University:
Glendale Community College
Course:
BIO160 | Introduction to Human Anatomy and Physiology
Academic year:

2022
Views:

239

Pages:

12
Author:

Zain Blankenship

Oogenesis • Monthly event that usually produces 1 haploid egg by meiosis • Right and left ovaries usually alternate every other month • Embryonic development of ovary: – female reproductive cells differentiate into oogonia and multiply by mitosis – before birth oogonia differentiate into about 2,000,000 primary oocytes • Birth to adolescence – most primary oocytes degenerate (atresia) during childhood – by puberty about 400,000 primary oocytes remain and will be all the eggs that will ever be produced by that person – after puberty, each month FSH stimulates maturation of groups of follicles and the primary oocytes complete meiosis I which produces secondary oocytes • Oocyte will only complete meiosis II if fertilized – after fertilization, releases 2nd polar body and becomes a zygote Oogenesis and Follicle Development Ovarian Cycle • Averages 28 days but may range from 20 to 45 days • Hormone cycle is under a hierarchy of control: – hypothalamus  pituitary  ovaries  uterus • Follicular phase (2 weeks) – menstruation occurs during first 3 to 5 days of cycle – uterus replaces lost endometrium and follicles grow • Postovulatory phase (2 weeks) – corpus luteum stimulates endometrial thickening – endometrium lost again if pregnancy does not occur Ovarian Cycle Ovarian Cycle Follicular Phase Initially high levels of FSH matures about 20-25 oocytes in primordial follicles into primary follicles. Granulosa cells of the maturing follicles secrete estrogen. Estrogen negatively feeds back on the hypothalmus (maintaining low levels of GnRH) and the pituitary maintaining a low level of LH and a reduction in FSH. Drop in FSH causes atresia (degeneration) of most of the 20-25 follicles. Granulosa cells of the surviving follicles produce more and more estrogen until it peaks at mid-cycle. The prolonged high concentration of estrogen stimulates the mid-cycle surge in FSH and LH due to increased sensitivity of the pituitary gonadotropes to GnRH (negative feedback from the high levels of estrogen leads to an up-regulation of GnRH receptors on the gonadotropes). Ovulation Spike of LH and FSH triggers ovulation: oocyte bursts out of mature follicle and is released from the ovary and is guided into a uterine (Fallopian) tube by the fimbriae. Postovulatory Phase Granulosa cells of the ruptured follicle develop into the corpus luteum as they become responsive to LH. The maturing corpus luteum secretes increasing amounts of progesterone and estrogen through the luteal phase. Progesterone inhibits pituitary gonadotropes and LH and FSH levels drop. Without high levels of FSH and LH, new follicles will not develop. Without developing follicles, estrogen levels drop. Fertilization and Pregnancy If the oocyte is fertilized it will implant into the endometrium of the uterus. The chorion membrane of the developing embryo develops into the placenta and produces human chorionic gonadotropin (HCG). HCG keeps the corpus luteum alive and secreting progesterone and estrogen. Progesterone and estrogen maintain the endometrium of the uterus and develop the mammary glands. Progesterone and estrogen inhibit FSH and LH which prevents development of follicles during pregnancy. No Fertilization Without HCG, the corpus luteum undergoes involution (dies) and turns into the corpus albicans. Progesterone and estrogen levels drop causing endometrial arteries to constrict. Without sufficient blood flow, the surface endometrium tissue dies and is shed. Low levels of progesterone and estrogen allow an increase in pituitary FSH and LH that leads to maturation of a new group of follicles. Figure 28.14 Corpus Luteum secretes Progesterone Ovulation of a Human Follicle Menstrual Cycle Proliferative phase Menstrual Cycle Menstruation Menstruation is the loss of the uterine endometrium over 4-5 days. Proliferative Phase Increasing estrogen levels from developing follicles increases the thickness of the remaining endometrium by increasing the amount of connective tissue under the epithelium. Secretory Phase Increasing progesterone levels from the corpus luteum develop spiral arteries and uterine glands throughout the endometrium. Glands produce a mucus that helps a fertilized zygote to attach to the endometrium. Permenstrual Phase Without implantation of a zygote, the corpus luteum dies and progesterone and estrogen levels drop. Lymphocytes invade the endometrium, spiral arteries constrict and the superficial tissue dies and is shed resulting in menstruation. Hormone Levels during Pregnancy Hormones of Pregnancy • HCG (human chorionic gonadotropin) – secreted by the chorionic membrane of the embryo within 9 days of fertilization – keeps corpus luteum alive and secreting progesterone and estrogen • Estrogen – increases to levels 30 times higher than before pregnancy – corpus luteum is source for first 12 weeks until placenta takes over – causes growth and enlargement of the uterus and mammary tissues Hormones of Pregnancy (continued) • Progesterone secreted by placenta and corpus luteum • suppresses secretion of FSH & LH preventing follicular development • prevents menstruation and thickens endometrium • stimulates development of breast tissue • HCS (human chorionic somatomammotropin) • called human placental lactogen • secreted from placenta in direct proportion to its size •  mother’s glucose usage and release of fatty acids • Other hormones related to pregnancy: • thyroid gland increases 50% in size and increases the basal metabolic rate (BMR) of the mother • parathyroid glands enlarge and stimulate osteoclasts to release additional calcium from the mother’s bones • aldosterone secretion  fluid retention and leads to  in mother’s blood volume and can increase blood pressure