Fertilization Flashcards
where does fertilization occur
in the ampulla region of the oviduct adjacent to the ampullary-isthmic junction
at ovulation, what stage is the oocyte in
meiosis has been re-arrested at metaphase II
what is the perivitelline space
space between vitelline membrane and zona pellucida
what is the corona radiata
internal layer of cumulus cells radiating out from the zona pellucida
what happens to cumulus
undergoes expansion –> changes from tightly packed cell mass to one with a looser association in which the cumulus cells are held together by a viscous hyaluronic acid matrix
what is required for transport of oocyte after ovulation
- epithelial cell ciliary beating
- currents of oviductal fluid
- oocyte must be contained within its expanded cumulus matrix
- temporary adhesion between complex and apex of cilia on fimbria
when does the sperm acquire motility and potential for egg binding/fertilization
during maturation in the epididymis (but can’t fertilize until later)
what is capacitation
the maturational changes that sperm undergo in the female tract (capacity to bind and fertilize the egg) –> modification of sperm plasma membrane and associated molecules –> exposure of surface receptors involved in egg recognition, increases fusogeniticy of membrane, alters mechanisms of calcium homeostasis
what does capacitation involve (5)
- removal of previously applied decapacitation factors
- lowering of cholesterol/phosopholipid ratio of the sperm plasma membrane
- hyperpolarization of the membrane resting potential in the region overlying the acrosome
- aggregation of lipid microdomains of the plasma membrane that contain zona pellucida-binding protein complexes
- phosphorylation of surface tyrpsine residues
what does the loss of cholesterol in capacitation do
- allows increased intracellular pH, which increases sensitivity of sperm for induction of acrosome reaction
- may also thin plasma membrane, exposing receptors involved in sperm-egg interaction
what is capacitation mediated by changes in
- hormones (increasing progesterone/prostaglandin from lutenizing follicle)
- contents of oviductal lumen from ovulated follicular fluid
- increasing oviductal fluid bicarb concentration
hyperactivated motility
- at the same time that sperm become capacitated, they develop a hyperactivated motility
- increased amplitude of tail movement
- reversible (can go back to activated/progressive motility)
- role in detaching sperm from the oviductal reservoir
first theory about whether a female controls the timing of sperm capacitation to coincide with ovulation
- events surrounding ovulation signal oviductal epithelium in sperm storage area –> secretes something that triggers capacitation and hypermotility –> decreases binding affinity for oviductal epithelium –> hyperactivated motility detaches it so it can move to site of fertilization
- tl;dr: female controls capacitation so that it is closely timed with the arrival of the oocyte
second theory about whether a female controls the timing of sperm capacitation to coincide with ovulation
- during estrus there is a fairly constant release of sperm from storage site to ampullary-isthmic junction (fertilization area)
- sperm become capacitated by changing conditions as they move up the oviduct
- if oocyte isn’t there in a reasonable time, these sperm die and are replaced by more
- ensures that there are always a population of capacitated sperm present when the oocyte arrives
- minor role for female control of capacitation timing only
what has longer life cycle - oocyte or sperm
sperm
once cumulus-oocyte complex enters ampulla, what happens
firmly adheres to oviduct epithelium and remains attached until after fertilization
why is hyperactivated pattern of motility following capacitation important
important in ability of sperm to detect and hone in on the oocyte –> hyperactivated sperm are better able to swim through the mucus found in the isthmus of the oviduct, turn and swim out of crypts in oviductal wall, and have an effective search pattern within confines of the oviduct
how sperm find oocytes (non-mammals)
- sperm chemotaxis –> products of the egg serve to attract the sperm
- most species: small peptides/proteins, lipids –> control intracellular calcium levels
- increasing levels of attractant cause membrane hyperpolarization, preventing Ca increase in the sperm –> low Ca causes linear swimming, sperm continue up attractant concentration gradient to egg
- decreasing attractant levels activate Ca channels –> increase intracellular Ca –> thrashing motility that gives non-linear progression –> reorientation
how sperm find oocytes (mammals)
- at longer distances, sperm display thermotaxis (swim up temp gradients)
- closer range: chemotactic responses
- follicular fluid is chemotactic for sperm
- products of oocyte/cumulus complex also act in short range, odorant receptors too
penetration of oocyte vestments
-once sperm has located oocyte, needs to penetrate cumulus oophorous, zona pellucida, vitelline membrane
penetration of cumulus oophorous
- only capacitated sperm, with hyperactivated motility reflex, are capable of penetrating cumulus cells and their hyaluronic matric
- hyaluronidase bound to sperm plasma membrane helps
penetration of zona pellucida
- capacitated sperm binds to ZP –> undergoes acrosome reaction
- zona and zona-binding proteins are species-specific
what is the acrosome reaction
- following capacitation in female tract, sperm are capable of undergoing the acrosome reaction
- involves fusion of the outer acrosomal membrane with the plasma membrane of the sperm
- results in exposure of the acrosomal content to the extracellular environment
what 2 substances induce acrosome reaction
- protesterone: sperm selection (induce AR?), prime sperm
- zona pellucida: sperm binding to it causes AR
