Pregnancy, Parturition and late foetal development: Flashcards
how is fetus nutrition provided?
- early- histiotrophic
- reliant on uterine gland secretions and breakdown of endometrial tissues
- switch at start of 2nd trimester- haemotrophic
- achieved through a haemochorial- type placenta where maternal blood directly contacts the fetal membranes (chorion)
what are the fetal membranes
extraembryonic tissue that forms a tough but flexible sac
encapsulates the fetus and forms the basis of the maternal-fetal interface
what are the types of fetal membrane?
amnion (inner fetal membrane)
chorion (outer fetal membrane)
allantois
what is the amnion?
arises from the epiblast (does not contribute to the fetal tissues)
forms a closed, avascular sac with the developing embryo at one eld
begins to secrete amniotic fluid for 5th week- forms a fluid-filled sac that encapsulates and protects the fetus
what is the chorion
outer fetal membrane
- Outer membrane surrounding conceptus unit
- Formed from yolk sac derivatives and the trophoblast
- Highly vascularized
- Gives rise to chorionic villi – outgrowths of cytotrophoblast from the chorion that form the basis of the fetal side of the placenta
what is the allantois?
- Outgrowth of the yolk sac
- Grows along the connecting stalk from embryo to chorion
- Becomes coated in mesoderm and vascularizes to form the umbilical cord.
how does the amniotic sac form?
Expansion of the amniotic sac by fluid accumulation forces the amnion into contact with the chorion, which fuse, forming the amniotic sac
what is the composition of the amniotic sac?
2 layers
amnion on inside
chorion on outside
what is the makeup of the placenta?
what are primary chorionic villi formed from?
cytotrophoblast
forms finger-like projections through syncytiotrophoblast layer into maternal endometrium
what is the use for primary chorionic villi?
provide substantial surface area for exchange
what are the phases of chorionic villi developement?
3 phases:
Primary: outgrowth of the cytotrophoblast and branching of these extensions
Secondary: growth of the fetal mesoderm into the primary villi
Tertiary: growth of the umbilical artery and umbilical vein into the villus mesoderm, providing vasculature.
what is the terminal villus microstructure?
- convoluted know of vessels and vessel dilation
- slows blood flow enabling exchange between maternal and fetal blood
- whole structure coated with trophoblast
- change through pregnancy
- in late pregnancy are thinner with less trophoblast separation from maternal blood
- this allows reduction in transfer distance later in pregnancy
how is maternal supplied to endometrium?
- Uterine artery branches give rise to a network of arcuate arteries.
- Radial arteries branch from arcuate arteries, and branch further to form basal arteries.
- Basal arteries form spiral arteries during menstrual cycle endometrial thickening.
what is the function of spiral arteries?
provide the maternal blood supply to the endometrium
how do spiral arteries re-model?
- Extra-villus trophoblast (EVT) cells coating the villi invade down into the maternal spiral arteries, forming endovascular EVT.
- Endothelium and smooth muscle is broken down – EVT coats inside of vessels (replace maternal endothelium/SM)
- Conversion: turns the spiral artery into a low pressure, high capacity conduit for maternal blood flow.
what is the structure of the placenta?
how does oxygen transport to fetus?
diffusional gradient (high maternal O2, low fetal O2 tension)
how does glucose transfer to fetus?
facilitated diffusion by transporters on maternal side and fetal trophoblast cells
how does water transfer to fetus?
placenta main site of exchange
some crosses amnion-chorion
majority by diffusion, some local hydrostatic gradients
how do electrolytes transport to fetus?
large traffic of sodium and other electrolytes across the placenta
combination of diffusion and active energy dependant co-transport
how does calcium transport to fetus?
actively transported across a concentration gradient by magnesium APTase calcium pump
how do amino acids transport to fetus?
reduced maternal urea excretion and active transport of amino acids to fetus
how does maternal supply change during pregnancy?
- Maternal cardiac output increases 30% during first trimester (stroke vol & rate)
- Maternal peripheral resistance decreases up to 30%
- Maternal blood volume increases to 40% (near term (20-30% erythrocytes, 30-60% plasma)
- Pulmonary ventilation increases 40%
what does the placenta and fetus absorb from maternal supply?
- Placenta consumes 40-60% glucose and O2 supplied
- But although fetal O2 tension is low, O2 content and saturation are similar to maternal blood.
- Embryonic and fetal hemoglobins: greater affinity for O2 than maternal hemoglobin
how does the circulatory system mature in late fetal development
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
how does the respiratory system develop?
primitive air sacs form in the lungs around 20 weeks
vascularisation from 28 weeks
surfactant production begins around week 20, upregulated towards term
fetus spends 1-4hrs/day making rapid respiratory movements during REM sleep
how does the GI system develop in late fetal development?
endocrine pancreas functional from start of 2T- insulin from mid-2T
liver glycogen progressively deposited- accelerates towards term
large amounts to amniotic fluid swallowed- debris and bile acids form meconium
how does the nervous system develop in late fetal development?
