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UNC-Chapel Hill BIOL 252 - Oogenesis and Sexual Cycle

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BIOL 252 1s Edition Lecture 25 Outline of Last Lecture I. Digestive SystemII. Reproductive SystemOutline of Current LectureI. Oogenesis and Sexual CycleII. OogenesisIII. FolliculogenesisIV. Sexual CycleV. Ovarian CycleVI. Menstrual CycleCurrent LectureI. Oogenesis and Sexual Cyclea. Sexual cycle: events that recur every month when pregnancy does not interveneb. Consists of two interrelated cycles controlled by shifting patterns of hormone secretioni. Ovarian cycle: events in ovariesii. Menstrual cycle: parallel changes in uterus c. Brain – hypothalamus -> GnRH (acts on anterior pituitary gland) => FSH & LH – act on gonads d. HPG Axis e. LH peak induces ovulation f. Follicles secrete estrogeni. As you increase follicle size, estrogen levels increase ii. After ovulation, increase in secretion of progesterone iii. Birth control mimics secretion of progesterone; prevents follicular development and ovulation II. Oogenesisa. Egg productionb. Produces haploid gametes (eggs/ova) by means of meiosisc. Distinctly cyclic event that normally releases one egg each monthd. Accompanied by cyclic changes in hormone secretione. Cyclic changes in histological structure of ovaries and uterus i. Uterine changes result in monthly menstrual flowThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.f. Oogenesis – Egg Developmenti. 2 million oogonia remain at birthii. By 6 months of age, oogonia have matured to become primary oocytesiii. Remain in developmental arrest until pubertyiv. At puberty, 400,000 primary oocytes but only about 480 will undergo ovulation (one ovum ovulated every 28 days from approximate age of 14 to 50 when menopause occurs)g. Follicle Development – folliculogenesis i. Primordial follicle => primary follicle => secondary1. Secondary: granulosa cells + zona pellucida, theca folliculi 2. Tertiary: antrum, cumulus oophorus, theca interna, theca externa3. Ovulation of mature follicle: bleeding into antrum, ovulated oocyte  corpus luteumh. Transform into primary oocytes: early meiosis Ii. Most degenerate (atresia) by time girl is bornii. What causes? Granulosa cell apoptosisiii. Egg: any stage from primary oocyte to time of fertilizationiv. By puberty: 200,000 oocytes remains1. Lifetime supply-only ovulate 480 timesi. Egg development resumes in adolescencei. FSH stimulates monthly cohorts of ~24 oocytes to complete meiosis Iii. Each oocyte divides into two haploid daughter cells of unequal size and different destinies1. Important to produce an egg with as much cytoplasm as possible2. If fertilized, must divide repeatedly and produce numerous daughter cells3. Secondary oocyte: large daughter cell that is product of meiosis I4. First polar body: one that ultimately disintegratesa. A means of discarding extra set of haploid chromosomes iii. Secondary oocyte: proceeds as far as metaphase II 1. Arrests until after ovulation2. If not fertilized, dies and never finishes meiosis3. Fertilized, completes meiosis II and casts off second polar bodyiv. Chromosomes of large remaining egg unite w/ those of spermIII. Folliculogenesisa. Development of follicles surrounding the egg that undergoes oogenesisi. Primordial follicles1. Consists of primary oocyte in early meiosis2. Surrounded by single layer of squamous follicular cells3. Follicular cells connected to oocyte by fine cytoplasmic processes for passage of nutrients and chemical signals4. Concentrated in cortex of ovary5. Adult ovary has 90-95% primordial follicles6. Monthly recruitment of about 24 follicles begins 290-day path to maturity that only one will finish (23 die)b. Primary folliclesi. About 140 days into cycle, recruited primordial follicles => primaryii. Contain larger, secondary oocytes and surrounding layer of cuboidal follicular cellsc. Secondaryi. Appear about 140 days in cycleii. Oocytes are even larger and follicular cells now in two or more layers1. Granulosa cellsiii. Zona pellucida: layer of glycoprotein gel secreted by granulose cells around oocyteiv. Theca folliculi: connective tissue around granulose cells condenses to form fibrous husk1. Theca externa: outer, rich in blood vessels2. Theca interna: hormone secreting layer producing androgens and granulosa cells convert to estradiold. Tertiaryi. Appear about 60 days before ovulation when granulosa cells begin secreting follicular fluid into small poolsii. As pool enlarge, merge forming single antrum1. Antral follicles: tertiary and mature2. Preantral: earlieriii. Cumulus oophorus: mound of granulosa cells on one side of antrum that covers oocyte and secures to follicular walliv. Corona radiate: innermost layer of cells in cumulus surrounding zona pellucida and oocyte1. Forms protective barrier around egge. Maturei. About 20 days before ovulation, one follicle becomes dominant (the one destined to ovulate)ii. Remainder degenerateiii. It captures and holds FSHiv. At about 5 days before ovulation it is large enough to be considered a preovulatory (graafian) follicleIV. Sexual Cyclea. Sexual cycle averages 28 days, varies from 20 to 45 daysb. Hormones of hypothalamus regulate pituitary glandi. Progesterone: dictates uterine lining, changes in endometriumii. Ovaries => inhibin and estradiol => negative feedback loop w/ anterior pituitary c. Pituitary hormones regulate the ovariesd. Ovaries secrete hormones that regulate the uteruse. Basic hierarchy of hormonal controlf. Hypothalamus® pituitary ® ovaries (gonads) ® uterusg. Ovaries exert feedback control over hypothalamus and pituitaryh. HPG Axisi. FSH enlarges; LH stimulates ovulation ii. FSH – follicles grow, secrete estrogen, negative feedback loop telling to stop production of FSHi. Before ovulation: follicular phasej. After ovulation: luteal phase k. Cycle begins with 2-week follicular phase i. Menstruation occurs during first 3 to 5 days of cycleii. Uterus replaces lost tissue by mitosis and cohort of follicles growiii. Ovulation around day 14: remainder of that follicle becomes corpus luteuml. Next 2 weeks: luteal phasei. Corpus luteum stimulates endometrial secretion and thickeningii. If pregnancy does not occur, endometrium breaks down in the last 2 daysiii. Menstruation begins and the cycle starts overV. Ovarian Cyclea. Three principal steps: follicular phase, ovulation, luteal phaseb. Reflects what happen in the ovaries and their relationship to hypothalamus and pituitaryc. Follicular phase


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