L11 - Placental development

Question 1

How does the placenta develop?

Answer 1

Begins to develop upon implantation of the blastocyst into the maternal endometrium. The syncytiotrophoblast invades decidua (endometrium).
The outer layer of the blastocyst becomes trophoblast which forms the outer layer of the placenta.
The outer layer is divided into 2 further layers.
The underlying cytotrophoblast and the overlying syncytiotrophoblast.
cytotrophoblast cells erode maternal spiral arteries and veins. spaces (lacunae) between fill up with maternal blood. followed by mesoderm that develops into fetal vessels

Question 2

The syncytiotrophoblast

Answer 2

The syncytiotrophoblast is amultinucleatedcontinuous cell layer that covers the surface of the placenta. It forms as a result of differentiation and fusion of the underlyingcytotrophoblastcells, a process that continues throughout placental development. The syncytiotrophoblast (otherwise known assyncytium), thereby contributes to the barrier function of the placenta.

Question 3

Compare cyto and syncytio trophoblast cells

Answer 3

CTB - Are undifferentiated stem cells compared to STB which are fully differentiated
CTB invade maternal blood vessels and destroys epithelium whereas STB is in direct contact with maternal blood
CTB give risk to STB
CTB reduce in number as pregnancy advances
STBs produce placental hormones

Question 4

Endocrine function - placenta

Answer 4

HCG maintains corpus luteum of pregnancy
progesterone and oestrogen
HPL - human placental lactogen - growth lactation

Question 5

What is the placental barrier?

Answer 5

Maternal blood in the lacunae in direct contact with syncytiotrophoblasts
Mono layer of syncytiotrophoblast/cytotrophoblast/fetal capillary epithelium is all that separates the fetal and maternal blood
Cytotrophoblasts decrease as the pregnancy advances (not needed)
The barrier thins as pregnancy advances leading to a greater surface area for exchange (over 10m2 )

Question 6

What transfers across placenta?

Answer 6

Gases 
water electrolytes
steroid hormones
proteins - only by pinocytosis 
maternal antibodies IgG transfer starts at 12 weeks and mainly after 34 weeks

Question 7

What are the parts of the decidua?

Answer 7

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

Question 8

What is vasa praevia?

Answer 8

velomentous cord insertion that runs across the cervical os. fetal blood vessels within the umbilical cord pass over the internal os.
if the woman starts contracting it could cause it to rip and bleed as the internal os dilates in labour the vessels are stretched and exposed so can rupture leading to massive fetal blood loss and death

Question 9

What can failure of trophoblastic invasion cause?

Answer 9

• Failure of trophoblastic invasion into maternal circulation at 12 and 18 weeks can lead to poor maternal/fetal mixing of blood, lack of oxygen and nutrients to the fetus, leads to fetal growth restriction, pre-eclampsia

Question 10

what is placenta praevia

Answer 10

Placenta praevia
massive bleeding in pregnancy
placenta lying in lower part of uterus

Question 11

what is placenta accreta?

Answer 11

Placenta accreta - Placenta unable to separate at birth – uterus can not contract down and massive bleeding.
The placenta has invaded too deep into the myometrium and thus after birth the placenta can not separate and stays within the uterus. As a consequence the uterus can not contract down and massive bleeding can occur leading to the requirement to do a hysterectomy.

Question 12

What is placental abruption?

Answer 12

Massive bleeding in pregnancy caused by separation of placenta, leading to disruption of the blood to the fetus - fetal distress and death