Pregnancy and placentation

Question 1

How is progesterone production maintained by the corpus luteum?

Answer 1

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

Question 2

How do we prolong luteal life?

Answer 2

controlling luteolysis

Method of CL breakdown varies between species

Question 3

How does passive luteolysis work?

Answer 3

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

Question 4

How do hCG levels change during early pregnancy?

Answer 4

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

Question 5

How does luteal regression occur in sheep?

Answer 5

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 F2𝛂 secreted by uterus 10-15 days after CL formation - CL breaks down
Ovary starts a new follicular phase

Question 6

How do we remove PGF2𝛂?

Answer 6

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

Question 7

What is trophoblast interferon (IFN𝛕)

Answer 7

Peptide secreted by trophoblast cells
Secreted 8-10 days after fertilisation
Oxytocin receptors expressed 10-12 days
CL and progesterone production maintained conceptus

Question 8

How does the mother develop to support the foetus?

Answer 8

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

Question 9

What happens if progesterone is lost during pregnancy?

Answer 9

Induces abortion

Placenta takes over progesterone production - this can vary between species from trophoblast cells after 6-9 weeks

Question 10

What is the average human progesterone secretion?

Answer 10

Non pregnant luteal phase = < 11ng/day

Pregnant = >200mg/day

Question 11

How is progesterone synthesised?

Answer 11

Maternal cholesterol converted to pregnenolone in the placenta which is then converted to progesterone also in the placenta and passed to mother and foetus

Question 12

What is an androgen?

Answer 12

Substrate for oestrogen production

Question 13

What role does the foetus play in androgen production?

Answer 13

Foetal adrenal glands convert progesterones to 19 androgen dihydroepiandrosterone (DHA), can also come from maternal adrenals
Foetal liver converts progestogens - androstenedione

Question 14

How are the androgens synthesised?

Answer 14

Foetal progesterone is converted into 17𝛂-hydroxyprogesterone which is converted into Dehydroepiandrosterone (DHA) (adrenal) + 16𝛂-hydroxyl-androstenedione (liver)

Question 15

In the placenta androgens produce which oestrogens?

Answer 15

Oestradiol 17𝛃: DHA from foetal or maternal adrenal
Oestrone: DHA from foetal or maternal adrenal
Oestriol: 16𝛂hydroxylated androgens from the foetal liver

Question 16

How is the foetus protected from such high levels of steriod?

Answer 16

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

Question 17

How does the blastocyst hatch?

Answer 17

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

Question 18

How does the blastocyst implant on the uterine wall?

Answer 18

Blastocyst trophoblast cells contact uterine epithelium and invade losing cell membranes and fuse becoming a network of syncytiotrophoblast which form the placenta

Question 19

What is the role of the placenta?

Answer 19

Mobilisation of oxygen
Provision of nutrition (salts + organic precursors)
Removal of waste products

Question 20

Describe a primate placenta

Answer 20

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

Question 21

Describe a sheep placenta

Answer 21

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

Question 22

Gestation length (days)
330-340
280-290
260-270
147-150
115 (3m,3w,3d)
61
60

Answer 22

Species
Horse
Human 
Cow 
Sheep
Pig 
Dog
Guinea pig