 This video will cover the following objective from the reproductive system, describe the location, structure, and function of the ovaries, define and describe the processes of oogenesis and folliculogenesis, describe the three phases of the ovarian cycle, follicular, ovulation, and luteal phase, describe the roles of FSH, LH, estrogen, and progesterone in the regulation of the ovarian cycle. The female gonads, known as the ovaries, are responsible for producing the ova and the sex steroid hormones, estrogen, and progesterone. The ovaries are a pair of oval-shaped organs located within the pelvic cavity, each is around 2-3 cm in length, approximately the size of an almond. The ovaries are supported by the broad ligament, an extension of the peritoneum that folds down into the pelvic cavity to envelop the ovaries, the uterine tubes, and the uterus. The ovaries are also supported by the ovarian ligament that anchors each ovary to the body of the uterus. Each ovary has an outer surface epithelium of cuboidal epithelial tissue, surrounding a layer of fibrous connective tissue called the tunica albuginia. The layer of tissue just deep to the tunica albuginia is the ovarian cortex. The ovarian cortex contains the oocytes that are maturing within structures called follicles. And then deep underneath the ovarian cortex, the inner region of the ovary is known as the medulla. So the ovarian medulla contains large blood vessels and lymphatic vessels and nerves. Here we see an electron micrograph of an ovarian follicle in the cortex of the ovary. This follicle is a secondary follicle, a follicle that has been stimulated by follicles stimulating hormone to start growing. And as it's growing, it starts to produce liquid that fills a space called an antrum. So a secondary follicle is a follicle that's starting to grow and develop an antrum. The process of eugenesis begins during prenatal development before birth in females when the eugenium divides by mitosis to produce primary oocytes. Then these primary oocytes enter into meiosis I, but immediately they will arrest at prophase I and meiosis I will not be completed until after puberty in response to rising hormone levels. Meiosis I will resume and be completed to produce a secondary oocyte and a polar body. The polar body is essentially just a waste product to remove excess chromosomes as the secondary oocyte will then go on to meiosis II. However, meiosis II will also become arrested. Meiosis II arrests at metaphase II and will not resume unless a sperm encounters the secondary oocyte within the uterine tube, in which case the secondary oocyte will complete meiosis II, producing another polar body that's just excess chromosomes that are being removed. And the secondary oocyte will mature into an ovum that will then receive the chromosomes from the sperm cell in the process of fertilization. Foliculogenesis is the development of ovarian follicles. Foliculogenesis starts with primordial follicles that are produced during prenatal development. Each primordial follicle is a single layer of granulosis cells surrounding the primary oocyte, and these granulosis cells are thin, flattened cells forming a simple squamous epithelium. In response to hormonal signals after puberty, primarily the follicle-stimulating hormone, will activate primordial follicles to mature into primary follicles. The primary follicle has a thicker layer of granulosis cells with a cuboidal shape, so the rounded granulosis cells first form a simple cuboidal epithelium surrounding the primary oocyte. Then as the granulosis cells continue to divide, the follicle starts to grow into multiple layers of cells, and an outer layer of connective tissue forms with cells in it known as sthica cells. These sthica cells will help to stimulate the production of estrogen, as granulosis cells are stimulated by a luteinizing hormone and produce a precursor to estrogen, an androgen hormone known as Andrastin dione. The granulosis cells can then convert that androgen hormone into estrogen. As the follicles start to produce higher levels of estrogen and the layers of the follicles start to grow into a more stratified epithelium, the cells of the follicle start to secrete a fluid and that fluid known as anterofluid starts to fill a space within the follicle called the antrum. Then the antrum continues to grow and when the antrum becomes very large, filling the majority of the volume of the follicle, a secondary follicle has matured into a tertiary follicle. The whole process of maturation from a primordial follicle to a tertiary follicle will take a couple of months, but then the tertiary follicles will either continue as the selected follicle that will enter ovulation and release the secondary oocyte, or the tertiary follicle will be degraded in a process known as atresia. So the tertiary follicles are the last stage, they're also known as mature follicles or antral follicles because the antrum is very large. They're also known sometimes as graphium follicles, but when the secondary follicle matures into a tertiary follicle, the primary oocyte is stimulated to complete meiosis I and form a secondary oocyte, and then that secondary oocyte will arrest at metaphase II within the tertiary follicle. And so it is the secondary oocyte that will be released from the ovary in the process of ovulation and after ovulation the remnant of the follicle that stays inside of the ovary matures into a structure known as the corpus luteum, and the corpus luteum will continue to produce steroid hormones, some estrogen, and a large amount of progesterone will be produced by the corpus luteum. The ovarian cycle is a roughly 28 day cycle that's divided into three phases. So during the follicular phase at the beginning of the ovarian cycle, secondary follicles are maturing into tertiary follicles, and those tertiary follicles compete to become the dominant follicle. So only one of the several tertiary follicles inside of an ovary will be selected to become the dominant follicle, and all of the other tertiary follicles will be degraded in a process known as atresia. But that one selected follicle, the dominant follicle, will go into the process of