Early pregnancy Flashcards
what is early transport of the zygote accomplished by (3)
- muscular contractions of oviduct (a1 contraction, b2 relaxation adrenergic receptors)
- ciliary beating on epithelial cells lining oviduct
- oviductal fluid movements in response to these
how do embryos move within oviduct (esp ampulla)
- to and fro rather than unidirectionally
- overall bias for movement toward uterus
what happens near the end of movement through oviduct
temporary arrest (days) at the ampullary-isthmic junction
what is purpose of arrest at ampullary-isthmic junction
- initiated by embryonic signals to oviduct
- ensures that it spends appropriate development time in the oviductal environment (vs being prematurely sent to the uterus where it would not survive)
what happens after arrest at ampullary-isthmic junction
- rapid transport through isthmus
- some species halt again at uterotubal junction prior to entry to uterus (tonic contraction of oviductal circular smooth muscle)
what is rate of transport under the influence of )3)
- steroid hormones
- paracrine factors
- autonomic nervous system (b2 = relaxation, a1 = contraction) –> adrenergic stimulation of oviductal smooth muscle
what does estrogen usually do to transport and what is the exception
- usually hastens transport
- slows it in the rabbit
how does embryo exert some control over oviductal transport
by secreting paracrine factors –> presence of embryo can either slow or hasten transport depending on the region
are embryos or unfertilized oocytes transported into uterus?
- most species: both embryos and unfertilized oocytes transported into uterus
- horse, donkeys, bats: only fertilized eggs (embryos) transported
what do equine embryos secrete from compact morula stage and why
- secrete PGE2
- acts locally to relax circular smooth muscle layer of oviduct to embryo can progress
does embryo of unfertilized oocyte enter uterus first
embryo
where do most embryos develop in cows and sheep
uterine horn on the same side as the CL (ipsilateral aide) –> little tendency to migrate to achieve maternal recognition of pregnancy
where do most embryos develop in horses
- single embryo migrates freely throughout uterus before it lodges at the base of one of the uterine horns
- must contact entire endometrium in order to survive
embryo migration in camelids
- equal distribution of ovulations from L and R ovaries
- 95% of pregnancies in L uterine horn
embryo migration in dog/cat/pig
- migrate freely through both uterine horns to achieve even spacing prior to implantation
- ensures adequate placenta available
what signals even spacing of embryos in dogs/cats/pigs
- chemical signals from embryo (histamine, estrogens, prostaglandin)
- stretching of uterine wall
one function of zona pellucida in embryo migration
prevention of premature embryo attachment during transport through oviduct (tubal pregnancy)
what must embryo do with zona following transport into uterus
escape zona pellucida to form a placenta and benefit from supportive uterine secretions prior to implantation (blastocyst stage –> blastocyst/zona hatching)
what has to occur for blastocyst/zona hatching to happen
- blastocyst or uterine endometrium secretes enzymes (plasma, trypsin) that weaken the zona
- blastocyst expands and contracts under influence of PGE
- results in splitting of zona –> blastocyst deforms and squeezes through gap
what can happen after blastocyst/zona hatching
cells of trophoblast can now interact directly with those of uterine epithelium (endometrium)
what happens to embryos following hatching
- exponential growth phase of elongation involving extraembryonic membranes
- elongation commences on day 11 (ewe/sow), 14-15 (cow)
- directed by products of endometrium secreted into hisotroph
bovine embryo elongation
- day 13: spherical, 3mm diameter
- day 17: 25cm filamentous structure
- day 18: extended into opposite uterine horn
- due to hyperplasia of cells in trophoblast
- embryo remains in horn ipsilateral to CL
pig embryo elongation
- rapid
- day 10: 2mm spheres
- day 11-12: 10mm cylinders
- start elongating 30-40mm/hr
- day 16: 100cm
- early lengthening through cellular remodeling and reorganization, not hyperplasia (then hyperplasia takes over)
purpose of embryonic elongation
having trophoblast contact all or close to as much of the uterine endometrium as possible to ensure maternal recognition of pregnancy
equine embryo elongation
- conceptus enclosed in a capsule
- does not elongate in early pregnancy like in other species
- ensures contact with large part of endometrium by moving around
what hormone is required for establishment of pregnancy and why
- progesterone from CL
- creates uterine environment in which early embryo can survive and establish itself for implantation and development of placenta, fetal development, birth
