Placentation Flashcards
Which species have a luteo-placental shift?
Horses, sheep, humans, cows
What is the luteo-placental shift
Shift in production of progesterone from luteal to placental
How does the luteoplacental shift affect the use of PGF2a to terminate pregnancy?
- If shift occurs, PGF2a can only be used to terminate pregnancy before the shift
- If there is no shift, then PGF2a can always be used to terminate pregnancy
What is a placenta?
The close attaachment of foetal membranes to the uterine wall to facilitate physiological exchange of gases, nutrients and waste products
Name the foetal membranes
- Amnion
- Yolk sac
- Allantois
- Chorion
Describe the amnion
- Innermost fluid filled membrane (reptile, bird, mammal)
- Allows symmetrical growth and protection of the foetus
- Temperature control and foetal movement
Describe the yolk sac
- Surrounds yolk in reptile and bird eggs
- In higher mammals has nutritional role in early pregnancy
Describe the allantois
- Sac from hindgut
- Connection to foetus becomes umbilical cor
- Collects waste products from foetus
Descibe the chorion
- Outer membrane
- Becomes foetal contribution to placenta
- All placentas are chorionic at some stage of development
What are the layers in the submucosa of the uterine wall?
Perimetrium, myometrium, endometrium
What are the 3 stages of implantation?
Apposition, adhesion, firm attachment or invasion of trophoblast into uterus
Describe apposition
Blastocyst or foetal membranes become closely apposed to uterine lining (epithelium)
Describe adhesion
Complex biochemical interaction between molecules on the trophoblast and epithlium
Outline firm attachment or invasion of trophoblast into uterus
On species, can be non-invasive or invasive
Describe non-invasive attachment of trophoblasts to uterus
- Cow, sheep
- Blastocyst undergoes elongation, long when attaches to uterus
- During apposition aligns with lining of uterus
- During attachment trophoblast layer in contact with uterine epithelium
Describe invasive attachment of trophoblasts to uterus
- Quickly after arrival of blastocyst to uterus
- Blastocyst remains spherical
- Invades uterine lining following apposition and attachment
What needs to occur to allow invasive attachment of the blastocyst?
- Specific window of time
- Presence of proteoglycan MUC1 needs to be down-regulated to allow attachment
What are the different methods of classification of placental structure?
- Foetal extraembryonic membranes
- Source of nutrition
- Maternal tissue shedding at birth
- Histological structure
- Macroscopic structure
What are the different types of nutrition possible in the placenta?
- Haemotrophic: nutrition from circulating maternal blood
- Histotrophic: nutrition from endometrium
What are the different types of maternal tissue shedding at birth?
- Deciduate vs non deciduate
- Deciduate: lining of uterus shed with lining of placenta at parturition
How can histological structure be used to calssifies placentas?
Different number of tissue layers interposed between maternal and foetal circulations
How can macroscopic anatomy of placentas be used for classification?
- Nutrient exchange areas
- Distribution of chorionic villi on chorionic sav and relationship with endometrium
- e.g. diffuse, cotyledonary, zonary
What are the placental classifications based on the number of tissue layers (Grosser classification)?
- Epitheliochorial
- Endotheliochorial
- Mesochorial
What are the 3 foetal layers of the placenta? (moving towards the maternal side)
- Foetal blood vessel endotehlium
- Foetal connective tissue/mesenchyme
- Foetal trophoblast
What are the 3 possible maternal layers of the placenta? (moving towards the foetal side)
- Maternal blood vessel endothelium
- Maternal connective tissue/mesenchyme
- Maternal epithelium
What layers are present in the epitheliochorial placenta?
All 3 foetal and all 3 maternal layers
Describe the epitheliochorial placenta
- 6 layers between blood circulations
- Present in sow and mare
- Least invasive type of placenta
- Chorion in contact with epithlium of maternal uterine wall
What layers are present in the endotheliochorial placenta?
- 3 foetal
- 1 maternal (maternal blood vessel endothelium)
Describe the endotheliochorial placenta
- Chorion in contact with endothelium of maternal blood vessel
- 4 layers present
- Present in bitch, queen
- Specialised version in ruminant
How many layers are present in the haemochorial placenta?
3 - only foetal layers
Describe the haemochoral placenta
- Endothelial cell layer around blood vessel incomplete
- Pool of maternal blood direclty in contact with the chorion
- Foetal layer thicker as more trophoblasts present
- Humans, primates, mice
- Most invasive
What are the classifications based on type of nutrient exchange area in the placenta?
- Diffuse
- Cotyledonary
- Zonary
- Discoid
What is the function unit of the foetal placenta?
