In week 2 of embryological development, two distinct cellular layers emerge from the outer cell mass.
What happens to the inner cell mass during week 2 of embryological development?
Inner cell mass becomes the bilaminar disk:
When does implantation begin and end?
- Commencement: day 6
- Completion: day 9
What are the 2 cavities in the embryo?
- Amniotic cavity
- Yolk sac
What has happened by the end of week 2 of embryological development?
- The conceptus has implanted
- The embryo and its two cavities are suspended by the connecting stalk within a supporting sac called the chorionic cavity
What is the fate of the embryonic spaces after the end of week 2?
- Yolk sac disappears
- Amniotic sac enlarges
- Chorionic sac is occupied by the expanding amniotic sac
What does implantation achieve?
- Establishes the basic unit of exchange
- Anchors the placenta
- Establishes maternal blood flow within the placenta
Identify and describe the 3 forms of villi created by implantation
- Primary villi: early finger-like projections of trophoblast
- Secondary villi: invasion of mesenchyme into core
- Tertiary villi: invasion of mesenchyme core by fetal vessels
What is meant by the statement 'implantation is interstitial'
Implantation is interstitial as the the uterine epithelium is breached and the conceptus implants within the stroma
Briefly, how does the placenta change as the foetus develops?
The placental membrane becomes progressively thinner as the needs of the foetus increase
What are chorionic villi?
- Chorionic villi are villi that sprout from the chorion to provide maximum contact area with maternal blood
- They contain foetal blood vessels
What is the placenta?
The placenta is an organ, specialised from the chorionic membrane, that connects the developing foetus to the uterine wall to allow nutrient uptake, thermo-regulation, waste elimination, and gas exchange
Identify 2 things that can go wrong with implantation
- Implantation in the wrong place
- Incomplete invasion
Identify 2 clinical conditions which can result from incomplete invasion
- Placental insufficiency
Identify 2 clinical conditions which result from implantation at an inappropriate site
- Ectopic pregnancy
- Placentae praevia
Describe implantation in ectopic pregnancy and the consequences of such
- Implantation at site other than uterine body (peritoneal/ ovarian)
- Very quickly becomes life-threatening emergency
Describe implantation during placenta praevia and the consequences of such
- Implantation in the lower uterine segment
- Can cause haemorrhage in pregnancy
- Requires C-section delivery
Describe how the invasion during implantation is controlled
- Transformation of the endometrium in the presence of a conceptus
- Conceptus becomes the decidua
- The decidual reaction provides the balancing force for the invasive force of the trophoblast
What is the decidual reaction?
- The decidual reaction is the set of changes in the endometrium of the uterus that prepare it for implantation of an embryo
- These changes include swelling of stromal cells due to accumulation of glycogen and other nutrients
What happens if the decidual reaction is sub-optimal?
Sub-optimal decidual reactions can lead to a range of adverse pregnancy outcomes
Describe the structure of the chorionic villus
Over the course of pregnancy, the 'placental' barrier becomes thinner.
Describe this in terms of the chorionic villi
- First trimester villus: thicker barrier (blue)
- Third trimester villus: barrier at optimal “thinness” (pink)
Identify the 2 classes of hormone produced by the placenta
Identify 2 steroid hormones produced by the placenta
Identify 4 protein hormones produced by the placenta
- Human chorionic gonadotrophin (hCG)
- Human chorionic somatomammotrophin (hPL/hCS)
- Human chorionic thyrotrophin
- Human chorionic corticotrophin
Describe the production, excretion and function of hCG
- Production: produced during first 2 months of pregnancy by synctiotrophoblast
- Excretion: excreted in maternal urine
- Function: mimics the action of LH and supports the secretory function of corpus luteum
What do the placental steroid hormones do?
- Responsible for maintaining the pregnant state
- Placental production takes over from corpus luteum by the 11th week
Explain how placental hormones influence maternal metabolism
- Progesterone increases appetite
- hCS increases glucose availability to foetus
Identify and describe the transport functions of the placenta
- Simple diffusion: H2O, electrolytes, gases, urea & uric acid
- Facilitated diffusion: glucose
Describe gas exchange in the placenta
- Simple diffusion
- Flow-limited, not diffusion-limited
Why is it important to maintain adequate uteroplacental circulation?
Foetal O2 stores are small therefore maintenance of adequate uteroplacental circulation is essential
There are specific “transporters” expressed by the syncytiotrophoblast.
Identify 3 substances which are transported
- Amino acids
The foetal (and newborn) immune system is immature and thus needs the transfer of passive immunity.
- Passive immunity is a receptor-mediated and immunoglobulin class-specific process involving the transfer of the IgG antibody only
- IgG concentrations in foetal plasma exceed those in maternal circulation
Identify 5 substances which can cross the placenta and their effects
- Thalidomide: limb defects
- Alcohol: FAS and ARND
- Therapeutic drugs e.g. warfarin, ace inhibitors, anti-epileptic drugs
- Drugs of abuse: dependency in the fetus and newborn
- Maternal smoking: smaller placenta, smaller babies
Teratogens can access the foetus by crossing the placenta.
Briefly highlight the effects of teratogenesis in the following period of development:
- Pre-embryonic – lethal effects
- Embryonic – extremely sensitive (narrow windows for some systems)
- Fetal – ± sensitive