Pregnancy, Parturition and Late fatal development Flashcards
Describe how fetal growth changes overtime? what drives the growth at each stage and what is it reliant on
Embryo-fetal growth during the 1st trimester is relatively limited
- Early nutrition is histiotrophic
- Reliant on uterine gland secretions and breakdown of endometrial tissues (by syncytiotrophoblasts)
Switch to haemotrophic support at start of 2nd trimester
- fetus derives nutrients from maternal blood
- Hence rate of growth of fetus increases exponential
Achieved via a haemochorial-type placenta where maternal blood directly contacts the fetal membranes.
- Develop in 12th week gestation
what are uterine galnd secretions a source of in histiotrophic nutrition
Endometrial milk-
How does early embryo derive nutrition?
Syncytiotrophoblasts invade endometrium
- uses breakdown products from tissue to support embryo
- secretions from uterine glands
- breakdown of maternal capillaries results in the syncytiotrophoblasts being exposed to the maternal blood
What is the amnion? what does it form eventually and when
- Derivative of the epiblast
- however does not go on to form part of the foetus- extraembryonic
- The first of the fetal membranes
- Forms the amniotic cavity through secretion from amniotic cells
- eventually becomes the amniotic sac which surrounds the developing foetus through 2nd/3rd trimester
What are the following derived from?:
- Yolk sac
- Amnion
- hypoblast
- epiblast
What are ALL the fetal membranes? give their features and contrast them
Extraembryonic tissues that form a tough flexible sac surrounds the fetus and forms the basis of the maternal-fetal interface
Amnion
> arises from the epiblast
> forms a closed, avascular sac with the 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 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
Yolk sac
> derived from hypoblast
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 ELSE is the allantosis important in the embryo
May contribute to the bladder of the embryo
May be important to remove toxins from embryo
Combination of allantosis, mesoderm and connecting stalk form the umbilical cord
How does the amniotic sac develop? what are the layers of the amniotic sac
- Expansion of the amniotic sac by fluid accumulation forces the amnion into contact with the chorion, which fuse, forming the amniotic sac
- Amniotic sac: 2 layers; amnion on the inside, chorion on the outside
What is the chorion?
- outer membrane in extraembryonic membranes
- surrounds the conceptus unit
What is the connecting stalk?
- part of extraembryonic tissue
- links the developing embryp to the chorion
What are the trophoblastic lacunae and how does it contribute to the origins of the placenta?
As the syncytiotrophoblasts invade endometrium, maternal capillaries and glands are broken down
Lumens of capillaries and glands start to fuse
This creates a continuous space through which maternal blood flows
become intervillous spaces or maternal blood spaces
Describe phases of chorionic villus formation
Primary: outgrowth of the cytotrophoblast through syncitiotrophoblasts 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 structure and function of chorionic villi?
Finger-like extensions of the chorionic cytotrophoblast, which then undergo branching
Provide substantial surface area for exchange
Describe the microstructure of terminal villi and describe it’s purpose
- Convoluted knot of vessels and vessel dilation
- Slows blood flow enabling exchange between maternal and fetal blood
- Whole structure coated with trophoblast
How does the structure of terminal villi change over pregnancy?
Early pregnancy:
- 150-200µm diameter,
- approx. 10µm trophoblast thickness between capillaries and maternal blood.
Late pregnancy:
> villi thin to 40µm, vessels move within villi to leave only 1-2µm trophoblast separation from maternal blood.
Describe maternal blood supply to the endometrium- give details of the anatomical branching.
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.
U A R B S
How does uterine blood supply change through the menstrual cycle?
Basal arteries during proliferative stage grow and spiralize to produce spiral arteries
if implantation does not occur, endometrium is lost and spiral arteries regress
if implantation does occur, spiral arteries stabilise and provide maternal blood supply to the fetus
Describe spiral artery remodelling. Draw if possible . what is the result of this remoddeling process?
Extensive during implantation and placental development
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 replaces these and coats inside of vessels
- EVT cells despiralise the capillaries to open them into straight channels
This leads to Conversion: turns the spiral artery into a low pressure, high capacity conduit for maternal blood flow
This process may underlie issues such as preeclampsia and intra-uterine growth retardation
Describe the structure of placenta
Maternal unit
- maternal blood supply and spiral arteries
- supply maternal blood spaces with blood
- drains via maternal vein
Fetal side
> chorionic villus
> invasion of fetal artery and vein into villi
How does nutrient exchange across the placenta occur?
