Chapter 27 The Reproductive System 1
Figure 27.19 Events of oogenesis. Before birth Meiotic events 2n Oogonium (stem cell) Mitosis Follicle development in ovary Follicle cells Oocyte 2n Primary oocyte Primordial follicle Infancy and childhood (ovary functionally inactive) 2n Primary oocyte (arrested in prophase I; present at birth) Primordial follicle Each month from puberty to menopause Primary follicle 2n Primary oocyte (still arrested in prophase I) Secondary follicle Spindle Vesicular (antral) follicle Meiosis I (completed by one primary oocyte each month in response to LH surge) First polar body n Secondary oocyte (arrested in metaphase II) Ovulation Meiosis II of polar body (may or may not occur) Polar bodies (all polar bodies degenerate) n n n n Second Ovum polar body Sperm Ovulated secondary oocyte Meiosis II completed In absence of fertilization, (only if sperm ruptured follicle becomes penetrates a corpus luteum and oocyte) ultimately degenerates. Degenerating corpus luteum 2
Figure 27.20 Schematic and microscopic views of the ovarian cycle: development and fate of ovarian follicles. Slide 1 1 Primordial 2 Primary follicles follicle 3a Theca folliculi 3a 3b 2 7 6 6 Corona radiata Secondary follicle 5 3b Primary oocyte Zona pellucida Antrum Secondary 4 oocyte Secondar y oocyte Zona pellucida Late secondary follicle Theca folliculi Forming antrum Antrum 6 Corpus luteum 5 Follicle ruptures; 4 Mature vesicular follicle (forms from ruptured secondary oocyte carries out meiosis I; ready follicle) ovulated to be ovulated 3
Ovulation Ovary wall ruptures, expels secondary oocyte with its corona radiata to peritoneal cavity Mittelschmerz - twinge of pain sometimes felt at ovulation 1 2% of ovulations release more than one secondary oocyte, which, if fertilized, results in fraternal twins Identical twins result from fertilization of one oocyte, then separation of daughter cells 4
Figure 27.20 Schematic and microscopic views of the ovarian cycle: development and fate of ovarian follicles. Slide 1 1 Primordial 2 Primary follicles follicle 3a Theca folliculi 3a 3b 2 7 6 6 Corona radiata Secondary follicle 5 3b Primary oocyte Zona pellucida Antrum Secondary 4 oocyte Secondar y oocyte Zona pellucida Late secondary follicle Theca folliculi Forming antrum Antrum 6 Corpus luteum 5 Follicle ruptures; 4 Mature vesicular follicle (forms from ruptured secondary oocyte carries out meiosis I; ready follicle) ovulated to be ovulated 5
Establishing the Ovarian Cycle During childhood, ovaries grow and secrete small amounts of estrogens that inhibit the hypothalamic release of GnRH At puberty Leptin from adipose tissue decreases the estrogen inhibition GnRH, FSH, and LH are released, act on ovaries In about four years, an adult cyclic pattern is achieved and menarche occurs 6
Hormonal Interactions During a 28-Day Ovarian Cycle Day 1: GnRH release of FSH and LH FSH and LH growth of several follicles, and estrogen release estrogen levels Inhibit the release of FSH and LH Stimulate synthesis and storage of FSH and LH Enhance further estrogen output 7
Hormonal Interactions During a 28-Day Ovarian Cycle Effects of LH surge Completion of meiosis I (secondary oocyte continues on to metaphase II) Day 14 LH Triggers ovulation Transforms ruptured follicle into corpus luteum 8
Hormonal Interactions During a 28-Day Ovarian Cycle Functions of corpus luteum Produces inhibin, progesterone, and estrogen These hormones inhibit FSH and LH release Declining LH and FSH ends luteal activity and inhibits follicle development Days 26 28: corpus luteum degenerates and ovarian hormone levels drop sharply Ends the blockade of FSH and LH The cycle starts anew 9
Figure 27.21 Regulation of the ovarian cycle. Hypothalamus Hypothalamus Slide 1 1 GnRH Travels via portal blood 4 Positive feedback exerted by large in estrogen output by maturing follicle. 4 GnRH 6 3 Slightly elevated estrogen and rising inhibin levels inhibit FSH secretion. 1 FSH LH 2 2 Anterior pituitary Thecal cells 2 Androgens LH surge Ruptured 5 follicle Granulosa cells 2 Inhibin Mature vesicular Convert androgens follicle Ovulated to estrogens secondary 2 oocyte Estrogens Early and Late follicular and midfollicular phases luteal phases 4 5 Progesterone Estrogens Inhibin 6 Corpus luteum Stimulates Inhibits 10
Uterine (Menstrual) Cycle Cyclic changes in endometrium in response to ovarian hormones Three phases 1. Days 1 5: menstrual phase 2. Days 6 14: proliferative (preovulatory) phase 3. Days 15 28: secretory (postovulatory) phase (constant 14-day length) 11
Plasma hormone level Plasma hormone level Figure 27.23 Correlation of anterior pituitary and ovarian hormones with structural changes of the ovary and uterus. Fluctuation of gonadotropin levels: Fluctuating levels of pituitary gonadotropins (follicle-stimulating hormone and luteinizing hormone) in the blood regulate the events of the ovarian cycle. LH FSH Dominant follicle Ovulation Corpus luteum Degenerating corpus luteum Ovarian cycle: Structural changes in vesicular ovarian follicles and the corpus luteum are correlated with changes in the endometrium of the uterus during the uterine cycle (d). Recall that only vesicular follicles (in their antral phase) are hormone dependent primary and secondary follicles are not. Follicular phase Ovulation (Day 14) Luteal phase Estrogens Fluctuation of ovarian hormone levels: Fluctuating levels of ovarian hormones (estrogens and progesterone) cause the endometrial changes of the uterine cycle. The high estrogen levels are also responsible for the LH/FSH surge in (a). Progesterone Functional layer Basal layer Days Menstrual flow Endometrial glands Blood vessels 1 5 10 15 20 25 28 Menstrual phase Proliferative phase Secretory phase The three phases of the uterine cycle: Menstrual: The functional layer of the endometrium is shed. Proliferative: The functional layer of the endometrium is rebuilt. Secretory: Begins immediately after ovulation. Enrichment of the blood supply and glandular secretion of nutrients prepare the endometrium to receive an embryo. Both the menstrual and proliferative phases occur before ovulation, and together they correspond to the follicular phase of the ovarian cycle. The secretory phase corresponds in time to the luteal phase of the ovarian cycle. 12
Uterine Cycle If fertilization does not occur Corpus luteum degenerates Progesterone levels fall Spiral arteries kink and spasm Endometrial cells begin to die Spiral arteries constrict again, then relax and open wide Rush of blood fragments weakened capillary beds and the functional layer sloughs 13
Premenstrual Syndrome - PMS Symtoms food cravings, headaches, depression, aggression, mood swings, sore breasts, etc. Cause excessive response of body to fluctuations of estrogen and progesterone and prostaglandins Estrogen mimics aldosterone Relief of PMS???? 14