since most species have short cycles, what must happen to establish and maintain a pregnancy
way to subvert or prevent the process of luteolysis that causes the cyclic demise of the CL
what is maternal recognition of pregnancy
process whereby the embryo signals its presence to the maternal system and prolongs the life of the CL to preserve its own existence
2 ways embryo prolongs life of CL
- supporting it with luteotropic signal
- producing antiluteolytic signal that prevents CL’s premature demise
ruminant maternal recognition
- luteolysis due to prostaglandin release from uterine endometrium in response to changing the effective estradiol/progesterone ratios toward end of luteal phase
- ratio change induces increased endometrial oxytocin receptors, increased central oxytocin release –> prostaglandin secretion
- back up system of luteal oxytocin release, counter-current exchange system
luteolysis in ruminants and how it relates to migration
- fairly unilateral phenomenon (prostaglandin from L uterine horn lyses CL on left ovary)
- single pregnancy (embryo) has to exert influence in the horn on the side of the CL –> don’t migrate to opposite horn
what does ruminant embryonic trophoblast secrete
interferon-tau (IFN-tau)
prevention of luteolysis in ewes
- trophoblast elongates rapidly from day 11, secretes IFN-tau from days 10-21 of pregnancy
- IFN-tau acts on endometrium to block transcription of estrogen receptors
- no increase in estrogen receptors –> no estrogen up-regulation of oxytocin receptor gene –> no release of PGF2a from endometrium –> CL continues to function
prevention of luteolysis in cow
- IFN-tau prevents up-regulation of endometrial oxytocin receptors
- unclear if this in indirect (via action on estrogen receptor like in sheep) or direct action on oxytocin receptor gene
what would you call system of maternal recognition in ruminants
antiluteolytic (protects CL by preventing release of prostaglandin)
prevention of luteolysis in pigs
- endocrine-exocrine theory of maternal recognition
- PGF2a secretion is usually endocrine, goes to uterine vasculature and then CL
- pregnant sow: direction changes –> discharged in exocrine manner into uterine lumen (sequestered)
- need at least 2 embryos in each horn
what would you call system of maternal recognition in pigs
antiluteolytic (prevents prostaglandin accessing the CL)
prevention of luteolysis in mare
- early pregnancy: conceptus produces small amounts of PGF2a and PGE –> uterine smooth muscle peristalsis
- embryo moves throughout uterus
- conceptus products act on endometrium to prevent PGF2a release
what would you call system of maternal recognition in mare
antiluteolytic
prevention of luteolysis in primates
- luteolysis mediated at level of ovary
- hCG produced by trophoblast is luteotropic signal
- acts directly on CL to support it by maintaining an “LH” signal (hCG has LH-like activity)
- induces neovascularization
what would you call the system of maternal recognition in primates
luteotropic
prevention of luteolysis in dogs and cats
- long luteal phase
- no current evidence for maternal recognition as described in other domestic species
- pseudopregnant cat: CL has natural lifespan while gestation is longer (prolactin is luteotropic)
- dogs: progesterone levels increased –> increased prolactin
prevention of luteolysis in lab rodents
- ultrashort cycle
- sterile mating gives pseudopregnancy
- function of CL needed for most but not all of gestation, need viable embryos for CL function
- initial rescue of non-functional CL due to mating induced prolactin secretion
- second rescue of CL in pregnant animals turns 12 day CL of pseudopregnancy into 17+ day CL of pregnancy –> prolactin, placental lactogens (luteotropic)
cow and maintenance of pregnancy beyond maternal recognition
- up to 150d: CL only
- 150-250d: CL and placenta
- 250-term: CL
bitch and maintenance of pregnancy beyond maternal recognition
CL throughout
queen and maintenance of pregnancy beyond maternal recognition
CL throughout
sow and maintenance of pregnancy beyond maternal recognition
CL throughout
goat doe and maintenance of pregnancy beyond maternal recognition
CL throughout
ewe and maintenance of pregnancy beyond maternal recognition
- first 55-60d: CL
- 60d-term: placenta
llama/alpaca and maintenance of pregnancy beyond maternal recognition
CL throughout
mare and maintenance of pregnancy beyond maternal recognition
- first 90-100d: CL
- 100d-term: placenta
what do placentas of some species produce instead of progesterone and why important
- some produce proogestens
- may not cross-react on assay (when placenta takes over, progesterone production levels within placenta may be far higher than those in systemic circulation)
