17. Placenta and intra-uterine growth restriction Flashcards
Fertilisation
Sperm and Ovum meet in Fallopian Tube (Uterine Tube) (usually ampulla) 12-24 hours after ovulation.
Fusion occurs and 2nd meiotic division occurs
Acrosome reaction makes ovum impermeable to other sperm
End- Zygote- has diploid (46 chromosomes)
Days 4-5
The morula develops a cavity and becomes known as a blastocyst.
Blastocyst thins out and becomes the trophoblast –start of the placenta
The rest of the cells move (are pushed up) to form the inner cell mass. This creates an embryonic pole.
The blastocyst has now reached the uterine lumen and is ready for implantation.
Day 6-7 bilaminar disc of the embryo
Inner cell mass differentiates into two layers: epiblast and hypoblast.
These two layers are in contact.
Hypoblast forms extraembryonic membranes and the primary yolk sac
Epiblast forms embryo
Amniotic cavity develops within the epiblast mass
Days 16+
Bilaminar disc develops further by forming 3 distinct layers (this process is known as gastrulation
Initiated by primitive streak.
The epiblast becomes known as ectoderm
The hypoblast is replaced by cells from the epiblast and becomes endoderm
The epiblast gives rise to the third layer the mesoderm.
The 3 germ layers of the embryo
The hypoblast degenerates. The epiblast gives rise to all three germ layers.
The embryo folds to create the adult pattern
The development of the placenta
Syncytiotrophoblast burrows into the myometrium of the uterus – the syncytiotrophoblasts invading the maternal spiral arteries and starting the formation of the primary/secondary and tertiary villi
Formation of the placenta
Syncytiotrophoblast invades decidua (endometrium)
Cytotrophoblast cells erodes maternal spiral arteries and veins
Spaces (lacunae) between the fill up with maternal blood
Followed by mesoderm that develops into fetal vessels
Aiding the transfer of nutrients, O2, across a simple cellular barrier
Cytotrophoblast cells (CTB)
Undifferentiated stem cells
Invade the maternal blood vessels and destroy the epithelium
Give rise to the syncytiotrophoblast cells (STB)
Reduce in number as pregnancy advances
Syncytiotrophoblast cells (STB)
Fully differentiated cells
Direct contact with maternal blood
Produce placental hormones
The placenta as an endocrine organ
Human chorionic gonadotrophin (HCG) maintenance of corpus luteum of pregnancy progesterone and oestrogen Human placental lactogen HPL growth, lactation carbohydrate and lipid Many more!
Placental barrier
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 )
Transfer across the placenta
Gases – oxygen and carbon dioxide by simple diffusion
Water and electrolytes
Steroid hormones
Proteins poor – only by pinocytosis
Transfer of maternal antibodies IgG -starts at 12 weeks – mainly after 34 weeks therefore lack of protection for premature infants
Named parts of the decidua
Topographical names
capsularis – overlying embryo and chorionic cavity
parietalis – side uterus not occupied by embryo
basalis – between uterine wall and chorionic villae
What is vasa praevia?
velomentous cord insertion that runs across the cervical os
The fetal vessels within the umbilical cord pass over the internal os. As the internal os dilates in labour the vessels are stretched and exposed and can rupture leading to massive fetal blood loss and death.
Diagnosed on Ultrasound using colour dopplers
Management deliver by Caesarean Section when the fetus is above 34 weeks.
Clinical aspects of the placenta
Position of the placenta within the uterus
Mainly fundal (at the top)
Anterior or posterior (front wall or back wall)
“low lying” or placenta praevia (near to the cervical os)
Placenta Praevia Massive bleeding in pregnancy Painless bleeding Fetal death Maternal death
