3: Pregnancy parturition and late fetal development Flashcards
Early embryo nutrition is
histiotrophic - reliant on uterine gland secretions and breakdown of endometrial tissues
When does embryo growth switch from histiotrophic to haemotrophic
start of 2nd trimester
- achieved through a haemochorial-type placenta where maternal blood contacts the foetal membrane
Fetal membrans
extraembryonic tissues that form a tough but flexible sac encapsulates the fetus and forms basis of maternal-fetal interface
Aminion (inner fetal membrane)
arises from epiblast (doesn’t contribute to fetal tissues)
forms closed, avascular sac with developing embryo at one end
begins to secrete amniotic fluid from 5th week - forms fluid filled sac that encapsulates and protects fetus
Chorion (outer fetal membranee)
formed from yolk sac derivatives and trophoblast
highly vascularised
gives rise to chorionic villi
Allantois
outgrowth from yolk sac
grow along connecting stalk from embryo to chorion
becomes coated in mesoderm and vascularises to form umbilical chord
How is the amniotic sac formed
Expansion of amniotic sac by fluid accumulation forces amnion into contact with chorion, which fuse forming amniotic sac
Amniotic sac
sac 2 layers
amnion on inside
chorion on outside
Chorionic villi
outgrowths of cytotrophoblast from chorion that form basis of fetal side of placenta
- cavity formed, coated by mesoderm then blood vessels inside cavity (primary secondary and tertiary chorionic vili)
Terminal villus
convoluted knot of vessels and vessel dilation
slows blood flow - enabling exchange between maternal and fetal blood
whole structure coated with trophoblast
Terminal vilus in early pregnancy
150-200 micro meters in diameter
approx. 10 micrometer trophoblast thickness between cappilaries and maternal blood
Terminal vilus in late pregnancy
villi thin to 40 micro meters
vessels move within villi to leave only 1-2 micro meters trophoblast separation from maternal blood
Maternal blood supply
Uterine artery - gives rise to network of arcuate arteries
Radial artery - branch from arcuate arteries and branch further to form:
Basal artery - form spiral arteries during menstrual cycle endometrial thickening
How is oxygen transferred from the mother to the fetus
Diffusional gradient (high maternal O2 tension, low fetal O2 tension)
How is glucose transferred from the mother to the fetus
facilitated diffusion by transporters on maternal side and fetal trophoblast cells
How is water transferred from mother to fetus
Placenta - main exchange site
some crosses amnion-chorion
majority by diffusion, but some local hydrostatic gradients present
How are electrolytes transferred from mother to fetus
Large traffic of sodium + electrolytes across placenta
combination of diffusion and active energy-dependent co-transport
How is calcium transferred from mother to fetus
actively transported against concentration gradient by magnesium ATPase calcium pump
How are amino acids transferred from mother to fetus
reduced maternal urea excretion and active transport of amino acids to fetus
What is organ formation in late fetal development driven by
corticosteroids
fetal circulatory system
placenta acts as a site of gas exchange
ventricles act in parallel rather than series
vascular shunts bypass pulmonary and hepatic circulation –> close at birth
Fetal respiratory system
Primitive air sacs form in lungs around 20 weeks - vascularisation from 28 weeks
Surfactant production begins around week 20, upregulated towards term
fetus spends 1-4h/day making rapid respiratory air movements during REM sleep
Fetal gastrointestinal system
endocrine pancreas functional from start of 2T, insulin from mid-2T
liver glycogen progressively deposited -accelerates towards term
large amounts of amniotic fluid swallowed - debris and bile acids form meconium
Fetal nervous system
Fetal movements begin late 1T
detectable by moth from ~14 weeks
Stress response from 18 weeks, thalmus-cortex connections form by 24 weeks
Fetus doesn’t show conscious wakefulness - mostly in slow-wave or REM sleep
