The placenta Flashcards
Where does the placenta come from?
- Begins to develop in second week of development
- Early development there is focus on ensuring development of foetal membranes
- Cannot have a healthy pregnancy without a healthy placenta
What are the foetal membranes?
- The sacs supporting the embryo/foetus
- The placenta
Why is week 2 known as the week of 2s?
- 2 distinct layers emerge from outer cell mass (syncytiotrophoblast and cytotrophoblast)
- And inner cell mass becomes bilaminar disc (epiblast and hypoblast)
What happens on day 6 of embryo development?
- Implantation begins
- Move into endometrial layer (maternal tissue)
- Blastocoele is genetically different structure
- Must be communication between cells of foetus and cells of mother to allow implantation
What happens on day 9 of embryo development?
- Early embryo is fully embedded in endometrium
- Placenta development is now vital to maintain integrity of embryo
What has happened by the end of the 2nd week of embryo development?
- Conceptus has implanted
- Embryo has 2 cavities: amniotic cavity and yolk sac
- Sacs are suspended by a connecting stalk within chorionic cavity
What is the fate of the embryonic spaces?
- Yolk sac disappears (part is pinched off to form primitive gut)
- Amniotic sac enlarges as embryo grows bigger
- Chorionic sac is occupied by expanding amniotic sac
What does implantation achieve?
- Establishes basic unit of exchange (villi)
- Anchors the placenta
- Establishes maternal blood flow within placenta
What is the function of the different types of villi?
- Primary villi: early finger-like projections of trophoblast
- Secondary villi: invasion of mesenchyme into core
- Tertiary villi: invasion of mesenchyme core by foetal vessels
Outline implantation
- Interstitial: uterine epithelium is breached and conceptus implants within stroma
- Placental membrane becomes progressively thinner as needs of foetus increase (large size of foetal brain requires more energy)
- Maternal blood separated from foetal capillary wall by layer of trophoblast
- 2 circulations never mix
What is a chorionic villus?
- Placenta is a specialisation of chorionic membrane
- Finger-like projections of trophoblast
- Inner connective tissue core containing foetal vessels
- Very good for exchange
- Foetal vessels can actively receive nutrients across cell membranes
What are the different types of implantation defects?
- Implantation in the wrong place (ectopic pregnancy, placenta praevia)
- Incomplete invasion (placental insufficiency, pre-eclampsia)
Outline ectopic pregnancy
- Implantation at site other than uterine body
- Fallopian tube most common site
- Can be peritoneal or ovarian
- Quickly becomes life-threatening emergency
Outline placenta praevia
- Implantation in lower uterine segment
- Can cause haemorrhage in pregnancy
- Can require C-section delivery
What controls how the embryo implants?
- Endometrium transforms to decidua in presence of conceptus
- Decidual reaction provides balancing force for invasive force of trophoblast
What happens to the decidua in ectopic pregnancy?
- There is no decidua in fallopian tubes
- Therefore no control
- Trophoblast can invade into intraperitoneal region
- Causes haemorrhage
What happens if decidual reaction is suboptimal?
- Can lead to a range of adverse pregnancy outcomes
- E.g. pre-eclampsia, preterm birth
What is a cotyledon?
- Functional units containing chorionic villi
- Allows transport
How does the chorionic villus change with time?
- First trimester villus creates thicker barrier
- Third trimester villus - barrier at optimal thinness to meet energy requirements of larger foetus
- Complete loss of syncytiotrophoblast
- Marginalisation of foetal capillaries so they sit right next to villus surface
What are the umbilical cord vessels?
- 2 umbilical arteries bring deoxygenated blood from foetus to placenta
- 1 umbilical vein brings oxygenated blood from placenta to foetus
What is the endocrine function of the placenta?
Produces:
- Human chorionic gonadotrophin
- Human chorionic somatomammotropin
- Human chorionic thyrotrophin
- Human chorionic corticotrophin
- Progesterone
- Oestrogen
Outline the production of hCG
- Produced during first 2 months of pregnancy
- Supports secretory function of corpus luteum
- Produced by syncytiotrophoblast
- Therefore is pregnancy specific
- Excreted in maternal urine
What is trophoblast disease?
- Cancer derived from trophoblast
- E.g. molar pregnancy and choriocarcinoma
- Leads to high hCG
What is the function of the placental steroid hormones?
- Progesterone and oestrogen
- Responsible for maintaining the pregnant state
- Placental production takes over from corpus luteum by the 11th week
Which placental hormones influence maternal metabolism?
- Progesterone leads to increased appetite
- hCS increases glucose availability to foetus
- This means that you can begin to see insulin resistance in mother
- Foetus is prioritised and gets preferential glucose for energy
Outline the transport functions of the placenta?
- Simple diffusion due to blood flow - e.g. water, electrolytes, urea, uric acid, gases
- Facilitated diffusion (applies to glucose transport)
Outline how gas exchange occurs over the placenta
- Simple diffusion
- Flow-limited, not diffusion-limited
- Foetal O2 stores are small therefore maintenance of adequate flow is essential
How does active transport occur over the placenta?
- Specific transporters are expressed by the syncytiotrophoblast
- Amino acids
- Iron
- Vitamins
Outline how passive immunity is transferred from mother to foetus
- Foetal and newborn immune system is immature
- Receptor-mediated process, maturing as pregnancy process
- Immunoglobin class-specific
- IgG only
- IgG concentrations in foetal plasma exceed those in maternal circulation
Outline the pathophysiology of placental transport
- Placenta is not a true barrier
- Substances can pass across placenta out of our control
- Teratogens can access foetus via placenta
- Unintentional outcomes from physiological process
- E.g. haemolytic disease of newborn secondary to Rhesus incompatibility of mother and foetus
What substances can cross the placenta and harm the foetus?
- Thalidomide - limb defects
- Alcohol - FAS and ARND
- Anti-epileptic drugs
- Warfarin
- ACE inhibitors
- Drugs of abuse
- Maternal smoking
Outline teratogenesis
- Timing is key
- Pre-embryonic exposure has lethal effects
- Embryonic period - embryo is highly sensitive
- After embryonic period risk of structural defects is very low
- Except CNS
