07. Implantation and Placenta

• embryo is considered a fetus after 12 weeks
• pregnancy is timed from the last menstrual period
• receptive endometrium is maintained by high progesterone and oestrogen levels
• receptive endometrium = highly vascularised
• receptive endometrium has pinopode formation

  • these are larger structures that provide adhesion for the blastocyst
• apposition = blastocyst loosely associates with uterine wall
• invasion = blastocyst attachment to the uterine wall triggers enzyme production - degrades and invades the glycogen rich endometrial stroma
• decidua = endometrium after changes during implantation

  • uterine NK cells enter
  • stromal fibroblasts become rounded and store glycogen and lipids - secret decidual proteins like prolactin to support pregnancy
• decidua completely surrounds implanted blastocyst by day 10
• the placenta provides no blood to blood contact - maternal blood enters chambers and contacts the chorion
• differentiation of the blastocyst is driven by growth factors and oxygen tension
• lacunae are the gaps in the placenta where the maternal blood flows
• primary villi form when trophoblasts invade into the decidua
• secondary villi form when the extra embryonic mesoderm invades the core of the primary villous
• stem villi = basal part of the villi, attached to chorionic plate
• branch villi = project from sides of stem villi
• terminal villi = swelling at the tips of branch villi containing terminal vessels
• chorionic villus thins to reduce the diffusion distance between teh mother and fetal blood
• spiral arteries are the mothers input into the placenta, and endometrial veins are the mothers output
• spinal arteries are normally resistance vessels, but we need to remove this resistance in the placenta
• extravillous trophoblasts plug the spinal arteries, reducing blood flow to the developing placenta - this creates a low oxygen environment and protects the fetus from oxidative stress
• haemotrophic phase = nutrition phase of the placenta, provides bloodflow
• 4 layers separate the maternal and fetal blood:

  • syncytiotrophoblasts
  • cytotrophoblasts
  • connective tissue
  • fetal capillary endothelium
• there is a flow of oxygen to the fetus due to increased fetal haemoglobin
• poor extravillous trophoblast invasion of the spinal arteries can result in loss of placenta function

  • early onset pre-eclampsia