Repro15 - The Placenta & Support of Pregnancy

Question 1

3 features of ‘the week of 2s’

2 Outer Cell Masses
2 Inner Cell Masses
2 Cavities

Answer 1

1. ) 2 Outer Cell Masses - syncytiotrophoblast and cytotrophoblast
   - form the placenta
2. ) 2 Inner Cell Masses - bilaminar disk (epiblast and hypoblast)
   - form the fetus and the fetal membranes
3. ) 2 Cavities - amniotic cavity and yolk sac
   - eventually, the yolk sac disappears and the amniotic sac enlarges and occupies the chorionic sac
   - forming the amniochorion

Question 2

5 features of implantation

What is it?
Maternal Blood Flow
Chorionic Villi
Placenta
Placental Membrane

Answer 2

1. ) What is it? - conceptus impants within the stroma of the uterine epithelium
   - this begins on day 6 after fertilisation
2. ) Establishes Maternal Blood Flow - end of week 2
   - syncytiotrophoblast erodes the maternal tissues allowing maternal blood from uterine spiral arteries to enter the lacunar network inside.
3. ) Chorionic Villi - establishes basic unit of exchange
   - site of exhange of substances with maternal circulation

4.) Anchors The Placenta - anchoring villi connect the maternal uterus to a cytotrophoblastic shell

5. ) Thinning Placental Membrane - easier exchange of substances as the needs of the fetus increases
   - eventually, one layer of trophoblast separates maternal and fetal blood but the two circulations never mix

Question 3

3 stages of chorionic villi

Primary
Secondary
Tertiary

Answer 3

1. ) Primary Chorionic Villi - formed by cytotrophoblast
   - finger-like projections which penetrate and expand into the surrounding syncytiotrophoblast
2. ) Secondary Chorionic Villi - contains a core of LCT which comes from extraembryonic mesoderm
3. ) Tertiary Chorionic Villi - contains embryonic vessels which form in the embryonic mesoderm

Question 4

4 implantation defects

Implantation in Wrong Place x2
Incomplete Invasion x2

Answer 4

1. ) Ectopic Pregnancy - implantation in wrong place
   - doesn’t occur in uterine body (usually fallopian tubes)
   - no decidua so no control of invasion
2. ) Placenta Praevia - implantation in wrong place
   - occurs too low in the uterus
3. ) Placental Insufficiency - leads to incomplete invasion
   - placenta unable to deliver adequate supply of nutrients and oxygen to the fetus
   - decidual reaction is suboptimal
4. ) Pre-eclampsia - leads to incomplete invasion
   - decidual reaction is suboptimal

Question 5

Endocrine function of the placenta

Proteins x4
Steroids x2
hCG
Influence on Maternal Metabolism

Answer 5

1.) Proteins Produced - hCG, hCS (somatomammotrophin),
hCT (thyrotropin), hCC (corticotrophin)

2. ) Steriods Produced - oestrogen and progesterone
   - responsible for maintaining the pregnant state
   - placenta takes over from CL by the 11th week
3. ) Human Chorionic Gonadotrophin (hCG)
   - produced by syncytiotrophoblast in first 2 months
   - supports secretory function of CL (replaces LH)
   - excreted in urine so used in pregnancy testing but can also be used in trophoblast diseases
4. ) Influence on Maternal Metabolism
   - increases appetite (progesterone)
   - increases glucose availability to fetus (hCS and human placental lactogen)

Question 6

Transport of substances in the placenta

Simple Diffusion
Facilitated Diffusion
Active Transport
Gas Exchange
Passive Immunity

Answer 6

1. ) Simple Diffusion - gases, water, electrolytes, urea
2. ) Facilitated Diffusion - glucose
3. ) Active Transport - AAs, iron, vitamins
   - transporters expressed by syncytiotrophoblast
4. ) Gas Exchange - efficiency depends on blood flow
   - adequate uteroplacental circulation needed for gas exchange as fetal oxygen stores are small
5. ) Passive Immunity - receptor mediated endocytosis
   - IgG only, goes against concentration gradient
   - sometimes, maternal antibodies can damage newborm RBCs due to rhesus incompatibility of mother and fetus

Question 7

Timing of teratogenesis

5 harmful substances to the placenta

Answer 7

Teratogenesis - process where congenital defects are produced in an embryo or fetus.

• pre-embryonic: lethal effects
• embryonic: very sensitive, leads to structural defects
• fetal: risk of structural defects very low, except in CNS

1. ) Thalidomide - limb defects
2. ) Alcohol - fetal alcohol syndrome (FAS), alcohol related neurological disease (ARND)
3. ) Therapeutic Drugs - anti-epileptics, ACE inhibitors, warfarin
4. ) Drug Abuse - causes dependency in newborn
5. ) Smoking - not a teratogen itself
   - affects size and structure of the placenta which can lead to placental insufficiency