Ovarian Cycle Lecture Notes PDF

OVARIAN CYCLE LECTURE NOTE INTRODUCTION The female is born with about a million of eggs. At menarche the body begins a process of maturing these eggs. Only a selected few fully mature over the female’s lifetime The ovarian cycle refers to the series of events that occur within the ovaries of female...

OVARIAN CYCLE LECTURE NOTE INTRODUCTION The female is born with about a million of eggs. At menarche the body begins a process of maturing these eggs. Only a selected few fully mature over the female’s lifetime The ovarian cycle refers to the series of events that occur within the ovaries of females during each menstrual cycle. It is a complex process that involves the maturation and release of an egg (ovum) from the ovary, as well as changes in the ovarian follicles and hormone levels. The ovarian cycle refers to the growth and maturation of an oocyte in preparation for fertilization and reproduction. The ovarian cycle starts from puberty until menopause. The ovarian cycle is been regulated by the hypothalamus, pituitary gland, and ovaries through the coordination of various hormones. It is a recurring expression of the synchronized interaction between the hormones of the hypothalamus-pituitary-ovarian system. The duration of the ovarian cycle can vary among individuals, typically ranging from 21 to 35 days but the length of the cycle is most often 28 days. This reserve declines as she grow older GOALS OF THE OVARIAN CYCLE The goals of the cycle are to: 1) Mature a follicle containing an ovum, leading to ovulation and fertilization. 2) Cause structural and functional changes in target tissues (uterus, oviducts, and vagina) and to facilitate fertilization and implantation. 3) Support early pregnancy. PHASES OF OVARIAN CYCLE The cycle of ovarian activity is divided into three main phases as follows: The follicular phase: is the part of the menstrual cycle when an egg matures in the ovaries. It is an ongoing process in the body of a woman during the reproductive years when hormones activate changes in the body that make pregnancy possible. This is the longest phase of the menstrual cycle. It lasts from 14 to 21 days. During this phase the ovaries house a developing egg they will later release during ovulation. It stays fairly consistent. The length of the follicular phase may vary at different stages of life. The length of the follicular phase depends on how long it takes the dominant follicle to form a fully matured egg. The female is born with about a million eggs. This reserve declines as she grow older. At menarche the body begins a process of maturing these eggs. Only a selected few fully mature over the female’s lifetime. During the follicular phase of the menstrual cycle, around 11 to 20 eggs begin developing, but only one matures completely. PHASES OF THE OVARIAN CYCLE CONT’D The ovulatory phase: this is the phase where a mature egg from the ovary is been released Ovulation is triggered by a complex interplay of hormones, primarily luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Ovulation is triggered by a complex interplay of hormones, primarily luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These hormones are produced by the pituitary gland in the brain and work together to stimulate the growth and maturation of a follicle (a fluid-filled sac) in the ovary. During the first half of the menstrual cycle, multiple follicles begin to develop in the ovaries. However, usually only one follicle becomes dominant and continues to grow, while the others degenerate. The dominant follicle produces increasing amounts of estrogen. Ovulation phase cont’d As the dominant follicle grows, it produces higher levels of estrogen. This increase in estrogen triggers a cascade of hormonal changes, leading to a surge in luteinizing hormone (LH). The LH surge, usually lasting for about 24 to 36 hours, triggers ovulation. The LH surge causes the dominant follicle to rupture, releasing the mature egg into the fallopian tube. From there, the egg begins its journey towards the uterus. If sperm is present in the fallopian tube during this time and fertilizes the egg, conception can occur. The ovulation phase is the most fertile time in a woman's menstrual cycle. It is the window of opportunity for conception because the egg is viable for fertilization for about 12 to 24 hours after its release. Ovulation usually occurs around the middle of the menstrual cycle, typically between days 12 and 16 in a 28-day cycle. The exact timing varies among individuals  FACTORS RESPONSIBLE FOR VARIATION IN OVULATION TIME Psychological/mental disturbance Stress hormonal imbalances  certain medical conditions Anxiety Change of environment Certain drugs Functions of the ovary i. Production of the female gamete (gametogenesis) ii. Supporting the growing gamete by steroidogenesis(which is the production of hormones by the ovarian cells). The luteal phase This is an essential part of the ovarian cycle in females. It occurs after ovulation and leading up to the onset of menstruation. It is named after the corpus luteum, a temporary endocrine structure that forms in the ovary after the release of an egg. It typically lasts for approximately 10-16 days, with an average length of 14 days. This phase is relatively constant in duration compared to the variable follicular phase. After ovulation, the dominant follicle transforms into a temporary endocrine gland known as the corpus luteum. The corpus luteum secretes two primary hormones: progesterone and estrogen. Progesterone levels rise significantly during the luteal phase, reaching their peak around 7-10 days after ovulation Luteal phase cont’d estrogen continues to be secreted by the corpus luteum, but at lower levels compared to the follicular phase If fertilization and implantation do not occur during the luteal phase, the corpus luteum gradually degenerates. As a result, the levels of progesterone and estrogen decrease, causing the endometrial lining to shed. This shedding leads to the onset of menstruation, marking the beginning of a new menstrual cycle. note; If