Describe the process for each molecule
(Ca? Amino acids? glucose? electrolytes?)
nutrient specific
Oxygen: diffusion (high maternal O2 tension, low fetal O2 tension)
Glucose: facilitated diffusion by transporters on maternal side and fetal trophoblast cells.
Water: placenta main site of exchange, though some crosses amnion-chorion. Majority by diffusion, though some local hydrostatic gradients.
Electrolytes: lots of sodium and other electrolytes across the placenta - combination of diffusion and active co-transport.
Calcium: actively transported against a concentration gradient by magnesium ATPase calcium pump.
Amino acids: reduced maternal urea excretion, converted to AA and active transport of amino acids making up urea to fetus
What is organ maturation coordinated by?
what else also increases?
fetal corticosteroids
- increase towards the end of pregnancy
- increase in lecithin (surfactant) and liver glycogen in parallel with increase in corticosteroids
Summarise and explain the phases of labour
Phase 1 - quiescence
- uterus is quiet
- no contraction
- cervical softening occurs
- late 1st trimester
Phase 2 - activation
- uterine preparation
- cervix becomes ready to dilate
Phase 3 - stimulation
- uterine contraction
- cervical dilation
- fetal and placenta expulsion
- (three stages of labour)
Phase 4 - involution
- restoration of uterus to original size
- repair of cervix
- onset of lactation
What are the 3 stages of labour? in phase 3
First Stage:
Contractions start and Cervix dilation.
Has 2 phases in it
- Latent Phase: Slow dilation of the cervix to 2-3cm
- Active Phase: Rapid dilation of the cervix to 10cm
longest phase
Second stage:
• Delivery of the fetus
- Commences at full dilation of the cervix (10cm)
- Maximal myometrial contractions
Third stage:
• Delivery of the placenta
- Expulsion of placenta and fetal membranes
- Post-partum repair
What happens during re-modelling of the cervix from 1st trimester to post partum repair
S R D P
Softening - begins in 1st trimester
- Measurable changes in compliance but retains cervical competence; cervix is still closed
↓
Ripening - weeks and days before birth
- Monocyte infiltration
- IL-6 and IL-8 secretion
- Hylaluron deposition
↓
Dilation - increased elasticity
- Increased hyaluronidase expression -> HA breakdown
- MMPs decrease collagen content
↓
Post-partum repair
• Recovery of tissue integrity and competency
• Ensures woman can become pregnant again
Describe the role of CRH in labour.
what happens to it’s levels and describe it function in labour
Current thinking: fetus determines timing of parturition through changes in fetal HPA axis
CRH levels rise exponentially towards the end of pregnancy
Decline in CRH binding protein levels, so CRH bioavailability increases
CRH functions 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
Describe the role of estrogen and progesterone in labour
High progesterone in 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 progesterone withdrawal
Rise in Estrogen Receptor Alpha expression
- Uterus becomes ‘blinded’ to progesterone action and sensitised to estrogen action
- Control of these changes unclear but likely leads to local changes in E:P ratio in uterine tissues
What is the Ferguson reflex? Draw it
maternal urge to push
Fetus starts to bear down on the cervix
Stretch receptors in cervix and vagina signal up to the hypothalamus
Firing of hypothalamus onto the posterior pituitary increases
Triggers release of oxytocin from posterior pituitary into maternal circulation
Acts on uterus and myometrium
Summarise the integrated regulation of labour. Draw it
Describe myometrial contractions during labour; how does it relate to the syncytium and what does it implicate in pregnancy
- Myometrial muscle cells form a syncytium (extensive gap junctions) to transmit contractions
- 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
Describe the process of fetal expulsion- summarise
Head engages with pelvic space 34-38wks
Pressure on fetus causes chin to press against chest (flexion)
Fetus rotates (belly to mother’s spine)
Head expelled first after cervix dilates
Shoulders delivered sequentially (upper first) followed by torso.
The final stage of labour
Describe the placental expulsion and repair process
Rapid shrinkage of the uterus after delivery causes area of contact of placenta with endometrium to shrink
Uterine shrinkage also causes folding of fetal membranes - which then peel off the endometrium
Clamping of the umbilical cord after birth stops fetal blood flow to placenta, then villi collapse
Hematoma formation between decidua and placenta
Contractions expel placenta and fetal tissues (occur with haematoma)
