Fertilisation and contraception Flashcards
What membrane changes occur during sperm maturation?
transfer of proteins into and onto the sperm plasma membrane
increase in membrane fluidity
essential for acrosome reaction
What flagellum changes occur during sperm maturation?
become more rigid
cAMP content of tail increases
When does capacitation begin?
predominantly within the female tract
due to proteolytic enzymes, cholesterol sinks and higher ionic strength
What characteristics do fully capacitated sperm exhibit?
hyperactivated motility
changes in membrane properties that subsequently allow the acrosome reaction to happen
Why is capacitation necessary?
they become more responsive to signals from oocyte
able to undergo acrosome reaction essential for fertilisation
What processes occur in capacitation?
stripping or modification of surface glycoproteins
changes in surface charge
reduction in membrane stability through loss of cholesterol and formation of lipid rafts
Hyperactivated sperm motility
involved high amplitude asymmetrical beating of the flagellum
helps sperm meet target
aids sperm penetration of zona pellucida
Cation channel of sperm (CatSper)
Ca2+ ion channel specific to sperm
activated by alkaline pH
essential for hypermotility + fertility
What processes aid the finding of the egg?
oocyte and cumulus cells release chemoattractants
surface sperm hyaluronidase digests cumulus cells to let sperm reach zona pellucida
What is the zona pellucida composed of primarily?
glycoproteins ZP1-4
ZP1
structural protein that corss links with others
ZP2
contains sperm binding domain for oocyte-sperm recognition + penetration of ZP
responsible for primary block to polyspermy
ZP3
complexed with ZP4
involved in primary sperm-egg binding
When does the acrosome reaction occur?
after sperm head binds to ZP
ZP proteins responsible for inducing reaction
What happens during the acrosome reaction?
acrosome swells and membrane fuses with sperm PM
acrosomal vesicle undergoes exocytosis leading to release of hyaluronidase + exposure of acrosin
Primary binding
sperm membrane binds ZP via ZP3/4 + species specific complementary binding partner
Acrosin
protease on inner membrane of sperm exposed by acrosome reaction
further digests ZP + oocyte PM
Secondary binding
sperm inner acrosomal membrane binds to ZP via ZP2 via acrosin on sperm
PLCζ
released into cytoplasm of oocyte following fusion
facilitates hydrolysis of membrane bound PIP2 to DAG + IP3
triggers calcium release + oocyte activation
What is the consequnce of PLCζ increasing intracellular calcium?
Ca induced Ca release
calcium oscillations which stimulate cortical granule release- ovastacin
PKC in gamete fusion
stimulates phosphorylation of other proteins essential for development of conceptus
How does ovastacin block polyspermy?
act on ZP to harden
inactivation of sperm receptors by cleaving ZP2
block to polyspermy
Formation of female pronucleus
Ca pulses stimulate resumption of meiosis
second polar body is extruded and female pronucleus forms
Formation of male pronucleus
sperm nucleus decondenses
protamines are replaced by histones
Syngamy
combination of two genomes
pronuclei come together, pronuclear membranes break down and replicated chromosomes align on mitotic spindle for first cleavage
What happens as a result of sperm-egg fusion?
increased oestrogen + progesterone
overrides monthly cycle and suppresses HGP axis
HCG produced by placenta
