UNC-Chapel Hill BIOL 252 - Oogenesis and Sexual Cycle (7 pages)

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Oogenesis and Sexual Cycle

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Oogenesis and Sexual Cycle


Oogenesis and Sexual Cycle

Lecture number:
Lecture Note
University of North Carolina at Chapel Hill
Biol 252 - Fundamentals of Human Anatomy and Physiology
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BIOL 252 1s Edition Lecture 25 Outline of Last Lecture I Digestive System II Reproductive System Outline of Current Lecture I Oogenesis and Sexual Cycle II Oogenesis III Folliculogenesis IV Sexual Cycle V Ovarian Cycle VI Menstrual Cycle Current Lecture I II Oogenesis and Sexual Cycle a Sexual cycle events that recur every month when pregnancy does not intervene b Consists of two interrelated cycles controlled by shifting patterns of hormone secretion i Ovarian cycle events in ovaries ii 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 estrogen i 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 Oogenesis a Egg production b Produces haploid gametes eggs ova by means of meiosis c Distinctly cyclic event that normally releases one egg each month d Accompanied by cyclic changes in hormone secretion e Cyclic changes in histological structure of ovaries and uterus i Uterine changes result in monthly menstrual flow These 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 III Oogenesis Egg Development i 2 million oogonia remain at birth ii By 6 months of age oogonia have matured to become primary oocytes iii Remain in developmental arrest until puberty iv 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 secondary 1 Secondary granulosa cells zona pellucida theca folliculi 2 Tertiary antrum cumulus oophorus theca interna theca externa 3 Ovulation of mature follicle bleeding into antrum ovulated oocyte corpus luteum h Transform into primary oocytes early meiosis I i Most degenerate atresia by time girl is born ii What causes Granulosa cell apoptosis iii Egg any stage from primary oocyte to time of fertilization iv By puberty 200 000 oocytes remains 1 Lifetime supply only ovulate 480 times i Egg development resumes in adolescence i FSH stimulates monthly cohorts of 24 oocytes to complete meiosis I ii Each oocyte divides into two haploid daughter cells of unequal size and different destinies 1 Important to produce an egg with as much cytoplasm as possible 2 If fertilized must divide repeatedly and produce numerous daughter cells 3 Secondary oocyte large daughter cell that is product of meiosis I 4 First polar body one that ultimately disintegrates a A means of discarding extra set of haploid chromosomes iii Secondary oocyte proceeds as far as metaphase II 1 Arrests until after ovulation 2 If not fertilized dies and never finishes meiosis 3 Fertilized completes meiosis II and casts off second polar body iv Chromosomes of large remaining egg unite w those of sperm Folliculogenesis a Development of follicles surrounding the egg that undergoes oogenesis i Primordial follicles 1 Consists of primary oocyte in early meiosis 2 Surrounded by single layer of squamous follicular cells 3 Follicular cells connected to oocyte by fine cytoplasmic processes for passage of nutrients and chemical signals 4 Concentrated in cortex of ovary 5 Adult ovary has 90 95 primordial follicles IV 6 Monthly recruitment of about 24 follicles begins 290 day path to maturity that only one will finish 23 die b Primary follicles i About 140 days into cycle recruited primordial follicles primary ii Contain larger secondary oocytes and surrounding layer of cuboidal follicular cells c Secondary i Appear about 140 days in cycle ii Oocytes are even larger and follicular cells now in two or more layers 1 Granulosa cells iii Zona pellucida layer of glycoprotein gel secreted by granulose cells around oocyte iv Theca folliculi connective tissue around granulose cells condenses to form fibrous husk 1 Theca externa outer rich in blood vessels 2 Theca interna hormone secreting layer producing androgens and granulosa cells convert to estradiol d Tertiary i Appear about 60 days before ovulation when granulosa cells begin secreting follicular fluid into small pools ii As pool enlarge merge forming single antrum 1 Antral follicles tertiary and mature 2 Preantral earlier iii Cumulus oophorus mound of granulosa cells on one side of antrum that covers oocyte and secures to follicular wall iv Corona radiate innermost layer of cells in cumulus surrounding zona pellucida and oocyte 1 Forms protective barrier around egg e Mature i About 20 days before ovulation one follicle becomes dominant the one destined to ovulate ii Remainder degenerate iii It captures and holds FSH iv At about 5 days before ovulation it is large enough to be considered a preovulatory graafian follicle Sexual Cycle a Sexual cycle averages 28 days varies from 20 to 45 days b Hormones of hypothalamus regulate pituitary gland i Progesterone dictates uterine lining changes in endometrium ii Ovaries inhibin and estradiol negative feedback loop w anterior pituitary c Pituitary hormones regulate the ovaries d Ovaries secrete hormones that regulate the uterus e Basic hierarchy of hormonal control V f Hypothalamus pituitary ovaries gonads uterus g Ovaries exert feedback control over hypothalamus and pituitary h HPG Axis i FSH enlarges LH stimulates ovulation ii FSH follicles grow secrete estrogen negative feedback loop telling to stop production of FSH i Before ovulation follicular phase j After ovulation luteal phase k Cycle begins with 2 week follicular phase i Menstruation occurs during first 3 to 5 days of cycle ii Uterus replaces lost tissue by mitosis and cohort of follicles grow iii Ovulation around day 14 remainder of that follicle becomes corpus luteum l Next 2 weeks luteal phase i Corpus luteum stimulates endometrial secretion and thickening ii If pregnancy does not occur endometrium breaks down in the last 2 days iii Menstruation begins and the cycle starts over Ovarian Cycle a Three principal steps follicular phase ovulation luteal phase b Reflects what happen in the ovaries and their relationship to hypothalamus and pituitary c Follicular phase extends from beginning of menstruation until ovulation Days 114 i Includes preovulatory phase from end of menstruation until ovulation ii Most variable part

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