Embryology

• Hypothalamus


  Secretes Gonadotropin releasing hormone (GnRH) to stimulate anterior pituitary gland to secrete gonadotropic hormones
• Gonadotropic hormones


  Released by anterior pituitary gland, include Follicle Stimulating Hormone and Luteinizing Hormone
• Follicle Stimulating Hormone (FSH)


  • Stimulates development of 15-20 primary-stage (preantral) follicles
  • Stimulates maturation of follicular granulosa cells surrounding the oocyte

  
• Luteinizing Hormone (LH)


  • Plays a role in the development of ova, induction of ovulation, and stimulation of estradiol and progesterone production by the ovaries
• Estrogen


  Produced by theca interna and granulosa cells of the follicles, assists in endometrial regrowth, ovulation, and calcium absorption, responsible for secondary sexual characteristics
• Progesterone


  Assists in endometrial re-growth and inhibition of FSH and LH release, also released by corpus luteum, prepares body for pregnancy
• Inhibin


  Inhibits FSH production
• Ovarian cycle


  Governs the preparation of the endocrine tissues and release of eggs

  
• Menstrual cycle


  Governs the preparation and maintenance of the uterine lining, 22-32 day cycle (28 days average)

  
• Follicular phase


  1. Rising levels of FSH and LH

  2. FSH stimulates growth of ovarian follicles

  3. Follicles release estrogen and low levels of progesterone

  4. Only one follicle reaches full maturity

  5. Other follicles become atretic

  
• Effects of estrogen production in follicular phase

  • Uterine endometrium enters proliferative phase
  • Thinning of cervical mucus
  • Stimulation of anterior pituitary to secrete LH
• Before ovulation


  1. High levels of estradiol stimulate secretion of high levels of FSH and LH

  2. FSH and LH cause maturation of one follicle

  3. LH surge causes oocyte to complete meiosis I and initiate meiosis II

  4. LH stimulates production of progesterone by follicular stromal cells (luteinization)

  5. LH causes follicular rupture and ovulation
• Ovulation


  1. FSH and LH cause maturation of one follicle

  2. High estradiol stimulates secretion of high FSH and LH

  3. LH surge causes ovulation (mature follicle ruptures and releases egg)
• Ovulation process


  • Mature graafian follicle (25mm diameter)
  • Abrupt increase in LH
  • Stigma forms at apex of ovary
  • Increased collagenase activity digests collagen fibers
  • Prostaglandin causes muscular contraction of ovarian wall to extrude oocyte
• Luteal phase


  1. Granulosa and theca interna cells form corpus luteum

  2. Corpus luteum produces estrogen and progesterone

  3. Progesterone causes uterine mucosa to enter secretory stage

  4. Progesterone inhibits release of further FSH and LH
• No fertilization


  1. Corpus luteum maximum development = 9 days after ovulation

  2. Corpus luteum degenerates and forms corpus albicans

  3. Estrogen and progesterone levels decrease causing menstruation

  
• Fertilization


  1. Human chorionic gonadotropin secreted by embryo prevents degeneration of corpus luteum

  2. Corpus luteum of pregnancy present until 4th month of pregnancy

  
• Menstrual cycle


  Changes that occur in the endometrium of the uterus, including menstrual, proliferative, and secretory phases

  
• Menstruation


  Sloughing off of the endometrium (compact and spongy layers), lasts 4-5 days

  
• Proliferative phase


  Secondary follicles in ovary mature, estrogen causes thickening of endometrium and formation of spiral glands
• Secretory phase


  Endometrial cells secrete small amount of fluid, uterine lining reaches greatest degree of development
• No fertilization


  Venules and sinusoidal spaces in endometrium become packed with blood cells, blood escapes from arteries and small pieces of stroma and glands break away causing menstruation
• Implantation


  Process by which blastocyst reaches uterus and embeds in thickened endometrium, occurs 5-7 days after fertilization

  
• Endometrial preparation for implantation

  • Secretory phase, 10-14mm thick, increasing progesterone levels
• Layers of endometrium after implantation

  • Compact layer, spongy layer, basal layer
• After fertilization


  1. Cleaving embryo maintained through exchange across zona pellucida

  2. Zona pellucida begins to break down by 6-7 days, blastocyst hatches

  
• Attachment


  1. Initial apposition and later adhesion of trophoblast to uterine epithelium

  2. Trophoblast cells begin to invade maternal tissues
• Day 9


  1. Blastocyst more deeply embedded, penetration defect closed by fibrin coagulum

  2. Vacuoles appear in syncytium forming lacunae

  
• Day 9


  Flat cells form exocoelomic membrane lining inner surface of cytotrophoblast, forms primitive yolk sac
• Days 11-12


  1. Blastocyst completely embedded, grows back over embryo healing wound

  2. Irregular lacunae appear in syncytiotrophoblast, filled with maternal blood from eroded vessels