Placenta General Flashcards
The placenta
-origin
- fx (2)
The placenta is a collection of extra-embryonic membranes (tissue) of fetal origin
- Of fetal origin, not maternal origin
The plancenta attaches to the uterine endometrium, and is involved in:
- Metabolic exchange between dam and fetus
- Endocrine regulation via hormone
production
Origin of placenta
The placenta initially develops from 2 unique layers in the hatched blast:
- Trophectoderm
- mesoderm
There is also a yolk sac developed from the primitive endoderm, and the allantois derived from the primitive gut
Note: amnionic cavity present Note: each fetus has own placenta (because placenta develops from blastocyst)
Placenta tissue layers
In a fully formed placenta, there are 3 major layers:
- amnion
closest to the fetus, amniotic vesicle/cavity
- allantois
middle layer (inner side of exterior membrane)
- chorion
outer layer (outer side of exterior membrane) In domestic species, the chorion and allantois eventually fuse, forming one double-layer membrane, the chorioallantois
Dog:
“Green
grass” is
the area of
exchange = fetal interface (part of chorioallantois)
Cattle: Amniotic vessel = feel for it, diagnose for pregnancy.
Tight junctions and sodium pump activation cause water to diffuse into the embryo, creating a blastocoele (a cavity filling with fluid) in the early blastocyst (5-12 days post fert)
The blastocyte
Blastocoele will increase in size, forming an expanded blastocyst (pushes on zona) Eventually, enzyme from trophectoderm cells + ‘pulsing’ of blast will cause a break in
the ZP = hatching blastocyst
The ICM (aqua) and trophectoderm (pink) differentially stained in an expanded blastocyst
Timing of embryonic stages
Location context for embryogenesis
Early development (up to morula stage) typically occurs in oviduct. Embryos have typically migrated into the uterus by blastulation
The amnion
The amnion is the layer closest to the fetus, forming the amniotic vesicle
- very thin, clear membrane
- formed by invagination of mesoderm +
trophectoderm
- membrane secretes amniotic fluid to cushion fetus
- umbilicus passes through this membrane
- amniotic plaques often present (inside surface) –
similar structure to skin, but function unknown
The yolk sac
The yolk sac is the most primitive membrane – very important in fish, reptiles and birds, but less so in mammals
- In mammals: transient structure, regresses during early gestation
- Source of primordial germ cells, involved in haematopoiesis, vasculogenesis,
nutrient/gas exchange
The chorioallantois (5)
Allantois
- Originates from primitive hindgut
- Fetal fluid waste collects in the allantoic cavity
- Grows significantly in size and often fuses completely with the chorion
Chorion
- Outermost layer, direct contact with maternal endometrium
- Calcification deposits on surface common
The maternal fetal interface (4)
The chorion is covered in chorionic villi – small projections, creating attachment to the endometrium
Placentae can be classified by their distribution of villi:
Chorionic villi are the site of exchnage between fetal + maternal circulation
The umbilicus
1 umbilical vein (oxy) = 2 umbilical arteries (deoxy)
Left = amniotic cavity
Inside the amnion = contains urachus (i.e. start of allantois)
Outside amnion = no urachus (because in allantoic cavity)
The umbilical vessels connect to the chorionic villi, where gas/nutrient exchnages occurs
Amniocentesis is the sampling fluid from the amniotic cavity with a needle for diagnostic testing.
Which fetal membranes would you pass through to perform an amniocentesis?
C
Metabolic functions of the placenta
The placenta transports gases, nutrients and waste products between fetal/maternal circulation
- Provides oxygen & nutrients from maternal circulation
- Actively converts substrates to create fetal nutrients
- Removes CO2 and waste products to maternal circulation
Placental insufficiency (abnormal dev/function) = intrauterine growth restriction (IUGR)
A number of transport mechanisms control movement of molecules across the placenta:
- Active transport via specific transport proteins, requires ATP to move against a
gradient
- Facilitated transport via transport proteins, with gradient
- Passive diffusion
- Endocytosis
Important to remember – many drugs and pathogens can also cross placenta!
The placenta plays a critical role as the ‘fetal lung’ – gas exchange occurs by diffusion
Endocrine functions of placenta
What happens after maternal recognition of pregnancy?
CL lifespan is prolonged = higher levels of P4 (progesterone block)
What about later in pregnancy?
In some species, placenta takes over aP4 production, with only small amounts produced by the CL
* Note: placenta will still produce P4 in sow, bitch, queen just not takeover. CL is main P4 source here.
* Placenta takeover VERY LATE in cow
To induce abortion, take away source of progesterone (either placenta or CL as well depending on species)
In those species with no placental takeover, gestation tends to be shorter. P4 concentrations can vary significantly over gestation, but remain high
Late in gestation, the placenta starts converting P4 to E2 – critical trigger for parturition
The plancental gonadotropins (3)
FSH like actions
Only the human/primate and equine placentas produce gonadotropins, named appropriately
Human chorionic gonadotropin
- Secreted from hatched blastocyst onwards
- Luteotropic – increases P4 production by existing CL
- Commonly used to induce ovulation
Equine chorioninc gonadotropin (eCG) aka pregnant mare serum gonadotropin (PMSG)
- Luteotropic – increases P4 production by existing CL
- Also causes additional ovulations to create accessory CLs
- Commonly used to stimulate follicle growth (superovulation)
Other placental endocrine productions
Placental lactogen & relaxin & pregnancy associated glycoproteins (PAGs)
