17. Fertilisation and Placental Development

Question 1

fertilisation - 4 days

Answer 1

zygote divided by mitosis
by 72 hours, forms morula (cluster of cells)
by 4 days, forms blastocyst

Question 2

days 4-5

Answer 2

morula develops a cavity = blastocyst
blastocyst thins out and becomes trophoblast
rest of the cells move to form inner cell mass - creates embryonic pole
blastocyst reaches uterine lumen and is ready for implantation

Question 3

days 6-7

Answer 3

bilaminar disc of embryo
inner cell mass differentiates into epiblast and hypoblast
layers are in contact

Question 4

hypoblast

Answer 4

forms extra embryonic membranes and primary yolk sac

Question 5

epiblast

Answer 5

forms embryo

amniotic cavity develops within mass

Question 6

days 16+

Answer 6

bilaminar disc develops further, into 3 layers
process = gastrulation
initiated by primitive streak
epiblast becomes ectoderm
hypoblast is replaced by cells from the epiblast to be the endoderm
epiblast also gives rise to mesoderm (between epiderm and endoderm)

Question 7

placental formation

Answer 7

syncytiotrophoblast invades ducidua (endometrium) - burrows into myometrium, starts formation or primary secondary and tertiary villi
cut-trophoblast cells erode maternal spiral arteries and veins
spaces between (lacunae) fill up with maternal blood
mesoderm develops foetal vessels
transfer of nutrients and O2 occurs over simple cellular barrier

Question 8

cut-trophoblast cells (CTB)

Answer 8

undifferentiated stem cells
invade maternal blood vessels, destroy epithelium
give rise to syncytiotrophoblast cells (STB)
reduce in number as pregnancy advances

Question 9

syncytiotrophoblast cells (STB)

Answer 9

fully differentiated cells
direct contact with maternal blood
produce placental hormones

Question 10

placental hormones

Answer 10

human chorionic gonadotrophin (hCG)

human placental lactogen (HPL)

Question 11

human chorionic gonadotrophin

Answer 11

maintains corpus luteum during pregnancy

progesterone and oestrogen

Question 12

human placental lactogen

Answer 12

important for growth and lactation

carbohydrate and lipids

Question 13

placental barrier

Answer 13

maternal blood in lacunae in direct contact with syncytiotrophoblasts
monolayer of STB/CTB/foetal capillary epithelium
barrier thins as barrier advances: greater surface area for exchange

Question 14

substances transferred across the placenta

Answer 14

gases (O2, CO2)
water and electrolytes
steroid hormones
proteins 
maternal antibodies (IgG)- mainly after 34 weeks (hence premature infants are less protected)

Question 15

named parts of the decidua

Answer 15

capsularis - overlying embryo and chorionic cavity
parietalis - side uterus, not occupied by embryo
basalis - between uterine wall and chorionic villi

Question 16

vasa praevia

Answer 16

velmentous cord insertion that runs across cervical os
foetal vessels within umbilical cord pass over internal os
as internal os dilates in labour, vessels are stretched and exposed
can rupture leading to massive foetal blood loss and death

Question 17

position of placenta

Answer 17

in uterus
mainly fundal (at top)
anterior or posterior
‘low lying’ or placenta praaevia (near cervical os)

Question 18

placenta praevia

Answer 18

placenta located close to internal cervical os
massive bleeding in pregnancy
maternal of foetal death

Question 19

failure of trophoblast invasion

Answer 19

into maternal circulation at 12-18 weeks
poor maternal foetal mixing of blood
foetus is deprived of nutrients and oxygen
foetal growth restriction and pre-eclampsia

Question 20

placenta accrete

Answer 20

placenta unable to separate at birth, stays in uterus
uterus cannot contract down
massive bleeding: hysterectomy may be required

Question 21

placental abruption

Answer 21

massive bleeding in pregnancy (often concealed)
not seen in vagina, uterus fills up with blood
extremely painful
foetal or maternal death