The chorionic villus
Describe the appearance of the chorionic villus
- Normally quite visible
- Interdigitation between villi of chorion and endometrium
Describe placentas with a diffuse nutrient exchange area
- Sow and mare
- Villi attach to crypts in endometrium
- Mare forms micro-cotyledons
Describe cotyledonary nutrient exchange areas
- Cow and ewe
- Cotyledons attach to caruncles to form a placentome
Describe zonary nutrient exchange areas
- Bitch and queen
- Band around middle of foetus on chorioallantoic membrane
Describe discoid nutrient exchange areas
- Humans and primates
- Plate shape/discoid arrangement of villi
Describe the placenta of the sow
- Epitheliochorial
- Diffuse
- Non-deciduate
- Embryotroph
- By day 24, allantois around the periphery of the embryo
Describe the uterine milk in sows
- Secretions important for development
- High P4 and E2
- Uteroferrin important for nutrition and Fe transport
Describe the placenta found in the ewe and cow
- Synepithliochorial
- Cotyledonary
- Non-deciduate
Explain synepitheliochorial placentation
- Specialised epitheliochorial pacenta
- Uninucleate trophoblast cells undergo division without cytokinesis = binucleate giant cells with small granules
- Migrate to maternal layer, fuse with maternal cells
- Produce hybrid cell
- Once fused, release contents into maternal circulation
What is contained within the granules of the binucleate cels?
Placental lactogen
Describe placentomes
- Foetal cotyledon + materna caruncle
- Can be convex or concave
- Are the exchange regions in the placenta
- Each placentome connected to umbilicus by a chorionic stalk
Describe a convex placentome
- Found in cow
- Cotyledon sits on top of and surrounds the caruncle
Describe a concave placentome
- Sheep, goat
- Cotyledon sits within the caruncle
Describe the placenta found in horses
- Epitheliochorial, diffuse, non-deciduate
- Forms endometrial cups
- Microcotyledons on chorioallantoic memrbane
What do the endometrial cups develop from?
Chorionic girdle cells
Describe the microcotyledons found in equine placentas
- Fit into mini-crypts in endometrium
- Small collections of villi
- See around week 8 gestation onwards
- Rough appearance of inside of placenta
Describe the formation of the chorionic girdle
- Choriovitelline forms in secnd week of pregnancy
- 4th week, chorioallantioc begins to form
- Around 30 days have both together
- Junction between these forms the chorionic girdle
- Yolk sac regresses (mostly disappeared at day 50), leaves girdle
What is the function of the chorionic girdle in horses?
Is what implants into the uterus (and forms endometrial cups)
Describe teh formation of the endometrial cups
- Cells from girdle invade maternal endometrium
- Destroy epithelium
- Bunch up and form endometrial cups
- Secrete equine chorionic gonadotrophin (eCG)
- LImited life span
- Invade at day 38-40
- Leukocytes gather around edge of cups
- Around day 100, these invade into cups and destroy, slough from lining of uterus
Describe the placenta in the dog
- Endotheliochorial
- Zonary
- Deciduate
What are the zones of the canine placenta?
- Transfer zone in middle (chorionic villi)
- Pigmented zone either side of transfer zone
What is the function of the pigmented zone?
Iron transfer
Describe the human placenta
Haemochorial, invasive, deciduate, discoid
Describe the decidual tissue of the human placenta
- Endometrial stromal cells proliferate and enlarge to become decidual cells
- Decidua is the maternal placenta and is shed when foetus is delivered
- Decidual tissue important in communication between mother and foetus
- Limits invasion of trophoblasts
What fluid predominates in human pregnancy?
Amniotic, not allantoic
What are the exchange zones in human placenta?
- Pools of maternal blood in these zones
- Is area where nutrient and waste exchange takes place
Between what structures does the umbilical cord run?
Placenta and foetus
What does the umbilical vein carry?
From palcenta, carrying oxygenated blood
Describe blood flow from the umbilical vein
- From placenta through umbilical vein
- Bypasses liver via ductus venosus, into cauda vena cava, into right atrium, left atrium, aorta , systemic circulation, cranial vena cava, into ventricle ductus arteriosus to bypass pulmonary and back to placenta via umbiliac arteries
What are the umbilical arteries derived from?
The interna iliac arteries
Where are the umbilical arteries located?
Either side of the foetal bladder
What is the true placenta?
The allantochorion
How is each piglet a separate unit in utero?
- Uterine horn convoluted forming pockets, each of which contains a piglet
- Each foetus has own foetal membranes
- Each end of the unit is necrotic
- Glue together towards end of pregnancy to form single tube along each horn
How is free-martinism avoided in pigs?
- Tips of each piglet unit are necrotic
- Prevents vascular anastomoses between units
What colour is the amniotic fluid?
Clear
What is the function of hormones produced by the placenta?
Prepare for lactation
How long can the uterine tract of the pig get during gestation?
2m long
Describe the expulsion of the placenta in the mare at birth
- Foetus breaks through cervial start of placenta, descends into vagina, umbilicus goes with it
- Placenta loosely attached to endometrium and so is pulled inside out
What may occur if some of the placenta is retained in mares?
