Placenta

• The placenta serves as the foetal

  • lungs
  • kidneys
  • gastrointestinal tract
• The placenta acts as a protective barrier to prevent pathogens passing from mother to baby
• The trophoblast (outer part of blastocyst from week 1) develops into placenta.
• Trophoblast splits into syncytiotrophoblast and cytotrophoblast
• Syncytiotrophoblast

  • direct contact with maternal blood
  • outer layer of multiple fused trophoblast cells
  • specialised systems for maternal/fetal transport
  • barrier to unwanted material
  • hormone secretion
• Cytotrophoblast

  • inner layer of trophoblast with several specialised functions
  • proteolytic enzyme secretion (acts on decidua)
  • endometrial invasion
  • villous formation
  • angiogenesis
• Uterus mainly supplied by uterine artery - branch of internal iliac artery
• Uterine artery branches

  A) arcuate

  B) radial

  C) basal

  D) spiral

  
• Arcuate artery runs circumferentially in myometrium
• Spiral arteries invaded by cytotrophoblast cells and remodelled to create a low resistance/high flow pathway. These supply a maternal blood pool, the intervillous space.
• Contributory mechanisms to maternal vascular development

  • growth factors
  • angiotensin II
  • cytokines
  • natural killer cells
• Spiral arteries funnel, generating a wide outflow into the intervillous space.
• During developing pregnancy, endothelium is replaced by endovascular cytotrophoblast cells, process facilitated by invaded natural killer and extravillous cytotrophoblast cells.
• • Frond-like villi grow into the intervillous space, well-perfused with fetal blood from the umbilical vessels
  • microvilli cover these villi, creating a huge surface area
  • acts as combination of lung and small intestine for the fetus
• maternal and fetal blood flows are not in direct contact.
• Maternal blood flow

  • intervillous space pressure 10 - 15 mmHg
  • flow 500 - 600 mL/min
  • uterine contractions compromise flow (e.g. in labour)
• Fetal blood flow

  • capillary pressure 30 mmHg
  • flow 450 mL/min at term
• The placenta is divided into 15-20 discrete sections called cotyledons
• The umbilical cord carries fetal blood. It has 2 arteries carrying deoxygenated blood to the placenta, and 1 vein carrying oxygenated blood away from the placenta.
• The umbilical arteries are supplied by the internal iliac arteries in the fetus.
• Placenta fetal surface

  • covered in smooth membranes
  • central umbilical cord insertion
• The amniotic sac has two layers: inner amnion and outer chorion.
• Placenta maternal surface

  • exposed rough surface
  • adherent to uterine wall
• After every delivery placental examination is required to confirm no cotyledons have been left behind.
• Ultrasound shows placental localisation
• Doppler shows umbilical cord insertion and can assess umbilical flow
• hCG

  • produced from blastocyst stage
  • supports pregnancy in 1st trimester via corpus luteum
• Progesteron produced independently by placenta
• Human placental lactogen - anti-insulin action makes more glucose available for fetus
• Transfer function

  • O2/CO2
  • nutrients: glucose, lipids
  • waste products: urea, bilirubin
  • hormones: cortisol
  • immunological: antibodies
  • drugs
• Anti-epileptics should not be used in pregnancy as e.g. sodium valproate can cross placenta and cause cognitive difficulties and congenital abnormalities in the fetus.
• Steroids and fatty acids cross the placenta by passive diffusion as they are fat soluble, small and unionised.
• Glucose crosses the placenta by facilitated diffusion, mainly via GLUT1 (insulin independent)
• Amino acids and minerals such as iron and calcium cross the placenta via active transport, with specific mechanisms for each
• Immunoglobulins cross the placenta via endo/pinocytosis (only IgG as it is the smallest)
• Water crosses the placenta by osmosis following electrolyte movement.
• Placental transfer of O2/CO2 is by simple diffusion. Fetal O2 carriage is aided by polycythaemia, and higher O2 affinity of HbF. O2 transfer is further aided by the Bohr effect (shift in Hb-O2 curve by changes in local environment including pH and pCO2)
• How does the Bohr effect help placental transfer?

  Small right shift in maternal circulation encourages O2 unloading

  Small left shift in fetal circulation encourages O2 uptake

  Double Bohr effect
• Placenta is generally an effective microbiological safety barrier, but is less efficient in early pregnancy (when fetal immune system is less developed)

  • risks include rubella, CMV, toxoplasmosis
  • HIV largely not transmitted during pregnancy in the Western world
  • maternal antibodies offer additional immune protection
• The maternal immune system sees the fetus as foreign tissue

  • delicate immunological truce prevents rejection
  • placental secretion of phosphocholine
  • trophoblast has reduced antigenic expression
  • suppression of maternal cytotoxic T cell activity
  • placental barrier to maternal lymphocytes