During the youth of a female, several hundred thousand graafian follicles may be present in the ovaries, each of which contains an oocyte surrounded by a single stratum of cells. By the time puberty is reached, however, most of these follicles have deteriorated and collapsed. Those that remain develop to maturity one at a time in a monthly cyclical process. A substance called the follicle-stimulating hormone that is emitted by the pituitary gland is the factor that facilitates the beginning of this process, ensuring the follicles growth and maintenance. The luteinizing hormone, which is also secreted by the pituitary gland, is also involved in the maturation of the graafian follicle, eventually stimulating the secretion of estrogens by the follicle and causing the vesicle to burst.