- Bacterial proliferation
- Bacterial toxins enter blood, damage laminae of hoof wall leading to laminitis
Differentiate between the amniotic cavity and the allantoic cavity
- Amniotic cavity is between amnion and foetus
- Allantoic cavity is within the allantois
Where does the umbilical cord arise?
From bilaminar omphlopleur (bounded by sinus terminalis of conceptus)
Describe the vessels within the umbilical cord
- 2 veins fuse to one vein at amnion
- 2 arteries that run into the placenta
Where does the umbilical cord enter the foetus?
Via the urachus
What happens in the umbilical cord at birth?
- Umbilicus snapped, but is full of arterial blood
- Smooth muscles very responsive to stretch
- Contract to prevent blood loss
- Is irreversible and cuts of blood supply so animal needs to breath for oxygen
List the functions of the placenta:
- Replaces the function of adult organs
- Lungs: delivery of gases
- Gut: delivery of nutrients
- Kidney: excretion of waste products
- Liver: storage/reservoir of energy
- Endocrine: placental derived hormones
- Protective: toxic substances, some immune function
Describe placental blood flow
Increase in uterine and umbilical blood flow increase as gestation progresses
Compare foetal and placental growth during gestation
- Foetus grows in size
- Placenta has some, but limited increase in weight
- Suggests foetal growth related to increased blood flow rather than increased size of placenta
Describe the cotyledonary microvascular density as pregnancy progresses
- Increases to match foetal growth
- Vascular density in both compartments of placentome increases
- Dramatic increase in cotyledon, less dramatic increase on caruncular side
Broadly, what are the transport mechanisms for nutrients and waste across the placenta?
- Passive
- Active
What affects the passive diffusion of gases, fatty acids, alcohol and drugs across the placenta?
- Blood flow (volume and nature)
- Structure (area for exchange)
- Morphology (tissue barriers against exchange, number of layers)
- Electrical gradient (slight, -ve charge encourages +ve charged ions)
- Concentration gradient (maternal to foetal blood gradient for exchange)
Compare the pH in adult and foetal blood
Higher [H+] hence lower pH in foetal
- 7.50 vs 7.35
Compare partial pressure of oxygen in adult and foetal blood
- Higher partial pressure of oxygen in adult
- Aids diffusion across from adult blood to foetal blood down
concentration gradient - 100 vs 25 mmHg
Compare oxygen saturation of Hb in adult and foetal blood
- Foetal Hb has higher affinity for oxygen than adult Hb
- Saturation of Hb occurs at lower partial pressures of oxygen (70% vs 95%)
Compare partial pressure of Co2 in adult and foetal blood
- Higher in foetus
- Aids diffusion to mother for removal
- 35 vs 50 mmHg
Compare amount of Hb in adult and foetal blood
- Higher in adult
- 15 vs 8 g/dl
What are the different types of placental blood flow? (going from least to most efficient)
- Concurrent
- Multivillous
- Crosscurrent
- Countecurrent
Describe the countercurrent blood flow found in rabbits
- Blood from umbilical artery splits into network of vessels
- Blood from mother flows into distended sinuses that fill with blood
- Maternal blood washes against foetal tissue, blood flows are in oppposite directions to each other
- Facilitates exchange
Rank the types of placenta using Grosser classification from lowest to highest transfer efficiency
- Epitheliochorial
- Endotheliochorial
- Haemochorial
- I.e. more layers = less efficient transfer
Describe the electrochemical gradient of the placenta
- Generated by endometrium and placenta
- May influence mineral transfer
- Is given with respect to maternal fluid
- Varies by species
Explain how the concentration used for glucose transfer is established
- INcreased O2 consumption later in gestation
- Drives increased gradient to encourage further maternal glucose transfer to foetus
- In maternal blood, higher glucose than foetus
- Increased energy useage maintains steep gradient to facilitate transfer
What nutrients are transferred using active transport?
Amino acids and glucose
What is the active transport of nutrients across the placenta determined by?
- Passive mechanisms
- density and activity of carrier molecules for amino acids and glucose
Describe amino acid transport
- Require transport from apical to basal membrane
- 2 stages: uptake, transport into trophoblast, release into foetal circulation
- Capacity, exhcange surface area and density increase with time to meet demands
- Range of transporters present
Describe placental glucose transport
- Specific transproters
- GLUT family
- GLUT-1 primary isoform, GLUT-3 important in rats and sheep
- High density of transporters on maternal aspect (microvillous membrane), lower density on basal membrane
- Density on basal membrane is the rate limiting step for transport
- IUGR can be caused by reguced glucose transport due to lack of transproters
What is IUGR?
Intra-uterine growth restriction
What are the types of anastomoses that can occur leading to Freemartinism?
- Arteriovenous
- Arterio-arterial
- Veno-venous
Explain how Freemartinism occurs in cattle
- Anastomosis of allantoic vessels
- Usually single chorionic cavity
