Estrogen is secreted by [D] Graafian vesicles. These vesicles, also known as ovarian follicles, are fluid-filled sacs within the ovary where oocytes (immature eggs) develop. As the follicle matures, it secretes increasing amounts of estrogen, primarily estradiol, under the influence of follicle-stimulating hormone (FSH) from the pituitary gland.

Once the follicle reaches maturity, it ruptures, releasing the mature egg into the fallopian tube in a process called ovulation. After ovulation, the remaining follicle transforms into the corpus luteum, a temporary endocrine structure. While the corpus luteum primarily secretes progesterone to support the uterine lining for potential pregnancy, it also produces estrogen.

However, the main source of estrogen throughout the menstrual cycle, particularly during the follicular phase, is the Graafian vesicles. Estrogen produced by these follicles plays a crucial role in regulating the menstrual cycle, promoting the growth and development of the uterine lining, and influencing secondary sexual characteristics in females.