Repro: The Placenta

Question 1

What actually breaks when the ‘waters break’ during labour?

Answer 1

The membranes of the chorionic sac and amniotic sac fuse together - called the chorion
This what breaks during labour

Question 2

What separates the maternal and fetal circulations?

Answer 2

Only the chorionic membrane separates the circulations. The chorionic villi are bathed in maternal blood

Question 3

What are the primary, secondary and tertiary villi of the placenta ?

Answer 3

The primary vili are early finger like projections of the trophoblast
The secondary villi invade the mesenchyme
The tertiary villi invade the mesenchyme core by fetal vessles

Question 4

How is the placenta anchored within the endometrium?

Answer 4

Via the outermost cytotrophoblast shell

Question 5

What is an ectopic pregnancy?

Answer 5

Implantation at a site other than the uterine body (most commonly the ampulla)
Implantation is invasive so can very quickly become a life threatening emergency

Question 6

What is placenta praevia?

Answer 6

Implantation into the lower uterine segment
The internal os can be occluded so requires a c section delivery
Can cause haemorrhaging

Question 7

What are pre-decidual cells?

Answer 7

Specialised maternal cells that control implantation.
They balance the invasive force of the trophoblasts

NB The decidual cells are only present in the endometrium therefore in ectopic pregnancy the invasion is not controlled so can rupture and continue to invade and cause haemorrhage

Question 8

Why do the spiral arteries of the endometrium need to be remodelled in pregnancy?

How are they remodelled?

Answer 8

Need to create a low resistance vascular bed to maintain the high flow to meet fetal demand (particularly for gas exchange late in gestation)

The trophoblast invades the spiral arteries and the maternal endothelium is diplaced by fetal tissue

Question 9

What abnormality occurs in pre-eclampsia?

Answer 9

A very common condition where the normal remodelling of the spiral arteries doesnt happen so there is too little invasion of fetal tissue.
Results in hypertension and proteinuria

Question 10

What are the different degrees to which membranes are shared by monozygotic twins?
Which is safest?

Answer 10

Both can have their own sacs and placenta - safest
They can share a chorion but have their own amnion
They can share the chorion and amnion - this is the most dangerous due to risk of twin to twin transfusion syndrome

Question 11

What are the differences between a T1 placenta and a term placenta?

Answer 11

The T1 placental barrier is thick as the demands of the fetus are low, whereas a term placenta has a thin barrier. A term placenta has increased surface area due to lots of branching vili.
A T1 placenta has a complete cytotrophoblast layer which acts as a stem cell layer for the overlying syncytiotrophoblast which is sheered from maternal circulation. However in term placenta the cytotrophoblast layer is lost

Question 12

What does the placenta synthesise?

Answer 12

Glycogen
Cholesterol - huge capacity as this is the precursor for oestrogen and progesterone
Fatty acids

Question 13

What hormones are produced by the placenta?

Answer 13

Steroid hormones: progesterone and oestrogen

Peptide hormones: hCG, human corionic somatomammotrophin, thyrotrophin and corticotrophin

Question 14

What is the function of hCG? Which structure produces it?

Answer 14

Produced during the first 2 moths of pregnancy and supports the secretory function of the corpus luteum until the placenta can take over.
Produced by the syncytiotrophoblast so is specific to pregnancy

Question 15

How does progesterone influence maternal metabolism?

Answer 15

it increased appetite in order to increase fat stores

Question 16

What type of gas exchange occurs at the placenta?

What is the limiting factor?

Answer 16

Simple diffusion
It is flow limited not diffusion limited therefore need high flow low resistance flow in the vili (fetal O2 stores are low so need good flow)

Question 17

How is passive immunity achieved?

Answer 17

Fetal immunity is immature even at birth. It is a receptor mediated process - IgG conc in fetal plasma exceed those in maternal circulation so its an active proccess.

IgG is only passed on although breast milk contains other antibodies. The fetus is likely to encounter the same infections as the mother does so same antibody passes on

Question 18

What are teratogens? Give examples

Answer 18

Agents that cause malformation of the fetus

eg rubella virus, syphilis, alcohol, ACEI, thalidomide, smoking

Question 19

What is rhesus blood group incompatibility?

Answer 19

Where the blood group of the fetus is positive and the mother is negative and visa versa. Can get leakage of fetal blood into the maternal circulation so mother mounts an immune response and during the next pregnancy the antibodies attack fetal RBCs

Can now give prophylactic treatment in the first pregnancy

Question 20

How does early pregnancy ‘support itself’?

Answer 20

The corpus luteum produces progesterone and oestrogen which maintains the endometrium

The endometrium supports the placenta

The placenta produces hCG to maintain the corpus luteum