5

• Female reproductive system


  • Shows regular cyclic changes
  • Changes are for preparation for fertilization and pregnancy

  
• Menstrual cycle


  A periodic vaginal bleeding with shedding of uterine mucosa (menstruation)

  
• Length of menstrual cycle

  • Average 28 days (can be 21-40 days)
  • Recorded from start of one menstrual period to start of next

  
• Cycle identification


  Numbered starting with first day of menstruation (e.g. LH surge occurs at day 14)
• Primordial follicles at birth

  • Many under ovarian capsule
  • Contain immature ovum
• Start of cycle

  1. Several follicles enlarge and become antral follicles

  2. One becomes dominant follicle (at day 6)

  3. Other follicles regress by apoptosis

  
• Dominant follicle


  Able to secrete significant amount of estrogen needed for final maturation

  
• When women injected with FSH, many follicles develop simultaneously
• Mature ovarian follicle


  • Has 2 functional cell layers: Granulosa cells (secrete estrogen) and Theca interna cells (secrete androgens)

  
• Primary source of circulating estrogen

  Granulosa cells of the ovaries

  
• Phases of menstrual cycle

  • Follicular phase
  • Ovulatory phase
  • Luteal phase

  
• Ovulation


  1. Mature follicle membrane fuses with ovarian capsule and expels ovum

  2. Ovum picked up by fimbria of oviduct and transported to uterus

  3. If fertilized, zygote implants in uterus

  4. If not fertilized, ovum transported out through vagina

  
• After ovulation


  1. Space in ruptured follicle filled with blood (corpus hemorrhagicum)

  2. Clotted blood replaced by yellowish, lipid-rich luteal cells (corpus luteum)

  3. Luteal cells secrete estrogen and progesterone

  
• Corpus luteum growth


  Depends on adequate blood supply and VEGF

  
• If pregnancy occurs


  Corpus luteum persists
• If pregnancy does not occur


  Corpus luteum involutes (degenerates) at day 24 and converts to corpus albicans

  
• In other mammals, more than one follicle ovulates and multiple births are the rule
• No new ova are formed after birth
• Ovarian development before birth

  • 7 million primordial follicles during fetal development
  • Many undergo atresia before and after birth

  
• Ovarian development at birth

  • 2 million ova
  • 1 million atretic
  • 1 million undergo first meiosis but arrested in prophase I

  
• Ovarian development at puberty

  • Less than 300,000 ova in both ovaries
  • 500 ova reach normal maturity per cycle, remainder degenerate

  
• Ovum maturation


  1. Just before ovulation, prophase arrested ovum completes first meiotic division

  2. Secondary oocyte with large cytoplasm and first polar body

  3. Secondary oocyte begins second meiotic division but arrested in metaphase II

  4. Completes second meiotic division upon fertilization

  
• Menstruation


  1. Endometrium is sloughed

  2. All endometrium except deep layer is sloughed

  
• Proliferative phase


  1. New endometrial cells grow by effect of estrogen from developing follicle

  2. Endometrium increases in thickness and uterine glands lengthen but do not become convoluted or secrete

  
• Proliferative phase


  Also called preovulatory or follicular phase

  
• Luteal phase


  1. Endometrium becomes highly vascularized and edematous under effect of estrogen and progesterone from corpus luteum

  2. Endometrial glands become coiled and tortuous and begin secreting fluid

  
• Luteal phase


  Also called secretory phase

  
• Menstrual phase


  1. If ovum not fertilized and implanted, endometrium undergoes necrosis due to corpus luteum regression and withdrawal of hormonal support

  2. Spiral arteries constrict and degenerate leading to spotty hemorrhages that become confluent and produce menstrual flow

  
• Injection of PGF2α produces endometrial necrosis and bleeding
• Proliferative phase


  Represents restoration from preceding menstruation

  
• Secretory phase


  Represents preparation of uterus for implantation of fertilized ovum
• Length of menstrual cycle phases

  • Secretory phase is constant at 14 days
  • Proliferative phase varies and can lengthen or shorten the cycle

  
• When implantation of fertilized ovum fails

  Endometrium is shed and a new cycle starts
• Menstrual flow


  • Duration is 3-5 days (can be 1-8 days)
  • 75% arterial and 25% venous blood
  • Amount ranges from slight spotting to ~80ml (average 30ml)
  • Amount affected by endometrial thickness, medications, and clotting disorders

  
• Anovulation is common in first 12-18 months after menarche and before menopause

  
• Anovulatory cycle


  1. No corpus luteum formed, so no progesterone

  2. Estrogen causes endometrial proliferation without secretory phase

  3. Bleeding occurs in less than 28 days, ranging from scanty to profuse

  
• Basal body temperature rise

  Starts 1-2 days after ovulation, due to increase in progesterone secretion

  
• Recording ovulation time and fertile period

  1. LH surge triggers ovulation ~9 hours later

  2. Ovum lives ~72 hours after ovulation but is fertilizable for much shorter time

  3. Fertile period is 48 hours (2 days) before ovulation