fertilization does occur, the developing embryo will secrete human chorionic gonadotropin (hCG), which signals the corpus luteum to persist and continue producing progesterone to support the pregnancy until the placenta takes over hormone production. Functions of hormone during the luteal phase The progesterone  it helps to thicken the endometrium (the inner lining of the uterus), It promotes its growth and vascularity (This prepares the uterus for implantation and enhances the chances of successful pregnancy). It inhibits the contraction of the uterine muscles, thus preventing premature contractions that could expel the fertilized egg. Oestrogen It works in conjunction with progesterone to further thicken the endometrium it enhances endometrium receptivity to a fertilized egg. It also promotes the secretion of cervical mucus that facilitates sperm transport and fertilization. Physiological changes during the luteal phase 1. Endometrial Changes: a. Secretory Phase: The endometrium enters the secretory phase during the luteal phase, characterized by increased vascularity and glandular development. This prepares the endometrium for possible implantation. b. Decidualization: Decidualization refers to the transformation of endometrial stromal cells into decidual cells. This process occurs under the influence of progesterone and prepares the endometrium to support a developing embryo. Physiological changes during the luteal phase cont’d 2. Cervical Mucus Changes: a) Under the influence of estrogen and progesterone, cervical mucus becomes thicker and forms a mucus plug, which acts as a barrier to prevent the entry of pathogens and sperm. b) The mucus plug also helps to maintain a stable environment within the uterus. 3. Basal Body Temperature (BBT): a) Progesterone's thermogenic effect leads to a slight rise in basal body temperature during the luteal phase. b) Monitoring BBT can serve as an indicator of ovulation and the initiation of the luteal phase. Diagram of ovarian cycle Hormones responsible for ovarian cycle The predominant hormones involved in the human female are : gonadotropin releasing hormone (GnRH from the hypothalamus) follicle stimulating hormone (FSH)  luteinizing hormone (LH) from the anterior pituitary estradiol (an estrogen) progesterone (a progestogen) inhibin from the ovary. CHANGES IN THE OVARY CHANGES IN THE OVARY Changes in the ovary during ovarian cycle: 1) menstruation 2) maturing follicle 3) mature follicle 4) ovulation 5) corpus luteum 6) disappearance of corpus luteum Changes in the body that indicate ovulation Changes in the cervical mucus: It becomes clearer, thinner, and more slippery, resembling the consistency of raw whites Increased basal body temperature (BBT): After ovulation, a woman's basal body temperature typically rises slightly due to increased progesterone levels. Ovulation pain: Some women experience mild pain or discomfort on one side of the lower abdomen during ovulation. Heightened sex drive: Some women may experience an increase in sexual desire or libido during the ovulation phase, possibly due to hormonal changes Ovarian cycle Interactions with the menstrual cycle The ovarian cycle and menstrual cycle are intricately interconnected and interact through hormonal signals and feedback mechanisms. This is as follow: 1. Hormonal Interactions Follicle-Stimulating Hormone (FSH): a) At the beginning of the menstrual cycle, FSH is released by the anterior pituitary gland. b) FSH stimulates the development of ovarian follicles, which contain immature eggs. - Estrogen: a) As the ovarian follicles grow, they produce estrogen. b) Estrogen has several effects, including thickening the uterine lining (endometrium) during the proliferative phase of the menstrual cycle. Ovarian cycle Interactions with the menstrual cycle cont’d - Luteinizing Hormone (LH): a) As the ovarian follicles mature, there is a surge in LH, triggered by a rise in estrogen levels. b) The LH surge stimulates ovulation, which is the release of a mature egg from the ovary. - Progesterone: a) After ovulation, the ruptured follicle forms the corpus luteum, which secretes progesterone. b) Progesterone prepares the uterine lining for potential implantation during the secretory phase of the menstrual cycle. Ovarian cycle Interactions with the menstrual cycle cont’d 2. Uterine Interactions Menstruation a) The menstrual cycle begins with the shedding of the previous cycle's uterine lining during the menstrual phase. b) This shedding occurs when the levels of estrogen and progesterone decrease due to the degeneration of the corpus luteum. Proliferative Phase c) As the ovarian follicles develop and produce estrogen, this hormone stimulates the growth and thickening of the endometrium in the uterus. d) The proliferative phase prepares the uterus for potential implantation of a fertilized egg. Ovarian cycle Interactions with the menstrual cycle cont’d Secretory Phase a) After ovulation, the corpus luteum forms and secretes progesterone, which further stimulates the growth and development of the endometrium. b) The secretory phase ensures that the uterine lining is highly vascularized and glandular, providing an optimal environment for embryo implantation. - Menstruation a) If fertilization and implantation do not occur, the corpus luteum degenerates, leading to a decrease in progesterone and estrogen levels. b) This decline triggers the shedding of the uterine lining during menstruation, marking the start of a new menstrual cycle. Note The ovarian cycle governs the development and release of an egg from the ovary while the menstrual cycle involves the changes in the uterus to support potential implantation. The hormones produced by the ovaries during the ovarian cycle, particularly estrogen and progesterone, influence the growth and shedding of the uterine lining during the menstrual cycle. The interactions between these cycles ensure the proper synchronization and preparation of the female reproductive system for potential fertilization and pregnancy. ASSIGNMENT WRITE EXIENSIVELY ON FACTORS THAT AFFECTS THE OVULATION.