Placentation, Parturition & Lactation Flashcards
List ways movement across placenta occurs
- Simple diffusion (gas - O2 & CO2)
- Facilitated diffusion (something is transported across placenta in a different form)
- Active transport (when something moves across the placenta in an unchanged form but requires energy)
Passive transport/simple diffusion across placenta
Water in by osmosis
Uric acid, urea, creatine out by osmosis (as healthy mother would have lower conc. than foetus)
Gas exchange
Facilitated diffusion
Glucose (main metabolic fuel for foetus)
Fetal blood sugar ~2/3 of maternal
Placenta also needs some glucose for itself
Some glucose is converted to glycogen and stored in placenta (readily available energy)
Some glucose is converted to lactate and transported across placenta as energy source
Active transport
Minerals - Fe, Cu, Ca, Mg
Against conc. gradient
Water soluble vitamins e.g. B & C
Some amino acids (foetus can only produce a few amino acids, the rest need to come from mother for protein synthesis and growth)
Blockage by the placenta
Maternal proteins (proteins cannot cross placenta) - foetus self synthesises proteins from amino acids
Lipids - most lipids are reconstituted by placenta then pass across in different form
Most maternal hormones are to big to cross placenta (e.g. insulin) - small hormones eg. oestrogen & other steroid hormones do pass placenta and is why newborns often have enlarged genitalia
Why does placenta take over progesterone production in some species?
Species with long gestation cannot maintain corpus luteum for that long so placenta takes over progesterone production
e.g. takes over at 3.5 months
How do oestrogen levels indicate foetal well being?
Oestrogen that the placenta produces is sourced from foetal androgens - healthy foetus produces a lot of androgens = high oestrogen levels
When does placental oestrogen production peak and why?
Production peaks pre-parturition - effects uterine quiescence
Causes smooth muscle contractions
Even though placenta is always producing oestrogen, progesterone levels are still high enough to block smooth muscle contractions
But just prior to parturition oestrogen levels produced by the placenta spike - causes smooth muscle contractions
Expels the foetus
Equine Chorionic Gonadotropin (eCG) in horses
Produced by the placenta - foetal placenta
Corpus lutea of pregnant mare produce high levels of progesterone
Levels then start to fall which is normally bad but in horses there is an extra surge of follicular growth That then turn into corpus lutea in the middle of their cycle
Means they get these accessory corpus lutea that start to produce progesterone
Whilst this is happening, the blastocyst has formed to stage where it is able to produce its own eCG
So in combination with this eCG and accessory corpus lutea, progesterone levels are high enough to maintain pregnancy until the end of gestation
Human Chorionic Gonadotropin (hCG) in humans/primates
Placenta takes over progesterone production at about 3.5 months
Surge in hCG during the period that placenta takes over progesterone production form corpus lutea
hCG is produced by blastocyst - has a role in maintaining corpus lutea for the amount of time needed for the placenta to take over and be able to produce enough progesterone
How is eCG used in reproductive technologies?
When eCG is injected into non-equine species is acts just like FSH - allows animals grow more follicles - produce more eggs and have more offspring
How is hCG used in reproductive technologies?
When hCG is injected into non-primates it acts like LH - induces ovulation
can be used to synchronise a herd - inseminate all in the one day
How does the foetus determine day of birth (list points)
Lack of space may induce a stress reflex
Cortisol is released
Parturition is initiated
How does the mother determines the time of day of birth
Circardian levels seem to influence birth timing
Parturition cascade
Foetus gets stressed because of lack of space
Cortisol is released
This then causes the production of a whole lot of enzymes that start to convert progesterone into oestrogen = this is the KEY event that cortisol triggers that brings about parturition
Oestrogen interrupts uterus quiescence
Progesterone levels must fall quickly and oestrogen levels must rise quickly to cause sudden activity and contractions - this is why progesterone is converted into oestrogen instead of being produced separately - much faster response
In species where corpus lutea are still present - luteolysis will occur at the same time that P4 is converted to oestrogen - stops production of P4
Placenta switches from making progesterone to making oestrogen
Oestrogen causes contractions
This puts more pressure on the foetus and causes the cervix to be stimulated
This feeds to pituitary and causes the release of oxytocin
Oxytocin causes strong muscle contractions - puts even more pressure on cervix - stimulates more release of oxytocin - more pressure (positive feedback loop) = really fast increase in pressure that is being put on the foetus to push it through the tract
At the same time oestrogen is causing secretions of the female reproductive tract to keep it lubricated
Cortisol is also causing an increase in the amount of PGF2 alpha - causes the breakdown of corpus luteum (progesterone source)
PGF2alpha also causes myometrium contractions
PGF2alpha also causes the release of the hormone relaxin - which relaxes the pelvic ligaments so that the pelvis can expand and expel the foetus
All these hormones work individually and in a positive feedback loop together to cause contractions
