The Placenta

Question 1

Where does the placenta come from?

Answer 1

• Begins to develop in the second week of development.
• Early development there us focus on ensuring development of the “foetal membranes”
  
  • Sacs supporting embryo
  • Placenta
• There cannot be a healthy pregnancy without a healthy placenta

Question 2

When does implantation begin?

Answer 2

Day 6-9 Blastocyst attaches which allows endometrium to make contact.

By the end of the second week, the conceptus has mechanically implanted.

Question 3

What is the structure of the conceptus at the end of the 2nd week?

Answer 3

Question 4

What happens to the embryonic spaces?

Answer 4

• Yolk sac disappears
• Amniotic sac enlarges
• The chorionic sac is occupied by the expanding amniotic sac.
• End up with an amniochorionic membrane. This is what ruptures during labour.

Question 5

What does implantation achieve?

Answer 5

Establishes the basic unit of exchange:

• primary villi - early finger-like projections of trophoblast
• secondary villi - invasion of mesenchyme into core
• tertiary villi - invasion of mesenchyme core by foetal vessels.

Anchor the placenta

Establish maternal blood flow within the placenta

Question 6

How are the maternal blood and foetal capillary wall separated?

Answer 6

One layer of trophoblast.

This means there are only two cells (trophoblast and capillary wall) between the two circulations.

Question 7

What is a chorionic villus?

Answer 7

The placenta is a specialisation of the chorionic membrane

Made from chorion frondosum

Finger-like projections

• Trophoblast
• Inner connective tissue (foetal vessels)
• Very good for exchange

Question 8

What are some implantation defects that are possible?

Answer 8

Implantation in the wrong place:

• Ectopic pregnancy
• Plaenta Previa

Incomplete invasion:

• Placental insufficiency
• Pre-eclapsia

Question 9

What is an ectopic pregnancy?

Answer 9

• Implantation at a site other than uterine body (most commonly Fallopian tube)
• Can be peritoneal or ovarian
• Can very quickly become a life-threatening emergency.

Question 10

What is placenta previa?

Answer 10

• Implantation in the lower uterine segment
• Can cause haemorrhage in pregnancy
• Can require C-section delivery

Question 11

How do we control invasion in implantation?

Answer 11

• Transformation of the endometrium in the presence of a conceptus
  
  • becomes the decidua
• The deciduous reaction provides the balancing force for the invasion force of trophoblast
  
  • Ectopic pregnancy
    
    • No decidua therefore no control
• If the decidual reaction is sub-optimal
  
  • Can lead to a range of adverse pregnancy outcomes.

Question 12

How does the chorionic villus change in structure?

Answer 12

Get rid of cytotrophoblast layer

Margination of foetal capillaries

Cause: The placental barrier to get thinner

Question 13

What umbilical vessels do we have?

Answer 13

Two umbilical arteries - deoxygenated blood from foetus to placenta.

One umbilical vein - oxygenated blood from placenta to foetus.

Question 14

What hormones are produced by the placenta?

Answer 14

Protein:

• Human chorionic gonadotropin (hCG)
• Human chorionic somatomammotrophin
• Human chorionic thyrotrophin
• Human chorionic corticotrophin

Steroid:

• Progesterone
• Oestrogen

Question 15

When is hCG secreted?

Answer 15

First two months of pregnancy

Question 16

What does hCG do?

Answer 16

• Supports the secretory function of corpus Luteum.
• Produced by suncytiotrophoblast therefore it pregnancy specific
• Excreted in maternal urine therefore used as the basis for pregnancy testing

Question 17

What is trophoblast disease?

Answer 17

This is a group of conditions in which tumors grow inside a woman’s uterus.

• Molor pregnancy (hydatiform mole)
• Choriocarcinoma

Question 18

What do placental steroid hormones do?

Answer 18

Maintain pregnancy state.

Placental production takes over from the corpus luteum by the 11th week.

Question 19

What hormones influence maternal metabolism?

Answer 19

Progesterone - increased appetite

hCS / hPL - increases glucose availability to foetus.

Question 20

How are things transported cross the placenta?

Answer 20

Simple diffusion

• Molecules moving down a concentration gradient
  
  • Water
  • Electrolyte
  • Urea and uric acid
  • Gases

Facilitated diffusion

• Applies to glucose transport

Active transport

• Specific ‘transporters’ expressed by the syncytiotrophoblast
  
  • Amino acids
  • Iron
  • Vitamins

Transfer of passive immunity.

• Foetal and newborn immune system is immature
• Receptor-mediated process, maturing as pregnancy progresses
• Immuniglobulin class-specific
• IgG only
• IgG concentrations in foetal plasma exceed those in maternal circulation

Question 21

What immunity gets transferred from mum to foetus?

Answer 21

Foetal and newborn immune system is immature.

Receptor-mediated process, maturing as pregnancy progresses.

IgG only - by birth the IgG concentration in foetal plasma exceeds those is maternal circulation.

Question 22

What substances are harmful to the placenta?

Answer 22

• Thalidomide - limb defects
• Alcohol -FAS and ARND
• Therapeutic drugs - anti-eplilectin, warfarin, ACEI
• Drugs of abuse - dependency in the foetus and newborn
• Maternal smoking

Question 23

What is teratogenesis?

Answer 23

The process by which congenital malformations are produced in an embryo or fetus.

Question 24

When is the foetus most sensitive to teratogenesis?

Answer 24

• Pre-embyonic - lethal
• Embryonic - very sensitive as there is a narrow window for the development of some systems.
• Foetal - lower risk of huge structural defects as most of the large system have already developed.
• After embryonic perid, the riskof structural defects is very low, except in the CNS

Question 25

What infections can cross the placenta?

Answer 25

Lots! e.g. Varicella zoster, Cytomegalovirus, plasmodium falciparum

Make sure not exposed and are immune to infections because they are associated with adverse pregnancy outcomes.