 In this video I will describe the structure and function of the male and female gonads, discuss the effects of the gonadal hormones, testosterone, estrogen, and progesterone. The hypothalamic pituitary gonadal axis or HPG axis starts with the hypothalamus producing GNRH to stimulate the anterior pituitary to release LH and FSH, the tropic hormones that stimulate the gonads. LH stands for luteinizing hormone and LH will stimulate the production of the steroid sex hormones. FSH stands for follicle stimulating hormone and FSH will stimulate the production of sperm and eggs. The name follicle stimulating hormone comes from the fact that FSH stimulates the growth of ovarian follicles. The ovarian follicles are support cells surrounding the immature egg cell. As the ovarian follicles grow and stimulate the maturation of the egg cell, they produce increasing levels of the estrogen known as estradiol. Estradiol is just a specific form of estrogen. Estradiol is produced in increasing levels during the follicular phase of the ovarian cycle and has effects all through the body, including in the uterus where it will stimulate the growth of the inner lining of the uterus known as the endometrium. But the production of estradiol is regulated by a negative feedback control mechanism where estradiol will bind to receptors in cells of the hypothalamus and anterior pituitary gland in order to inhibit the production of GNRH, LH, and FSH. Around the 14th day of the ovarian and uterine cycle, the control mechanism regulating the production of estradiol switches from a negative feedback to a positive feedback mechanism where estradiol will stimulate the production of increasing levels of GNRH, leading to even higher levels of LH and FSH. As the levels of LH surge, this stimulates the process of ovulation, which is the release of the immature egg cell known as a secondary oocyte from the ovary. The immature egg cell will then travel through the ovaduct, also known as the fallopian tube or uterine tube, following ovulation. The remnant of the ovarian follicle is called the corpus luteum, and the corpus luteum produces the steroid hormone progesterone in response to luteinizing hormone. So this is where the name luteinizing hormone comes from, that LH stimulates the process of ovulation leading to the production of the corpus luteum, and the corpus luteum secretes the hormone progesterone. Progesterone will then bind to receptors within cells all throughout the body. In the uterus, this will help to maintain the endometrium and stimulate the secretion of the uterine glands. In the hypothalamus and anterior pituitary progesterone will have a negative feedback mechanism to decrease the production of GNRH, LH, and FSH. The uterus is regulated by the steroid sex hormones estrogen and progesterone. These hormones fluctuate over roughly a 28-day cycle. We can start looking at the cycle with the beginning marked by the initiation of menstruation, the shedding of the uterine lining, or endometrium. Following menstruation, increasing levels of FSH stimulate the ovarian follicles to produce estrogen, and estrogen stimulates the growth of the endometrium during what's known as the proliferative phase of the uterine cycle. Following ovulation, the corpus luteum of the ovary produces progesterone in response to luteinizing hormone. Then progesterone will stimulate secretions from the uterine glands during the secretory phase of the uterine cycle, and progesterone helps to maintain the endometrium. But as progesterone levels fall towards the 28th day of the uterine cycle, declining levels of progesterone stimulate the process of menstruation. In men, the HPG axis regulates the production of the steroid sex hormone testosterone, and also regulates the process of sperm production. The HPG axis starts with GnRH from the hypothalamus, stimulating the production of FSH and LH from the anterior pituitary. LH binds to receptors on the surface of cells in the testes known as the LIDIG cells, or interstitial cells of LIDIG, and LH stimulates the LIDIG cells to produce testosterone. Testosterone then travels through the blood, all through the body, binding to receptors within cells, stimulating the masculinizing effects like increased muscle mass, increased growth, stimulates the maturation of hair follicles to produce the facial hair, stimulates the enlargement of the larynx to produce the deeper voice in men. But locally within the testes, testosterone will help to stimulate the production of sperm, and within the hypothalamus and anterior pituitary, testosterone will have a negative feedback control mechanism decreasing the levels of GnRH, FSH, and LH. But testosterone's negative feedback effect is primarily regulating the control of luteinizing hormone release, and luteinizing hormone is the primary hormone regulating the production of testosterone. Follicle stimulating hormone binds to receptors on the surface of sirtoli cells. Sirtoli cells are support cells in the testes that help to support the production of sperm. One of the ways that sirtoli cells help to stimulate sperm production is by producing the androgen binding protein that binds to testosterone and helps to increase the local concentration of testosterone within the testes. Sirtoli cells also produce a hormone known as inhibit. Inhibit travels through the blood to reach cells in the anterior pituitary, where it will decrease the production of FSH. So inhibit is a negative feedback mechanism to control the production of FSH, whereas LH is primarily controlled by the negative feedback for testosterone.