lecture 16: placentation Flashcards
1
Q
What is viviparity?
A
- birth of live-young
- advantages – foetal development and survival
- requires specialised maternal-foetal interface – placenta
- foetal and maternal components
- exchange of gases, nutrients, and wastes
- hormones
- limits foetal invasion
- immunological interface
2
Q
What is the role of placenta?
A
- nutritive exchange
- histotrophic vs haemotrophic (cells or blood)
- gas: O2 and CO2
- sugars, amino acids, lipids etc
- waste products, e.g. urea
- hormones
- maternal recognition of pregnancy
- uterine contractility and secretion
- modulation of maternal and foetal physiology
- immunological interface
- foetus has “foreign” paternal genes → MHC etc so foetus is an allograft. why isn’t it rejected by an immune response?
3
Q
How do we get formation of extra-embryonic membranes (mouse/human)?
A
- trophectoderm → chorionic ectoderm → chorion and placenta
- inner cell mass:
- embryonic ectoderm, mesoderm, endoderm → embryo and foetus
- extraembryonic ectoderm → amnion, chorion and placenta
- extraembryonic mesoderm → yolk sac and allantois, amnion
- extraembryonic endoderm → yolk sac and allantois
- endoderm surrounding a cavity - (yolk sac cavity)
- mesoderm spread all the way around
- pro-amniotic cavity
- extra-embryonic coelom
- in most eutherian mammals the yolk sac will regress, lose its main function, come away from the coelom
- formation of allantois → forms about the time when the mesonephric kidney starts to function
- embryo inside amniotic cavity
- connected via primitive umbilical cord to chorio-allantois (future placenta)
4
Q
What is the classification of placentation?
A
- tissues
- chorio-vitelline
- chorio-allantoic
- macroscopic structure
- microscopic relationship between foetal and maternal tissues – invasiveness
- non-invasive e.g. pig, horse, sheep, cow
- invasive - eccentric e.g. dog, rat, rabbit
- invasive - interstitial e.g. human
- placentation evolved in lots of different types of mammals at lots of different times
5
Q
What are the major placenta types?
A
- discoid, e.g. human, mouse
- single disc in which the foetal and maternal blood come together
- zonary, e.g. dog
- donut around the uterus
- cotyledonary e.g. sheep, cow
- lots scattered over the uterine placenta
- sub placentas
- in each one is foetal and maternal tissue
- interdigitate
- diffuse e.g. pig, horse, camel
- interactions all over the surface
6
Q
What is Grosser’s classification of placental types?
A
- horse: epithelio-chorial
- non-invasive
- sheep: synepithelio-chorial
- slightly invasive - part of maternal tissue lining has been eroded
- dog: endothelio-chorial
- syncytial structures forming
- human: haemo-chorial
- very invasive
- syncytium
- pool of blood
- large blood sinuses with foetal villi dangling in them
7
Q
What is the placental blood flow in the human?
A
- blood flow is clearly vital to the functioning of the placenta
- maternal blood spaces → foetal tissue going into that → connected via umbilicus to the embryo
- maternal spiral arteries: blood vessels that supply the placental surface, very distinctive anatomical structures
8
Q
What is seen when you make a cast of foetal placental capillary bed?
A
- very dilated capillaries
- drive blood at reasonably high speed, high pressure into these capillaries which are close to the maternal system
- hits this big space
- blood goes slowly, spends time there, exchange
- gets carried back by narrow vessels
9
Q
What is the placenta of sheep?
A
- synepithelio-chorial
- maternal tissue forming a cup around the foetal tissue
- tissue with maternal blood vessels interdigitating with tissue with foetal blood vessels
- counter-current exchange
- maternal and foetal flow going in opposite directions
10
Q
What are placental hormones?
A
- hCG
- LH activity - maintains CL
- immune suppression
- hPL (hCS)
- prolactin/growth hormone activity
- increased breakdown of adipose tissues
- progesterone and oestrogen
- modulates endometrium: MRP; implantation; secretory activity; immunological modulation; etc
- suppresses gonadotrophins
- myometrium; mammary development
- maternal amino acid metabolism
11
Q
What are placental oestrogens?
A
- oestrone
- oestradiol
- oestriol
- converted from androgens e.g. testosterone via aromatase
- different number of alcohol or ketone groups
- oestradiol is potent
- oestrone and oestriol are weak but present in large amounts so that balances up
12
Q
In what way is steroidogenesis in pregnancy a combination of maternal, placental and foetal activity?
A
- production of progesterone in the placenta which goes through the maternal system
- DHA - a weak androgen from the maternal adrenals → androgens → testosterone
- oestradiol, oestrone, progesterone → acting on mother
- some of progesterone and pregnenolone goes in the foetal system
- foetal liver making oestriol → placenta → maternal circulation
- urinary tests for oestrogens can tell you a lot
- if oestriol levels are lower than normal often a sign that the foetal liver is not forming as it should → sign the foetus is not doing well
13
Q
What is the placental exchange membrane?
A
- consumes 30% of O2 supplied
- uterine artery supplying freshly oxygenated blood
- in through placental exchange
- loses oxygen, picks up CO2
- coming out with reduced oxygen and increased CO2
- umbilical artery supplying deoxygenated blood to the placenta
- picking up oxygen
- not very high percentage oxygen
- it works
- placenta is highly metabolic so it requires a lot of the oxygen
- foetuses have a form of haemoglobin that can grab oxygen from the maternal system
- haemoglobin in the face of acid tends to release oxygen
14
Q
What is the metabolism of the placenta?
A
- placental transfer
- main energy from glucose and lactate in humans
- active transfer of specific materials – amino acids, lipids, vitamins, etc
- placental metabolism
- uses 30% of glucose and oxygen supplied by mother
- highly active in protein synthesis
- conjugation and inactivation of maternal hormones
- foetal haemoglobin
- bohr effect - pH change as CO2 exchanged increased O2 transfer
- note: placenta highly metabolically active - uses 30% of oxygen supplied
15
Q
What is circulation before birth?
A
- lung is fluid filled and has low oxygen
- it has constricted arterioles restricting blood flow
- oxygenated blood draining from placenta mixes with depleted blood from body
- most blood shunts through foramen ovale and ductus arteriosus
- functionally running on a two chambered heart
16
Q
What is circulation after birth?
A
- oxygen opens capillaries in lung increasing blood flow
- increased flow into Left Atrium closes flap over Foramen ovale
- increased oxygenation of blood in ductus arteriosus causes contraction and closure
- standard four chambered heart : left and right side
- placenta gone
