What is the primary role of the hormone estrogen in the female reproductive system?

The primary role of estrogen in the female reproductive system is to regulate the menstrual cycle. Estrogen is vital for the development and maintenance of the female reproductive tissues as well as the secondary sexual characteristics. During the menstrual cycle, estrogen levels fluctuate and play a crucial role in the preparation of the endometrial lining for potential implantation of a fertilized egg. It works in tandem with other hormones to trigger the proliferation of the endometrium during the follicular phase and helps to maintain menstrual regularity.

As the cycle progresses, estrogen's rise leads to positive feedback on the pituitary gland, promoting the secretion of luteinizing hormone (LH) which is critical for ovulation. However, its main function is not solely to stimulate ovulation directly, as that is primarily the role of LH. The involvement of estrogen in stimulating the production of follicle-stimulating hormone (FSH) is indirect and occurs as a result of the complex hormonal interactions throughout the cycle. It's also important to note that estrogen does not increase testosterone levels; instead, it plays a contrasting role in the balance of androgens in the female body.

Thus, the central role of estrogen in regulating the menstrual cycle makes it fundamental to female reproductive health and function.