Ex 3 - Fertilization and early embryonic development Flashcards
When do oocytes (2n) begin meiosis?
During fetal development
*They then arrest and remain dormant until after puberty
Meiosis I –> Meiosis II
Meiosis I resumes when an individual follicle is recruited –> paired chromosomes divide and one polar body is extruded –> Meiosis II begins when a follicle is recruited –> it is then arrested in metaphase II
What stage of meiosis is the oocyte in at ovulation? When does meiosis resume?
metaphase II
Meiosis resumes only if the oocyte is fertilized by a sperm
(nb: in the dog, oocyte is ‘immature’ when it ovulates - still in meiosis I)
Oocyte release and fertilization locations
Oocyte and its assoc’d cells are released from the follicle at ovulation –> picked up by the infundibulum (oviduct) –> transported to the ampulla (ciliary action & sm m contraction)
Where does fertilization occur?
ampulla/isthmus junction
Where are the sperm reservoirs?
cervic and/or uterotubular junction
*sperm can last a few days in these areas
What is sperm capacitation?
changes in glycoproteins on surface of sperm cell
alteration of lipid structure of sperm plasma membrane
hyperactive motility
Describe sperm fertilization of the oocyte
- sperm penetrates the cumulus cells surrounding oocyte
- sperm then binds to ZP3
- Influx of Ca++ trigger the acrosome reaction –> sperm fuse with oocyte membrane
- hyper motile sperm burrow through the ZP –> enters the perivitelline space –> sperm is internalized into cytoplasm of oocyte
What is “Zona Hardening”?
sperm and oocyte binding causes release of phospholipase from the sperm –> this triggers release of Ca++ from internal stores –> release of cortical granules –> these alter the ZP so no other sperm can bind
what is the sperm binding site on ZP?
ZP3
what is a zygote?
a diploid cell resulting from the fusion of two haploid gametes; a fertilized ovum
How is the zygote formed?
meiosis II resumes (stim by Ca++) –> 2nd polar body extruded –> M & F pronuclei are fused –> chromosomes are duplicated (mitosis) –> cell cleaves into blastomeres
*This is what we look for during in vitro fertilization
How often do blastomere cells divide?
every 12-24 hours
- each division results in smaller individual cells
What do we call the developing embryo when it reaches 16-32 cells?
Morula
- ZP is still intact
What is the fluid-filled cavity in the embryo called?
blastocele
What is the outer layer of the embryo called?
Trophoblast
- this is what forms the placenta
What is the purpose of the inner cell mass?
forms the fetus
How long does it take the embryos to exit the oviduct and enter the uterus?
- pigs?
- cows?
- mares?
- 3.5 days
- 4-7 days
- 5.5-6.5 days
What must equine embryos produce to allow passage through UTJ?
Embryos must produce PGE2!
This relaxes the sm muscle of the oviduct and allows embryo passage
*Unique to horses
Zona hatching in ruminants and mice
development of embryo results in pressure –> ZP cracks –> blastocyst stage escapes and squeezes out
Zona hatching in equine
Expand and the zona thins –> ZP is shed
- Equine embryos (in vivo) develop a glycoprotein capsule
- this protects the embryo after zona is shed
What hormone is required by ALL FEMALES to maintain pregnancy?
P4!!
What is the initial source of P4?
CL
*in some spp, P4 is eventually produced by placenta
Can we give P4 to maintain a pregnancy?
little evidence to support routine use of supplemental exogenous P4 in order to maintain pregnancy
*use of exogenous P4 is common in mares, “Regu-mate”
Maternal recognition of pregnancy (MRP)
LA embryos must signal the uterus to prevent secretion of PGs and luteolysis
*a decrease in P4 will result in pregnancy failure
MRP - bovine/ovine
Embryos secrete IFNtau –> inhibits production of oxytocin receptors in the endometrium
(oxytocin can’t stimulate PFG2a synthesis)
Embryos elongate to till the uterine hormones ipsilateral to the CL
- elongation allows IFNt to contact the entire endometrium to signal MRP –> this is imp. b/c PGFs travel via counter-current transport system in these guys
- IFNt must contact the uterine horn that contains the CL
(PGF is only released into the IPSILATERAL horn)
MRP - pigs
produce E2 which re-routes PFG secretion into the uterine lumen –> rather than release into systemic circulation
MRP - equine
unknown in horses!
The embryos remain spherical and migrate throughout the entire uterine lumen, multiple times a days
This migration signals MRP to the entire endometrium –> important as equine PGF travel via systemic circulation
What are dizygotic twins?
aka Fraternal twins or non-identical
*Arise from two ovulations that were fertilized separately
What are monozygotic twins?
aka Identical twins
*Arise from a single oocyte –> developing embryo splits
**horses –> twins often do not survive
**sheep/goats –> twins/triplets are common
Is it possible to split embryos to create identical twins via new technology?
yes