ovulation. So ovulation is when the secondary oocyte is released from the ovary and exits the ovary and then will travel into the uterine tube. The process of ovulation is stimulated by a surge of luteinizing hormone, and luteinizing hormone will stimulate the follicle to produce large amounts of estrogen during ovulation, and then after ovulation the remnant of the follicle known as the corpus luteum will produce large amounts of progesterone and estrogen in response to luteinizing hormone. So the luteal phase of the ovarian cycle is when the corpus luteum is producing progesterone, and progesterone will then have effects on the uterus to help prepare for implantation of the embryo. However, if an embryo never implants, the corpus luteum will stop producing progesterone as it becomes the corpus albicans. So corpus luteum is literally the yellow body, and it's yellow from large amounts of progesterone that are being produced within the corpus luteum. But as progesterone production declines, the color fades to white, and corpus albicans is literally the white body. Then corpus albicans will further degrade as it's not being stimulated by luteinizing hormone, as luteinizing hormone levels fall at the end of the ovarian cycle as we enter into the beginning of the next ovarian cycle. The ovarian cycle is regulated by the endocrine system. The hypothalamus produces ganatotropin-releasing hormone to stimulate the anterior pituitary gland to produce luteinizing hormone and follicle-stimulating hormone. Then follicle-stimulating hormone will stimulate the process of folliculogenesis, and luteinizing hormone will stimulate the production of estrogen. Estradiol is the primary form of estrogen that's produced, and estradiol will then have effects in the uterus. It will stimulate the endometrium of the uterus to grow to help prepare for implantation of an embryo. Estradiol will also have effects in the anterior pituitary and hypothalamus to decrease the production of GNRH, LH, and FSH. Estradiol has the negative feedback function of decreasing GNRH, LH, and FSH production to help maintain a homeostatic set point for LH and FSH. At approximately the 14th day of the ovarian cycle, the negative feedback mechanism we saw during the follicular phase of the ovarian cycle ends and switches to a positive feedback mechanism, where estradiol will stimulate increasing production of GNRH, stimulating increased production of LH and FSH. In particular, the levels of luteinizing hormone drastically rise. This is known as the LH surge, and this LH surge triggers the mechanism of ovulation, where the secondary oocyte is released from the tertiary follicle and travels out of the ovary into the uterine tube. Following ovulation is the luteal phase of the ovarian cycle, where the corpus luteum in the ovaries produces progesterone in large amounts in response to luteinizing hormone. Then progesterone will have an effect in the uterus, progesterone will stimulate secretions from the glands in the endometrium, the inner lining of the uterus, and progesterone will help to maintain the endometrium, which prevents the process of menstruation. However, progesterone has a negative feedback effect to decrease the levels of GNRH, LH, and FSH that are being produced. As the negative feedback causes declining levels of luteinizing hormone, progesterone levels will decline, and the declining levels of progesterone will stimulate the transition to menses, initiating the sloughing off of the stratum functionalis, the inner layer of the endometrium in the uterus. Hormonal regulation of the ovarian cycle during the follicular phase involves high levels of follicle stimulating hormone, activating folliculogenesis to produce secondary follicles that are secreting estradiol, then the secondary follicles mature into tertiary follicles, and one selected tertiary follicle grows as the other tertiary follicles are degraded by atresia. Then as we approach the 14th day of the ovarian cycle, a positive feedback mechanism takes over, driving increasing levels of follicle stimulating hormone and luteinizing hormone, and the surge of luteinizing hormone stimulates the mechanism of ovulation, where the secondary oocyte is released from the tertiary follicle, following ovulation, luteinizing hormone stimulates the corpus luteum to produce progesterone, and a smaller amount of estrogen. But as progesterone has a negative feedback effect to decrease the levels of luteinizing hormone, luteinizing hormone levels fall, causing progesterone levels to decrease at the end of the luteal phase, progesterone levels fall, and the falling progesterone will trigger the end of the luteal phase as the corpus luteum degrades, and this will also trigger the beginning of menstruation, the shedding of the inner lining of the uterus. Here we see the histology of the ovary, where there are numerous small primordial follicles in the outermost superficial layer of the ovarian cortex. Then in the layer just deep to the primordial follicles, there are several primary follicles and secondary follicles. The primary follicles contain a layer of cuboidal granulosa cells surrounding the primary oocyte, and the secondary follicles contain a developing anterum where fluid is starting to accumulate inside of the follicle. Then here we can see a large tertiary or mature or graphene follicle where the anterum is a large fluid filled space occupying the majority of the follicle volume. Here's another image of the ovary where you can see several primordial follicles with a simple squamous epithelium of granulosa cells, and then you can see a primary follicle here where the granulosa cells have a cuboidal appearance, and then the secondary follicle here has a developing anterum of fluid filled space and an outer layer of theca cells. Here's a high magnification view focused on a tertiary or mature graphene follicle where there's a large anterum fluid filled space occupying the majority of the follicle. Then surrounding the anterum is a layer of cuboidal shaped granulosa cells forming a stratified epithelium. There's numerous layers of granulosa cells, and then surrounding the granulosa cells are the theca cells.