Placental Function

Question 1

Where does the placenta come from?

Answer 1

• Begins to develop in the second week of development
• Early development there is focus on ensuring development of the “fetal membranes”
– i.e. the sacs supporting the embryo/fetus
– and the placenta
• There cannot be a healthy pregnancy without a healthy placenta - starts from process of implantation - weds to be just right

Question 2

Decsribe the beginning of implantation

Answer 2

Blastocyst hatches from within zona pelluicda. Outer cell mass contact with endometrium. Syncitioprophoblast contact endometrium surface epithelium. Entire conceptus buried in endometrium by week 2 . Gives acces to glands and blood vessels

Question 3

What happens by the end of week 2

Answer 3

• the conceptus has implanted 
• the embryo and its  two cavities
– amniotic cavity &
– yolk sac will be
• suspended
– connecting stalk within a 
• supporting sac
– chorionic cavity

Question 4

Wat is the fate o the embryonic sacs

Answer 4

• The yolk sac disappears • The amniotic sac enlarges
• The chorionic sac is occupied by the expanding amniotic sac
Amniotic sac warapped around embryo . Grows with embryo. The amniotic sac grows to fill space in chorionic sac. Given amniochorion membrane (this ruptured in labor)

Question 5

What does implantation achieve

Answer 5

• establishes the basic unit of exchange - the chorionic villus - folds of chorionic membrane to give big SA for transport
– primary villi: early finger-like projections of trophoblast
– secondary villi: invasion of mesenchyme into core
– tertiary villi: invasion of mesenchyme core by fetal vessels
• anchor the placenta
• establish maternal blood flow within the placenta

Question 6

Decsribe implantation continued

Answer 6

• implantation is interstitial
– the uterine epithelium is breached and the conceptus implants within the stroma
• the placental membrane becomes progressively thinner as the needs of the fetus increase
• in the human one layer of trophoblast ultimately separates maternal blood
from fetal capillary wall - minimise barrier to 2 cellular layers - optimum exchange
– But the two circulations never mix (foetus is genetically distinct from mother)

Question 7

What is a chorionic villus?

Answer 7

• The placenta is a specialisation of the chorionic membrane 
• Chorion frondosum 
• Finger-like projections
– Trophoblast 
– Inner connective tissue core
• Fetal vessels 
– Very good for exchange
Cytotrophobast are a stem cell layer for sycitiotrophoblast 0 they will merge to keep expanding the syncitiotopphoblast.  Then get mesenchymal in the core of villi - mesoderm derivatives - fetal capillaries begin to develop in the centre of chorionic villi.

Question 8

What are plantation defects

Answer 8

• I Implantation in the wrong place
– Ectopic pregnancy
– Placenta praevia 
• II Incomplete invasion
– placental insufficiency
– pre-eclampsia

Question 9

What is an ectopic pregnancy and placenta prevail

Answer 9

 implantation at site other than uterine body (most commonly Fallopian tube) - barrier to zygote
 can be peritoneal or ovarian
 can very quickly become life- threatening emergency

 implantation in the lower uterine segment
 can cause haemorrhage in pregnancy
 can require C-section delivery

Question 10

How is invasio controlled

Answer 10

• Transformation of the endometrium over the course of implantation in the presence of a conceptus
– Becomes the decidua
Interaction between trophoblast and decidua
• The decidual reaction provides the balancing force for the invasive force of
the trophoblast - balance between invasion and managing invasive force. Checks invasion is sufficiently deep to maintain pregancy, but also makes sure it doesnt go too far
– ectopic pregnancy = no decidua therefore no control
• If the decidual reaction is sub-optimal
– Can lead to a range of adverse pregnancy outcomes

Question 11

What are examples of how invasion goes wrong

Answer 11

See slide 0 Shallow invasio - over zealous decidual reaction - preeclampsia -
When trophoblast invades too far, into myometrium - placenta accretion

Question 12

Describe the gross morphology of a placenta

Answer 12

See slide

Question 13

Describe the structural changes of the chorionic villus between. The second and thirst trimester

Answer 13

Fist trimester - thicker barrier - more layers to cross - lots of cytorophoblast ready to grow s in TIII - placenta stil growing.
By third trimester - need barrier optimised and thinner - get rid of complete cytoprophoblast layer - dotted around to repair hold. Only a few. Eliminates one cellular layer. Then capillaries pushed out to edge of villi, 2 cellular layers up against each other. Minimise barrier for transport a

Question 14

Decsribe how maternal blood contacts the chorionic villus

Answer 14

Thick main stem villi - branch out. At the tip of the villus trees there are anchoring. Villi - open ended tip - spill cytotrophoblast cells out of tips to create big wide trophoblast cell around placental membrane around amniochorion - enclose conceptus within decidua.. endometrial veins and arteries eel endometrium. These vessels are transformed into high flow low resistance circulation - hey spill maternal blood into blood lake” in cotyledon.

