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
Describe the physiological changes to circulation in pregnancy for BOTH mother and fetus
Maternal changes:
- Maternal cardiac output increases 30% during first trimester (stroke vol & rate)
- TPR decreases up to 30%
- Blood volume increases to 40% as RBC and plasma also increase
- Pulmonary ventilation increases 40%
Placenta and Fetus:
- 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.
Describe the Organ maturation process of the Circulatory and Respiratory system
In 2nd/3rd trimester
Circulatory system
• begins with tube of mesoderm pumping blood from day 22
• placenta acts as site of gas exchange for fetus
• ventricles act in parallel rather than series
• vascular shunts bypass pulmonary & hepatic circulation -> closed at birth
• allows efficient pumping of placental blood
Respiratory system
• start as lung buds
• 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 (practicing)
Describe the Organ maturation process of the GI and Nervous system.
GI: when does pancreas and liver function start ans what happens to amniotic fluid
Neuro: movement and consciousness
Gastrointestinal system
• gut tube formed from endoderm
• endocrine pancreas functional from start of 2T, insulin from mid-2T
• liver (day 23) glycogen progressively deposited - accelerates towards term
• large amounts of amniotic fluid swallowed -debris and bile acids form meconium (delivered after birth)
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
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
