placentation and parturition Flashcards
What form of embryonic nutrition leads to the breakdown of maternal endometrial
capillaries to form trophoblastic lacunae?
histiotrophic
haemotrophic nutrition
start of 2nd trimester
what type of placenta do humans have?
haemochorial
histiotrophic
Histotroph is the nutritional material accumulated in spaces between the maternal and fetal tissues, derived from the maternal endometrium and the uterine glands. This nutritional material is absorbed by phagocytosis initially by blastocyst trophectoderm and then by trophoblast of the placenta
hemotrophic nutrition
hemotrophic nutrition is the exchange of blood-borne materials between the maternal and fetal circulations
Trophoblastic lacunae
Large spaces filled with maternal blood formed by breakdown of maternal capillaries and uterine glands
Become intervillous spaces aka maternal blood spaces
what is the chorion?
The chorion is a double-layered membrane formed by the trophoblast and the extra-embryonic mesoderm
what is the difference between the amnion and the chorion
The amnion is found on the innermost part of the placenta. It lines the amniotic cavity and holds the amniotic fluid and the developing embryo. … The chorion, on the other hand, is the outer membrane that surrounds the amnion
formation of amnion
ballon analogy
arises from epiblast
Forms a closed, avascular sac with the developing embryo at one end
Begins to secrete amniotic fluid from 5th week – forms a fluid filled sac that encapsulates and protects the fetus
chorion
formed from yolk sac derivatives
highly vascularised
gives rise to chorionic villi
what is the umbilical chord formed from?
allantois
outgrowths of yolk sac
connecting stalk from embryo to chorion
once coated in mesoderm and vascularised becomes umbilical chord
chorionic villus formation
Cytotrophoblast forms finger-like projections through syncitiotrophoblast layer Into maternal endometrium
only cytotrophoblasts have ability to proliferate
villus microstructure
convulated know of vessels
slow blood flow: enabling exchange
coated in trophoblast
what changes occur to terminal villus over pregnancy?
effect of these changes?
1 - thins out to 40um diameter (from 200)
2 - trophoblastic covering also thins to only 1-2um separation from maternal blood
enable a rapidly growing embryo in later stages which requires a lot more oxygen, blood flow, nutrients
distance of diffusion reduces
maternal blood supply
ovarian artery > arcuate> radial> basal> spiral
ovarian artery > _____> radial>_____>_______
ovarian artery > arcuate> radial> basal> spiral
_____ > arcuate>____> ___l> spiral
ovarian artery > arcuate> radial> basal> spiral
endovascular EVT formation
Extra-villus trophoblast (EVT) cells which coat villi invade down
the spiral arteries
form endovascular extra-villus trophoblast cells
this replaces maternal endothelium, smooth muscle
evt coats inside of the vessel
what is conversion?
maternal spiral arteries invaded by villus trophoblast, replaced by endovascular EVT
conduit for maternal blood flow
calcium exchange across placenta
Calcium: actively transported against a concentration gradient by magnesium ATPase calcium pump
maternal changes
Maternal blood volume increases to 40% (near term (20-30% erythrocytes, 30-60% plasma
cardiac output increased by 30%: stroke volume / rate
Pulmonary ventilation increases 40%
placenta/ fetus oxygen exchange
Placenta consumes 40-60% glucose and O2 supplied
why is the fetus able to consume 40-60% of oxygen?
this is a sickening fact!
fetal / embryonic haemoglobin has a higher affinity for o2 than maternal
what forms chorionic villus and what is its function?
invasion of chorionic cytrophoblast cells into maternal endometrium
surface for exchange
why does the fetus make rapid respiratory movements when it doesnt need to?
perhaps to practise breathing reflex
to aid diaphragm development
circulatory system
Placenta acts as site of gas exchange for fetus
Ventricles act in parallel rather than series
vascular shunts bypass pulmonary & hepatic circulation -> close at birth
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 (1st poo lining baby’s intestines during pregnancy)
nervous system
Fetal movements begin late 1T, detectable by mother from ~14 weeks
Stress responses from 18 weeks, thalamus-cortex connections form by 24 weeks
Fetus does not show conscious wakefulness – mostly in slow-wave or REM sleep
resp system
Primitive air sacs form in lungs around 20 weeks, vascularization from 28 weeks
Surfactant production begins around week 20, upregulated towards term
Fetus spends 1-4h/day making rapid respiratory movements during REM sleep
3 aims of labour?