Question 15

Describe the fetal circulation vessels

Answer 15

• Two umbilical arteries
– Deoxygenated blood from fetus to placenta
• One umbilical vein
– Oxygenated blood from placenta to fetus
High pO2 in maternal blood lake, low in fetal circulation.

Question 16

What are hormones reduced by the placenta

Answer 16

Steroid hormones - produced by placentas - takes over from corpus luteum:

• progesterone
• oestrogen

Protein hormones:
human chorionic gonadotrophin (hCG) human chorionic somatomammotrophin human chorionic thyrotrophin
human chorionic corticotrophin

Question 17

What is the role of hcg in pregnancy

Answer 17

Maintains corpus luteum until placenta can take over
• produced during the first 2 months of pregnancy
• supports the secretory function of corpus luteum
• produced by syncytiotrophoblast therefore is pregnancy specific
• excreted in maternal urine therefore used as the basis for pregnancy testing
• trophoblast disease
– molar pregnancy (hydatidiform mole)
– choriocarcinoma

Question 18

What is the role of steroid hormones

Answer 18

• progesterone and oestrogen
• responsible for maintaining the pregnant state - suppression of hpo axis - development of Mary glans - preparing for both of baby
• placental production takes over from corpus luteum by the 11th week

Question 19

How do placental hormones influence maternal metabolism

Answer 19

• Progesterone
– increased appetite - fat stores to help lactation
• hCS / hPL
– increases glucose availability to fetus - mothers tissue sbecome insulin resistance - glucose crosses placenta very quickly and easily

Question 20

What are the transport function of the placenta

Answer 20

• Simple diffusion
– molecules moving down a concentration gradient
• water • electrolytes • urea & uric acid • gases
• Facilitated diffusion
– applies to glucose transport

Question 21

Decsribe gas exchange in the placenta

Answer 21

• simple diffusion
• flow-limited, not diffusion-limited - need to make sure flow is optimised
• fetal O2 stores, are small therefore maintenance of adequate flow essential
• NB!!
– adequate uteroplacental circulation

Question 22

Decsribe active transport in the placenta

Answer 22

• specific “transporters” expressed by the syncytiotrophoblast
– amino acids
– iron
– vitamins

Question 23

Decsribe the transfer of passive immunity

Answer 23

• fetal (and newborn) immune system is immature
• receptor-mediated process, maturing as pregnancy progresses
• immunoglobulin class-specific
• IgG only
• IgG concentrations in fetal plasma exceed those in maternal circulation
Oly receptors for IgG
Foetus “borrows” antibodies from mother. As pregnancy proceeds, much more immunoglobulins cross placenta

Question 24

Describe the basis of the pathophysiology of placental transport

Answer 24

• the placenta is not a true “barrier”
• teratogens can access the fetus via the placenta - Teratogens = agents athat can interfere with normal physiological processes
• unintentional outcomes from physiological process
– Haemolytic disease of the newborn secondary to Rhesus incompatibility of
mother and fetus
Transfer of antibodies - anti thesis antibodies ban go across placenta and trigger destruction of RBCs in the baby

Question 25

What are harmful substances to the placenta

Answer 25

• thalidomide
– Limb defects - stops limb buds from growing to whole length
• Alcohol
– FAS and ARND
— alcohol can go straight through placenta
• therapeutic drugs
– Anti-epileptic drugs
– Warfarin
– ACE inhibitors
• drugs of abuse eg opioids, cocaine, etc, transported across placenta a
– Dependency in the fetus and newborn
• maternal smoking - not necessarily teratogen, but can have negative effect on placenta

Question 26

Scribe teh timing of teratogenesis

Answer 26

• Timing is key 
• Pre-embryonic
– lethal effects 
• Embryonic (weeks 3-8)
– ++ sensitive
– narrow windows for some systems 
• Fetal
– +/- sensitive 
- less bc structures just growing and maturing lower risk of structural defects 
• After embryonic period, risk of structural defects very low 
– Except CNS

Question 27

Describe infectio in pregnancy

Answer 27

See slide