Pregnancy and placentation Flashcards
How is progesterone production maintained by the corpus luteum?
Lifespan of the CL is limited to 2 weeks in non- pregnant cycle, extended if pregnancy occurs
Continued progesterone synthesis by CL essential in early gestation - until placenta takes over progesterone production
Placental shift can result in abortion
How do we prolong luteal life?
controlling luteolysis
Method of CL breakdown varies between species
How does passive luteolysis work?
LH decrease in luteal phase of cycle
Loss of luteotrophic action causes spontaneous CL regression
Blastocyst trophoblast cells produce luteotrophic factor
Chorionic gonadotrophin (hCG), binds to LH receptor
How do hCG levels change during early pregnancy?
Circulating hCG is detectable 9 days after ovulation
hCG is used for pregnancy testing
hCG falls at week 12 and CL regresses as placenta takes over progesterone production
How does luteal regression occur in sheep?
Corpus luteum remains for either: duration of luteal phase of ovarian cycle, early pregnancy
If no pregnancy, corpus luteum must regress at the end of the luteal phase of the ovarian cycle
Prostaglandin F2๐ secreted by uterus 10-15 days after CL formation - CL breaks down
Ovary starts a new follicular phase
How do we remove PGF2๐?
Released from uterus by oxytocin
Oxytocin receptors only expressed in the uterus during luteal phase
Conceptus needs to block receptors days 12-20 to maintain CL
What is trophoblast interferon (IFN๐)
Peptide secreted by trophoblast cells
Secreted 8-10 days after fertilisation
Oxytocin receptors expressed 10-12 days
CL and progesterone production maintained conceptus
How does the mother develop to support the foetus?
Maintainence of the lining of the uterus (endometrium)
Developement and hypertrophy of the uterine musculature for expulsion of the foetus at term
Development and maintainence of the mammary glands for post partum lactational nutrition
Take over of metabolism is controlled by pregnancy hormones
- steroidal hormones
- progesterone and oestrogens
What happens if progesterone is lost during pregnancy?
Induces abortion
Placenta takes over progesterone production - this can vary between species from trophoblast cells after 6-9 weeks
What is the average human progesterone secretion?
Non pregnant luteal phase = < 11ng/day
Pregnant = >200mg/day
How is progesterone synthesised?
Maternal cholesterol converted to pregnenolone in the placenta which is then converted to progesterone also in the placenta and passed to mother and foetus
What is an androgen?
Substrate for oestrogen production
What role does the foetus play in androgen production?
Foetal adrenal glands convert progesterones to 19 androgen dihydroepiandrosterone (DHA), can also come from maternal adrenals
Foetal liver converts progestogens - androstenedione
How are the androgens synthesised?
Foetal progesterone is converted into 17๐-hydroxyprogesterone which is converted into Dehydroepiandrosterone (DHA) (adrenal) + 16๐-hydroxyl-androstenedione (liver)
In the placenta androgens produce which oestrogens?
Oestradiol 17๐: DHA from foetal or maternal adrenal
Oestrone: DHA from foetal or maternal adrenal
Oestriol: 16๐hydroxylated androgens from the foetal liver
How is the foetus protected from such high levels of steriod?
All steroids arriving from mother/placenta are conjugated to sulphates by the foetus in the foetal liver and adrenals
These conjugate steroids are biologically inactive
The placenta de-sulphates steroids releasing free biologically active hormones into the water - permits high levels of steroid in the mother but low levels of active steroid in the foetus
How does the blastocyst hatch?
As it moves down the oviduct towards the uterus implantation is prevented by the zona pellucida.
At the uterus the blastocyst hatched by digesting a hole in the zona pellucida
Blastocyst becomes a free floating embryo dependant on the uterus for survival
How does the blastocyst implant on the uterine wall?
Blastocyst trophoblast cells contact uterine epithelium and invade losing cell membranes and fuse becoming a network of syncytiotrophoblast which form the placenta
What is the role of the placenta?
Mobilisation of oxygen
Provision of nutrition (salts + organic precursors)
Removal of waste products
Describe a primate placenta
Discoid
Can attach anywhere in the uterus
Single site exchange
Extremely invasive - sits in a pool of maternal blood
Multiple small placental vessels merge to form the umbilical cord
Has three layers of separation - 1. Chorionic epithelium 2. chorionic interstitium 3. Chorionic capillaries
Describe a sheep placenta
Uterine caruncles
Comprised of: maternal caruncle, foetal cotyledon which together form a placentome
Made up of : 1. Maternal capillaries, 2. maternal interstitium, 3. maternal epithelium, 4. Chorionic epithelium, 5. Chorionic interstitium, 6. Chorionic capillaries
Epitheliochorial, non-invasive, 6 layers, minimal blood loss at parturition
Gestation length (days) 330-340 280-290 260-270 147-150 115 (3m,3w,3d) 61 60
Species Horse Human Cow Sheep Pig Dog Guinea pig
