Implantation 2

Question 1

What is the fetal conbribution of the normal process of implantation?

Answer 1

• Trophectoderm differentiation during blastogenesis
• After fertilization, zygote undergoes cell divisions to produce smaller cells called blastomeres
• Compaction (tight aligning of cells) results in the inner cell mass and outer cells differentiate into trophectoderm that eventually become the trophoblast and placenta
• Space develops in morula between inner and outer cells to make the blastocyst and zona pellucida disappears
• Blastocyst rolls along uterine cavity and orients the embryonic pole to face the endometrial lining (usually)
• Embryo then attaches to the uterine wall at day 5-6 post-fertilization

Question 2

Describe the molecular regulation of the first true differentiation event in human development.

Answer 2

• Formation of the trophectoderm from the inner cell mass
• Regulated at the molecular level by a switch resulting in trophectoderm rather than inner cell mass
  
  • involves decrease in Oct4 with an associated increase in Cdx2
    
    • Oct4 maintains pluripotency
    • Cdx2 acts as a molecular switch to promote trophoectoderm differentiation
  • Tead4 is upsream of Cdx2
  • Sox2 and Gata3 also important

Question 3

What is the maternal contribution to implantation?

Answer 3

• Proper decidualization is very important
  
  • Estrogen priming
  • Early steps induced by progesterone
    
    • stromal cells become decidualized and take on a different histologic appearance (larger with lipid and glycogen vacuoles) and altered function
• Implantation occurs in the endometrium in the secretory phase
• Decidua impedes movement of invasive trophoblast by forming physical barier and cytokines that promote attachment rather than invasion
• Early decidua also provides nutirents to developing embryo and aids in immune tolerance

Question 4

Where does normal human blastocyst implantation occur?

Answer 4

In the anterior or posterior wall of the body of the uterus

Question 5

What are the molecular mediators of blastocyst attachment?

Answer 5

• L-selectin on the trophoblast surface interacts with carbohydrate receptors on the uterine epithelium
• Integrins and laminin facilitate attachemnt
• Fibronectin regulates migration
• Trophoblast differentiation signals also part of the process