Placenta & Foetal Membranes

Question 1

Where does the early foetal nutrition come from?

Answer 1

Diffusion from zona pellucida
Blastocyst fluid (blastocele)
Yolk sac

Up until day 12

Question 2

Where does the long-term foetal nutrition come from?

Answer 2

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

Question 3

Where does fertilisation occur?

Answer 3

Ampulla of uterine tube

Question 4

What is decidualisation?

Answer 4

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

Question 5

What else occurs at the same time at decidualisation?

Answer 5

At day 7, trophoblast also differentiates into 2 types of cells:

1. Cytotrophoblast
2. Syncytiotrophoblast

(blastocyst also splits into 2 layers called epiblast and hypoblast)

Question 6

What is invasion?

Answer 6

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

Question 7

What happens during complete implantation?

Answer 7

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

Question 8

When can you get bleeding occurring in pregnancy and why?

Answer 8

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

Question 9

Where can abnormal implantation take place?

Answer 9

Most commonly in uterine tubes (>95%)

Other sites in peritoneal cavity into vascular tissues e.g. rectouterine pouch and bowel

Question 10

What can ectopic pain mimic?

Answer 10

Appendicitis pain

Question 11

Where should implantation of the blastocyst ideally occur?

Answer 11

Into upper uterine wall covered in decidua known as the decidua capsularis

Question 12

What are the decidual layers?

Answer 12

1. Decidua basalis: decidua under blastocyst - maternal placenta
2. Decidua capsularis: decidua covering blastocyst
3. Decidua parietalis: remainder of uterine lining

Temporary and are shed following birth

Question 13

What are the functions of the placenta?

Answer 13

1. Endocrine i.e. hormone production of HCG, progesterone and oestrogen (HPO suppression)
2. Transfer of nutrition and waste affected by molecular weight, solubility and charge
3. 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)

Question 14

What different types of transport occur across the placenta?

Answer 14

Simple diffusion of O2/CO2
Facilitated diffusion of glucose
Active transport of ions/electrolytes
Pinocytosis/transcytosis of larger proteins e.g. IgG

Question 15

What are lacunar networks?

Answer 15

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)

Question 16

What is chorionic villi and how do they develop?

Answer 16

1. Primary (week 2): cells from cytotrophoblast grow into syncytiotrophoblast forming primary villi
2. Secondary (week 3): extra-embryonic mesenchyme grows into primary villi forming secondary villi
3. 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

Question 17

What is placental anchoring?

Answer 17

Cytotrophoblast grows through and surrounds the syncytiotrophoblast forming a shell that chorionic villi attach to via anchoring villi and to decidua basalis

Question 18

What are the 4 layers that separate foetal and maternal blood?

Answer 18

1. Maternal blood
2. Syncytiotrophoblast
3. Cytotrophoblast (continuous)
4. Foetal capillary endothelium/wall (+ mesenchyme)

Question 19

What is essential for embryonic survival?

Answer 19

Presence of branched chorionic villi bathed in maternal blood

Question 20

Where does the placenta normally sit?

Answer 20

Between endometrium and outer myometrium in a layer called the junctional zone (JZ) or inner myometrium

Question 21

What changes occur to the placenta membrane at 20 weeks?

Answer 21

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

Question 22

What placental dysfunctions can occur?

Answer 22

1. Position/development
2. Growth may be uncontrolled causing gestational trophoblastic disease or choriocarcinoma
3. Transport of unwanted substances/pathogens
4. Blood flow compromised

Question 23

What unwanted substances can placenta transport?

Answer 23

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.)

Question 24

What problems can occur with placental position and development?

Answer 24

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

Question 25

What blood flow problems can occur with the placenta?

Answer 25

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

Question 26

What can impairment of placental blood flow cause?

Answer 26

Foetal growth problems

Foetal death

Question 27

What are the extra-embryonic foetal membranes?

Answer 27

1. Amnion (innermost)
2. Yolk sac
3. Allantois
4. Chorion (outermost)

Question 28

What forms the chorion?

Answer 28

Formed from syncytiotrophoblast, cytotrophoblast and extraembryonic mesoderm and forms part of placenta - extraembryonic coelom is the chorionic cavity

Question 29

What eventually ends up happening to the extra-embryonic foetal membranes in late pregnancy?

Answer 29

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

Question 30

What should be present in the umbilical cord?

Answer 30

2x arteries (deox)
1x vein (ox)
Wharton’s jelly
Amnion cover

Question 31

Why should you check umbilical cord after delivery?

Answer 31

If there is defects e.g. only 1 artery, the baby may have associated defects esp. in CVS

Question 32

What does the placenta look like at the maternal surface?

Answer 32

Rough with lumps called cotyledon and intervillus spaces/grooves between them separating them - decidua basalis closest to this side

Question 33

What does the placental look like at the foetal surface?

Answer 33

Smooth as a result of transparent overlying smooth amniochorionic membrane with umbilical cord producing umbilical vessels that radiate to form chorionic vessels under amnion

Question 34

What has to happen to the placenta at birth?

Answer 34

Amniochorionic membrane must rupture to let the foetus out

Question 35

What does the umbilical cord look like?

Answer 35

Torturous with false knots throughout it with a smooth outer membrane to give it more protection and prevent it ripping