The placenta

Question 1

What are the functions of the placenta?

Answer 1

• Provides nutrients and removes wastes
• Partial immunological barrier
• Synthesises and secretes hormones required for maintenance of pregnancy and foetal growth:
  
  • Progesterone and oestrogen
  • Prostaglandins
  • hCG, hPL
  • CRH, GH, ACTH, GnRH, TSH
  • Relaxin

Question 2

Where does blood exchange occur in the foetus?

Answer 2

• Foetal circulation passes through the umbilical cord and enters chorionic villi - this is where exchange happens

Question 3

How is the placenta formed? (9 steps)

Answer 3

1. After implantation, syncytiotrophoblast cells for multinucleated wall
2. Decidualisation response has damaged uterine blood vessels
3. Blood from the uterine arteries pools in lacunae of syncytiotrophoblasts
4. Lacunae merge
5. Cytotrophoblast sends cells to anchor placenta to the endometrium - forms primary chorionic villi
6. Week 3 - foetal mesoderm layer expands under cytotrophoblast - forms secondary chorionic villi
7. Blood vessels enter villi - becomes tertiary chorionic villi
8. Cytotrophoblasts form cytotrophoblastic shell:
   
   1. Villi attached to this: anchoring villi
   2. Villi branchin of this: branching villi
9. Foetal blood flow established by week 4

Question 4

What is the conversion process?

Answer 4

• The differentiation of cytotrophoblast cells
• Mature maternal blood flow by week 10-12, results in formation of extravillous trophoblast cells:
  
  • Cytotrophoblast cells detach from villi and migrate into maternal endometrium and myometrium
  • Some migrate along spiral arteries - these become endovascular extravillous trophoblasts (these condvert vessels into low pressure, high capacity circulation)

Question 5

What are the complications of conversion failure?

Answer 5

• Villi will break and will cause poor diffusion
• It can lead to an increase in pressure that is too high:
  
  • Pre-eclampsia
  • Intrauterine growth restriction

Question 6

What is histotrophic nutrition?

Answer 6

• Occurs before implantation -> early implantation
• Endometrial fluids (from endometrial glands) provide nutrition
• Substances pass through thin trophoblast and distributed via diffusion
• Blood vessels develop as mesoderm develops

Question 7

What is haemotrophic nutrition?

Answer 7

• Blood-borne via placenta - complete by week 14
• NOTE: maternal and foetal circulation are separate
  
  • Uterine a./v. between mother and placenta
  • Umbilical a./v. between foetus and placenta

Question 8

How are different molecules transported across the placenta?

Answer 8

• Simple diffusion: low-molecular weight, non-polar molecules
• Facilitated diffusion: glucose, lactate
• Active transport: amino acids, Fe, Ca, P, vitamins

Question 9

Outline the different sections of the foetal membrane and spaces.

Answer 9

• Chorion
• Amnion
• Yolk sac
• Atlantois

Question 10

What is the chorionic membrane?

Answer 10

• Mesodermal inner cells mass + trophoblast (both parts)

Question 11

What is the amnion?

Answer 11

• Transparent sac filled with amniotic fluid - protects embryo and provides optimal conditions
• Amniotic membrane: ectodermal and mesodermal inner cell mass

Question 12

What is the yolk sac?

Answer 12

• Endodermal and mesodermal layers under the trophoblast

Question 13

What is the allantois?

Answer 13

• Allantoic endoderm at caudal end of embryo - grows out, surrounded by mesoderm, to form allantois

Question 14

What is the fusion of allantoic and chorionic mesoderm?

Answer 14

• This marks the site of chorio-allantoic placenta and the umbilical cord

Question 15

Development of chorion, yolk sac and amnion throughout pregnancy

Answer 15

• Chorion and yolk sac decrease in size
• Amnion increases

Question 16

What are the functions of amniotic fluid?

Answer 16

• Protection - cushions foetus from injury
• Temperature regulation
• Prevents compression of umbilical cord
• Allows feotus to grow and move
• Fluid reservoir for development of hollow structures

Question 17

How much amniotic fluid is there?

Answer 17

• 10mL @ 8 weeks -> 1L @ 32 weeks
  
  • Due to water exchange between foetus/placenta/mother
  • Can be used as a marker for in utero developmental defects, physiological responses to foetal hypoxia and other metabolic disturbances

Question 18

Define polyhydramnios and oligohydramnios

Answer 18

• Polyhydramnios:
  
  • ​Too much amniotic fluid
  • Treated antenatally with serial amniocentesis
• Oligohydramnios:
  
  • Not enough amniotic fluid
  • If membranes are intact, usually treated with delivery

Both can be associated with major congenital anomales or adverse perinatal outcomes