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
what is organ maturation coordinated by?
fetal corticosteroids
what is the point 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
labour has the characteristics of pro-inflammatory reaction
how does labour have the characteristics of pro-inflammatory reaction?
immune cell infiltration
inflammatory cytokine and prostaglandin secretion
what are the phases of labour?
quinescence
activation
stimulation
involution
what are the 3 stages of labor?
- all occur in phase 3 of labour
- first stage
- contractions start
- cervix dilated
- latent phase (slow dilation of cervix to 2-3 cm)
- active phase (rapid dilation of cervix to 10cm)
- second stage
- deliver of fetus
- commences at full dilation of cervix
- maximal myometrial contractions
- deliver of fetus
- third stage
- delivery of placenta
- expulsion of placenta and fetal membranes
- post-partum repair
how long does first delivery take?
8-18 hours
how long do subsequent deliveries take?
5-12 hours
how does the cervix change during pregnancy?
remodeling of cervix occurs
- Cervix has a critical role in retaining the fetus in the uterus.
- High connective tissue content:
- Provides rigidity
- Stretch resistant
- Bundles of collagen fibres embedded in a proteo-glycan matrix
- Changes to collagen bundle structure underlie softening, but mechanism unclear.
how is labour initiated?
- Fetus determines timing of parturition through changes in fetal HPA axis
- CRH levels rise exponentially towards end of pregnancy
- Decline in CRH binging protein levels, so CRH bioavailability increases
what are the functions of CRH in labour?
- promotes fetal ACTH and cortisol release
- Increasing cortisol drives placental production of CRH -> Positive feedback!
- stimulates DHEAS production by the fetal adrenal cortex -> substrate for estrogen production
what are the levels of progesterone and estrogen in labour?
- High progesterone through pregnancy maintains uterine relaxation
- Serum estrogen:progesterone ratio may shift in favour of estrogen – this is unclear
- As term approaches, switch from PR-A isoforms (activating) to PR-B and PR-C (repressive) isoforms expressed in the uterus -> functional prog. withdrawal
- Rise in Estrogen Receptor Alpha expression
- Uterus becomes ‘blinded’ to progesterone action and sensitized to estrogen action
- Control of these changes unclear but likely leads to local changes in E:P ratio in uterine tissues.
what is the role of oxytocin in labour?
- NonapeHSptide (9aa) hormone synthesized mainly in the utero-placental tissues and pituitary.
- 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
- Signals through G-coupled oxytocin receptor (OTR/OXTR)
- Pre-labour: progesterone inhibits OXTR expression -> uterus relaxed
- Rise in estrogen promotes large increase in uterine OXTR expression
what are the functions of oxytocin in labour?
- Increases connectivity of myocytes in myometrium (syncytium)
- Promotes formation of gap junctions to myometrium can act as a syncytium
- Destabilise membrane potentials to lower threshold for contraction
- Enhances liberation of intracellular Ca2+ ion stores
what are the primary prostaglandins synthesised in labour?
PGE2
PGF2alpha
PGI2
how do estrogen levels drive prostaglandin action in uterus in labour?
- Rising estrogen activates phospholipase A2 enzyme, generating more arachidonic acid for PG synthesis
- Estrogen stimulation of oxytocin receptor expression promotes PG release.
what do prostaglandins do during labour?
PGE2 – cervix re-modelling
- Promotes leukocyte infiltration into the cervix, IL-8 release and collagen bundle re-modelling
PGF2alpha – myometrial contractions
- Destabilises membrane potentials and promotes connectivity of myocytes (with Oxytocin)
PGI2 - myometrium
- Promotes myometrial smooth muscle relaxation and relaxation of lower uterine segment
Other factors: peptide hormone relaxin and nitric oxide (NO) implicated in cervix re-modelling
what is the hypothesis for the regulation of labour?
how do myometrial contractions cause the formation of the birth canal?
- Myometrial muscle cells form a syncytium (extensive gap junctions)
- Contractions start from the fundus, spread down upper segment
- 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 happens to the uterus after fetal delivery?
rapid shrinkage of uterus
causing area of contact of placental with endometrium to shrink
also causes folding of fetal membranes- peel off the endometrium
what is the effects of clamping of the umbillical cord after brith?
stops fetal blood flow to placenta -> villi collapse
hematoma formation between decidua and plaenta
contractions expel placenta and fetal tissues
what state does the uterus remain in after delivery?
contracted
to facilitate uterine vessel thrombosis
uterine involution and cervix repair restore the non-pregnant state, shielding uterus from commensural bacteria and restore endometrial cyclicity in response to hormones
how does expulsion of the fetus occur?
