Final answer:
At puberty, the hypothalamus triggers the release of FSH and LH from the pituitary gland, which in turn stimulate the follicles to develop further by promoting estrogen production.
Step-by-step explanation:
At puberty, the hypothalamus signals the pituitary gland to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormones play a crucial role in stimulating the follicles to develop further. FSH specifically encourages the growth of ovarian follicles, while LH promotes the production of estrogen by the granulosa and theca cells within these follicles. As a result, during the follicular phase, the tertiary follicles grow in diameter and secrete estrogen, which is necessary for the maturation of the ovaries and other reproductive organs, as well as the development of secondary sexual characteristics.