_oocyte. Primary oocytes are dormant until puberty. When puberty begins, the release of FSH (follicle_stimulating_hormone) causes one or more primary oocytes to undergo maturation each month.
As a primary oocyte undergoes maturation, its epithelium changes from squamous to cuboidal and it begins producing estrogens. It is, combined with its follicular cells, now called a primary_follicle. At the end of meiosis-I, the division of the cytoplasm is unequal. One daughter cell (the polar_body) is too small to survive. The other daughter cell retains the bulk of the cytoplasm and develops into the secondary_oocyte (of the secondary_follicle). Many primordial follicles become primary follicles, but very few primary follicles complete development to become secondary follicles.
The secondary follicle will continue to mature. The increase in size of the follicular cells causes an increase in estrogen levels. When the follicle matures, it is called a vesicular (antral) follicle. Vesicular follicles were formerly called tertiary_follicles or Graafian follicles. These follicles have a well-developed fluid-filled chamber (antrum) and a fertilization membrane (zona_pellucida), which surrounds the oocyte. The zona pellucida, in turn, is surrounded by a group of granulosa cells, which are known as the corona_radiata.
Increasing estrogen levels cause fluid to accumulate in the antrum of the vesicular follicle. Fluid pressure will eventually push the oocyte and its corona radiata from the vesicular follicle. Fluid pressure in the antrum peaks approximately 14 days into the 28-day cycle, and ovulation occurs. The ruptured follicle then begins to collapse. The remaining granulosa cells will infiltrate the cavity and proliferate.
If the ovum is fertilized, the secondary oocyte will complete meiosis-II, producing one large ovum and a second polar body. The polar body will disintegrate and, after the ovum completes meiosis-II, the egg and sperm chromosomes will combine to form the diploid nucleus. The zygote (fertilized_egg) will start its development.