Safe expulsion of the fetus at the correct time
Expulsion of the placenta and fetal membranes
Resolution/healing to permit future reproductive events
role of prostaglandins during labour
Immune cell infiltration
Inflammatory cytokine and prostaglandin secretion
all aid in labour
which has characteristics of a pro - inflammatory reaction
first stage of labour
contractions start
latent phase: slow dilation of cervis 2/3 cm
active phase - rapid dilation 10cm
second stage
delivery of fetus
maximal myometrial contractions
third stage
Explusion of placenta and fetal membranes
Post-partum repair
cervix remodelling
Changes to collagen bundle structure underlie softening, but mechanism unclear
from rigid, stretch resistant to softening
Ripening : Monocyte infiltration and IL-6 and IL-8 secretion
Hylaluron deposition
Dilation : Increased hyaluronidase expression -> HA breakdown
MMPs decrease collagen content
post partum repair
recovery of tissue integrity and competency
what is MMPs?
matrix mellano proteases
Initiation of labour - how does it start?
fetus determines
as there are changes in the fetal HPA axis- CRH levels rise exponentially
decrease in CRH bidning protein levels so more bioavailable CRH
what is CRH?
corticotrophin releasing hormone
what is the role of CRH?
3 things it does
1) promotes fetal ACTH and cortisol release
2) drives placental prodcution of CRH - POSITIVE feedback loop
3) stimulates DHEAS production by fetal adrenal cortex
what is a substrate for oestrogen production and is produced by the fetal adrenal cortex
this substrates production is stimulated by CRH
DHEAS Dehydroepiandrosterone sulphate (DHEAS) is an androgen primarily produced by the adrenal cortex
what maintains uterine relaxation throughout pregnancy?
HIGH progesterone
describe functional progesterone withdrawal
term approaches
shift from activating signalling progesterone isoform A to progesterone B, C isoforms.
These are repressive isoforms expressed in uterus and cause the uterus to become blind to progesterone action
simultaneous rise in estrogen receptor alpha expression
summarise progesterone withdrawal as term approaches
PR-A switch to PR-B, PR-C; from activating receptors to repressive receptors
suppression of alpha oestrogen receptor expression is also removed
Estrogen can then act to transform the myometrium to a contractile phenotype
what is ferguson reflex?
The Ferguson reflex is a neuroendocrine reflex in which the fetal distension of the cervix stimulates a series of neuroendocrine responses, leading to oxytocin production
what increases oxytocin production
Uterine oxytocin production increases sharply at onset of labour
Expression increase is driven by increase in estrogen levels.
Release promoted by stretch receptors -> Ferguson reflex
what is the oxytocin signalling?
Signals through G-coupled oxytocin receptor (OTR / OXTR)
what inhibits oxytocin signalling pre labour?
Pre-labour: progesterone inhibits OXTR expression -> uterus relaxed
what promotes uterine OXTR expression?
Rise in estrogen promotes large increase in uterine OXTR expression
3 functions of oxytocin
Increases connectivity of myocytes in myometrium (syncytium)
Destabilise membrane potentials to lower threshold for contraction
Enhances liberation of intracellular Ca2+ ion stores
what is relevance of liberation of Ca2+ ion stores by oxytocin?
aids myometrial contraction
Primary Prostaglandins synthesized during labour? are PGE2, PGF2alpha and PGI2
PGE2,
PGF2alpha
PGI2
what drives prostaglandin action?
Rising estrogen levels
2 ways
rising estrogen levels drives prostaglandin action?
. Rising estrogen activates phospholipase A2 enzyme, generating more arachidonic acid for PG synthesis
2. Estrogen stimulation of oxytocin receptor expression promotes PG release.
action of PGE2
cervix re-modelling
IL8 release
collagen bundle re-modelling
leukocyte infiltration into cervix
PGF2 alpha action
. destabalises membrane potentials
promotes connectivity of myocytes
PGI2
myometrium
between contraction and relaxation allows for myometrial smooth muscle relaxation of lower uterine segment
what factors implicated in cervix re-modelling other than prostaglandin
peptide hormone relaxin
nitric oxide
what forms a syncytium
myometrial muscle cells - extensive gap junctions
contraction start at fondus
what does brachystatic mean?
muscle contractions in fetal expulsions are like this
what does it do?
Muscle contractions are brachystatic –fibres do not return to full length on relaxation
This causes lower segment and cervix to be pulled up forming birth canal
what causes area of contact of placenta with endometrium to shrink?
Rapid shrinkage of the uterus after fetal delivery
uterine shrinkage causes what two things?
folding of fetal membranes
area of contact of placenta with endometrium to shrink
clamping of umbilical cord after birth does what?
stops fetal blood flow to placenta - villi collapse
Hematoma formation between decidua and placenta
Contractions expel placenta and fetal tissues
uterine incolution and cervix repair restore to what?
Shielding uterus from commensural bacteria
Restore endometrial cyclicity in response to hormones
what facilitates uterine vessel thrombosis
uterus remaining contracted
