Implantation and Maternal Recognition of Pregnancy

How long is the blastocyst in the free living stage in humans and rodents?

short lived - 4-6 days

How long is the blastocyst in the free living stage in sheep?

long free phase - 16 days

What is an example of an animal that can be in diapause?

pandas

What is histiotrophic nutrition?

material secreted by the endometrium into the uterus to support free living blastocyst

What is haemotrophic nutrition?

establishment of adjacent foetal and maternal circulations in the placenta for exchange

At what stage of embryo development is histiotrophic nutrition needed?

free living blastocyst stage

At what stage of embryo development is haemotrophic nutrition needed?

after implantation

Why is it important for the endometrium to be in the secretory phase at the fertilisation / implantation phases?

for histiotrophic nutrition to support free living stage of blastocyst

Why can an embryo NOT implant in the pre-receptive endometrium (2)?

• mucin coat covering uterus

• long microvilli with high surface charge

What happens to the endometrium to allow implantation (receptive endometrium) (3)?

• mucin thins

• microvilli shorten and lower surface charge

• integrin-extracellular matrix interactions

Why can an embryo NOT implant in the refractory endometrium?

systems to start next menstrual cycle have already started

What determines the degree of splitting in monozygotic twins (3)?

• timing of the split (e.g. 2/3 days old vs time of implantation)

• split earlier = separate membranes / sacs

• split later = shared

What are the names of the 2 membrane surrounding a foetus?

• amniotic - inner membrane

• chorionic - outer membrane

What are the 4 main stages of implantation?

1. hatching

2. apposition

3. adhesion

4. invasion

What stages of implantation are similar in the rodent and human?

apposition and adhesion

What stages of implantation are different in the rodent and human?

human much more invasive

In what type of species is the apposition stage of implantation particularly important (2)?

• polytoccus species (multiple embryos - e.g. mice)

• embryos communicate to ensure spacing

What signal is thought to be used by embryos in polytoccus species to ensure spacing at the apposition stage of implantation?

oestrogen analogue

How do embryos attach to the endometrium for implantation (3)?

• carbohydrate tethering on endometrium (lectin)

• heparin binding EGF-like growth factor on endometrium and EGF and heparan suphate proteoglycans on trophoblast = stimulates invasion

• integrins on endometrium bind to ECM (e.g. fibronectin)

Why is the molecular dialogue for implantation poorly understood (3)?

• complex with redundancy

• species specific

• in vitro modelling difficult - lots of cross talk between different cell types

What signalling stimulates embryo invasion during the attachment phase (2)?

• endometrium - heparin binding EGF-like growth factor

• trophoblast - EGF and heparan suphate proteoglycans

How is an embryo tethered to the endometrium during attachment?

lectin expressed on endometrium = binding

What is an example of rodents not being a good model for other species (2)?

• Lif expressed at same time as implantation in mice

• not seen in other species

What is a key implantation gene in mice and how was this discovered (4)?

• Lif

• Lif K/O mouse is fertile but fail to implant

• Lif K/O blastocysts viable in WT mice

• Lif supplementation = implants

What happens during invasion (5)?

• decidualisation response (cells secretory)

• oedema (cytokines)

• angiogenesis

• tissue breakdown and remodelling

• cell-cell interactions important to control degree of invasion

Why is there angiogenesis and tissue breakdown and remodelling during invasion?

establish new vascular supplies in close proximity

Which cells on the embryo actually burrow into the endometrium for invasion?

syncytiotrophoblasts - exterior cells, multi-nucleate, terminally differentiated but functional

Why is maternal recognition of pregnancy so important?

for embryo to survive - progesterone and corpus luteum must be maintained

What are the 3 different systems for maternal recognition of pregnancy?

• luteotrophic (e.g. human)

• anti-luteolytic (e.g. sheep)

• coitus initiates maintenance of corpus luteum (e.g. mice)

What does hCG stand for?

human chorionic gonadotrophin

What cells release hCG and why (3)?

• trophoblast cells (in humans and horses)

• signals pregnancy to mother

• luteotrophic system

Why is it important that hCG is highly glycosylated?

confers greater stability / longer half life

What cells does hCG bind to?

LH receptors on large luteal cells of corpus luteum (to maintain progesterone)

What hormone does hCG have high homology to?

LH (and FSH)

How does maternal recognition of pregnancy work in sheep (4)?

• anti-luteolytic mechanism

• interferon tau secreted by trophoblast

• inhibits endometrial PGF2alpha

• normally, PGFalpha2 causes luteolysis

How does luteolysis work normally in sheep and cows (e.g. no pregnancy present) (2)?

• pulsatile oxytocin = PGF2alpha secretion by endometrium

• PGF2alpha is luteolytic hormone

What signal is secreted by trophoblasts in sheep for maternal recognition of pregnancy?

interferon tau (blocks PGF2alpha)

What type of ovulation is seen in rates and mice?

spontaneous

How is the corpus luteum maintained in rodents to support a pregnancy?

coitus stimulates nocturnal prolactin surge = maintains corpus luteum

What is the Whitten effect in rodents?

females housed together are anovulatory but will synchronously ovulate when a male is added due to male pheromone

What evidence is there that the coitus maintains the corpus luteum and pregnancy in rodents?

pseudopregnancy can be initiated using infertile or vasectomised mice

What is the luteal-placental shift?

point where placenta takes over progesterone secretion from the corpus luteum and can maintain pregnancy

Why do rodents not have a luteal-placental shift?

very short pregnancy (~20 days)

What is one of the triggers of parturition in mice?

luteal regression - corpus luteum maintains progesterone throughout due to short pregnancy (luteolytic response)

How did we discover when / if the luteal-placental shift occurs in different species (2)?

• very unethical experiments

• ovary / corpus luteum removal at different gestational ages to see effect on foetus / pregnancy

What are some examples of animals that require progesterone from both the placenta AND the corpus luteum throughout pregnancy (no luteal-placental shift) (2)?

• cow

• pig

What are some examples of animals that have a luteal-placental shift (3)?

• humans

• sheep

• horses