Usually only one follicle reaches maturity during each reproductive cycle in the human female. It is then 15-18 mm in diameter.
In polytocous mammals, several follicles reach maturity and ovulate simultaneoously.
It is also an endocrine gland, producing estrogens and giving rise after ovulation to corpus luteum. It appears first as a primordial follicle with the oocyte surrounded by a single layer of flattened follicular cells, then by a single layer of cuboidal or columnar cells (called a primary follicle).
Follicles mature at intervals depending on the type of reproductive pattern in response to the folicle-stimulating hormone (FSH) from the anterior lobe of the pituitary.