Placenta & Foetal Membranes Flashcards
Where does the early foetal nutrition come from?
Diffusion from zona pellucida
Blastocyst fluid (blastocele)
Yolk sac
Up until day 12
Where does the long-term foetal nutrition come from?
Maternal circulation via placenta from day 12 until term because the blastocyst attaches to endometrium at days 6-7 and implants into it at days 12-13
Where does fertilisation occur?
Ampulla of uterine tube
What is decidualisation?
A process that occurs at day 7 where the endometrium gets ready for pregnancy: blastocyst triggers the decidual reaction of uterine lining where decidual are uterine cells that have accumulated glycogen and lipid in response to progesterone
What else occurs at the same time at decidualisation?
At day 7, trophoblast also differentiates into 2 types of cells:
- Cytotrophoblast
- Syncytiotrophoblast
(blastocyst also splits into 2 layers called epiblast and hypoblast)
What is invasion?
A process that occurs at day 8 where the synchytiotrophoblast (multinucleated invasive cell mass) erodes endometrium causes apoptosis and proteolysis (not blood vessels yet) and produces HCG which maintains the corpus luteum - decidual cells degenerate during implantation providing their nutrients to the embryo
What happens during complete implantation?
On days 9-10, epithelial layers of uterus reform over implanted blastocyst, lacunae appear in the syncytium due to invasion of endometrial glands and maternal capillaries and the lacuna fluid provides nutrition for embryo by diffusion later becoming filled with maternal blood
When can you get bleeding occurring in pregnancy and why?
Around days 9-12 (coinciding with when you would expect your normal menstrual cycle but slightly earlier) because the syncytiotrophoblast will be invading through blood vessels
Where can abnormal implantation take place?
Most commonly in uterine tubes (>95%)
Other sites in peritoneal cavity into vascular tissues e.g. rectouterine pouch and bowel
What can ectopic pain mimic?
Appendicitis pain
Where should implantation of the blastocyst ideally occur?
Into upper uterine wall covered in decidua known as the decidua capsularis
What are the decidual layers?
- Decidua basalis: decidua under blastocyst - maternal placenta
- Decidua capsularis: decidua covering blastocyst
- Decidua parietalis: remainder of uterine lining
Temporary and are shed following birth
What are the functions of the placenta?
- Endocrine i.e. hormone production of HCG, progesterone and oestrogen (HPO suppression)
- Transfer of nutrition and waste affected by molecular weight, solubility and charge
- Immunity: most IgG Abs easily cross over giving passive immunity to baby but this can be problematic e.g. Rhesus (IgM cannot move across which is what ABO blood group Abs are so mother does not attack baby if blood group differs)
What different types of transport occur across the placenta?
Simple diffusion of O2/CO2
Facilitated diffusion of glucose
Active transport of ions/electrolytes
Pinocytosis/transcytosis of larger proteins e.g. IgG
What are lacunar networks?
Spaces called lacunae open up in syncytiotrophoblast at days 10-12 and adjacent lacunae fuse to form lacunar networks (prominent at embryonic pole) - maternal spiral arteries and veins open into them and maternal blood returns via endometrial veins via thin barrier made up of extraembryonic mesoderm (maternal and foetal circulations dont actually mix)
What is chorionic villi and how do they develop?
- Primary (week 2): cells from cytotrophoblast grow into syncytiotrophoblast forming primary villi
- Secondary (week 3): extra-embryonic mesenchyme grows into primary villi forming secondary villi
- Tertiary (week 3): blood vessels develop in extraembryonic mesoderm so there is communication with umbilical a. and v. so blood is present in foetal vessels by end of week 3
What is placental anchoring?
Cytotrophoblast grows through and surrounds the syncytiotrophoblast forming a shell that chorionic villi attach to via anchoring villi and to decidua basalis
What are the 4 layers that separate foetal and maternal blood?
- Maternal blood
- Syncytiotrophoblast
- Cytotrophoblast (continuous)
- Foetal capillary endothelium/wall (+ mesenchyme)
What is essential for embryonic survival?
Presence of branched chorionic villi bathed in maternal blood
Where does the placenta normally sit?
Between endometrium and outer myometrium in a layer called the junctional zone (JZ) or inner myometrium
What changes occur to the placenta membrane at 20 weeks?
It becomes thinner during gestation to cope with growing demand of developing foetus via cytotrophoblast degeneration by 20 weeks and in some cases intervening tissues ending up as 2 layers in some areas - placental capacity may be exceeded in post-date pregnancies which is why birth needs to be induced
Arborisation results in massive expansion of placental SA
What placental dysfunctions can occur?
- Position/development
- Growth may be uncontrolled causing gestational trophoblastic disease or choriocarcinoma
- Transport of unwanted substances/pathogens
- Blood flow compromised
What unwanted substances can placenta transport?
Smoking compounds Drugs e.g. cocaine Alcohol (FAS) Infectious agents e.g. Rubella Abs e.g. Rhesus (IgG) if foetus is Rh +ve but mother is Rh-ve (in 2nd preg.)
What problems can occur with placental position and development?
Uncontrolled invasion either partly into uterine myometrium SM layer (accrete) or fully through it (percreta) which can cause bleeding at pregnancy as not all placenta will come out
Inappropriate site e.g. ectopic or previa (low insertion over cervix or internal os which can also cause tearing/bleeding during late pregnancy and cannot delivery vaginally)
Abruption where placenta separates from uterine wall with sub-placental haemorrhage - can have concealed haemorrhage under placenta which you cant see but mum will start to become hypovolaemic
What blood flow problems can occur with the placenta?
Inadequate placentation causing pre-eclampsia: failure of normal invasion of trophoblast cells leading to maladaptation and constriction (rather than dilation/widening) of maternal spiral arterioles and poor uteroplacental blood flow
Impairment due to maternal vascular disease or gestational age (post-dates)
Mechanical e.g. IVC compression
Volume due to maternal haemorrhage or low hydration
What can impairment of placental blood flow cause?
Foetal growth problems
Foetal death
What are the extra-embryonic foetal membranes?
- Amnion (innermost)
- Yolk sac
- Allantois
- Chorion (outermost)
What forms the chorion?
Formed from syncytiotrophoblast, cytotrophoblast and extraembryonic mesoderm and forms part of placenta - extraembryonic coelom is the chorionic cavity
What eventually ends up happening to the extra-embryonic foetal membranes in late pregnancy?
Amnion grows as baby grows, obliterating the chorionic cavity and getting so close to chorion that it fuses with it forming the amniochorionic membrane
Uterine cavity obliterated as decidua capsularis joins parietalis
What should be present in the umbilical cord?
2x arteries (deox)
1x vein (ox)
Wharton’s jelly
Amnion cover
Why should you check umbilical cord after delivery?
If there is defects e.g. only 1 artery, the baby may have associated defects esp. in CVS
What does the placenta look like at the maternal surface?
Rough with lumps called cotyledon and intervillus spaces/grooves between them separating them - decidua basalis closest to this side
What does the placental look like at the foetal surface?
Smooth as a result of transparent overlying smooth amniochorionic membrane with umbilical cord producing umbilical vessels that radiate to form chorionic vessels under amnion
What has to happen to the placenta at birth?
Amniochorionic membrane must rupture to let the foetus out
What does the umbilical cord look like?
Torturous with false knots throughout it with a smooth outer membrane to give it more protection and prevent it ripping
