implantation and placentation

steps of pregnancy

1. fertilization

2. development in the oviduct, migration to the uterus

3. implantation into the uterus

4. placentation

5. fetal development

loss of child from conception to childbirth

• 20-40% → pre-implantation

• 30% → 1st trimester (<12 weeks)

• 1-2% → 2nd trimester (12-24 weeks)

• 0.4% → stillbirth (3rd trimester)

uterine cell types and organization

• epithelium

• endometrium

  • functionalis

  • basalis

• myometrium

• perimetrium

hormonal and uterine characteristics

• estradiol

  • slow increase with peak just before ovulation

  • smaller, broader peak throughout luteal phase

• progesterone

  • broad peak throughout luteal phase

uterine changes throughout reproductive cycle

1. follicular phase

   • growth of uterine epithelium (estradiol)

   • proliferation of endometrial/stromal cells (progesterone, estradiol)

   • growth of spiral arteries

2. pre-menstrual/menstrual phase

   1. thinning of spiral arteries (low progesterone)

   2. 
      

      • necrosis and shedding of uterine epithelium and proximal layer of endometrium

      • basal layer of endometrium and embedded uterine glands remain to seed next cycle of growth and proliferation

corpus luteum

• remains of ovulated follicle

• produces the progesterone required to maintain the uterus in an embryo-receptive state

• maintained by the embryo

  • human chorionic gonadotropin (hCG) binds to LH receptor

diapause

suspension of development before implantation that certain species do if the egg is fertilized during fall, when the mother is entering hibernation

steps of implantation

1. apposition

2. attachment

3. invasion

factors implicated in blastocyst-uterine attachment

• HB-EGF (heparin-binding epidermal growth factor)

• ErbB4 (receptor for HB-EGF)

• MUC1 (present in mucin covering luminal epithelium)

invasion of embryo into uterine tissue

1. synctiotrophoblast (STB/SCT) breaches basement membrane

2. luminal epithelium breakdown

3. deeper invasion

   • primitive cytotrophoblast expands away from uterine lumen

growth of villous columns

• future fetal blood vessels pushes into uterine endometrium, displacing cytotrophoblast and synctiotrophoblast

• brings maternal and fetal blood in proximity