LECTURE 28 - fertilisation and contraception Flashcards
1
Q
What is the role of the Bulbourethreal Gland (Cowper’s gland)?
A
- pre-ejaculate
- lubrication
- neutralisation
2
Q
What is ovulation?
A
Cumulus-oocyte complex is picked up by ciliated fimbriae on the end of the uterine tube
3
Q
How is sperm transported in the female tract?
A
- ~200 million sperm deposited in upper vagina
- seminal plasma = short term buffering
- cervical mucus is least vicious during the 9-16 days of menstrual cycle
~100,000 sperm enter the uterus
~1000 sperm enter the uterine tube with 50% chance of choosing uterine tube with oocyte
4
Q
Where does fertilisation occur and when can it occur?
A
- occurs in ampulla region of uterine tube
- sperm remain capable of fertilisation for ~5 days within female tract
- oocyte remains viable for ~24 hours
- ovulation prediction kits (OPSKs) - looking for LH as huge surge occurs before ovulation
5
Q
Describe the sperm interaction with egg vestments
A
- remote detection of oocyte-cumulus complex
- penetration of cumulus
- zona binding
- acrosome reaction
- zona penetration
6
Q
Explain penetration of the cumulus
A
- approx. 3,000 cells embedded in gelatinous matrix (hyaluronic acid)
- closely apposed cells form tight, organised layers
- sperm penetrate and can disperse the cumulus
7
Q
What is the zona pellucida?
A
- extracellular protein matrix which surrounds all mammalian eggs
- 4 glycoproteins in humans (ZP1-4)
- important for sperm-egg binding and induction of acrosome reaction (AR)
- persists post-fertilisation
8
Q
What is the acrosome reaction?
A
- permits zona penetration
- exposes new membrane for oocyte fusion
- only capacitated sperm can undergo AR
- hyper activated motility
9
Q
What is fusion?
A
- sperm penetrates ZP and occupies the perevitelline space
- equatorial segment of sperm head fuses with oocyte plasma membrane
- sperm nucleus is encased by a vesicle composed of internalised oocyte membrane
- large increase in the free [Ca2+] - sweeps across egg from point of sperm fusion
10
Q
What is izumo?
A
- sperm membrane receptor for fusion
- detectable on sperm surface only after AR
- KO completely abolishes fusion
11
Q
What is Juno?
A
- receptor for Izumo on the oocyte plasma membrane
- KO abolishes function
12
Q
How are oocytes activated?
A
- within 1-3 mins of fusion - large rise in [Ca2+] which sweeps across egg from point of sperm entry - lasts 2-3 mins
- followed by Ca2+ oscillations every 3-15 mins which may last hours
- Ca triggered by phospholipase C zeta - sperm specific PLC
13
Q
How does oocyte activation cause the release of meiotic block?
A
- M-phase Promoting Factor (MPF) = cyclin-dependent kinase (cdk1) plus cyclin B - blocks metaphase to anaphase transition
- MPF is stabilised by Cytostatic Factor (CSF)
- raised Ca levels suppress CSF activity and destroy cyclin B by activating anaphase-promoting complex/cyclosome (APC/C) - causes expression of ubiquitin (E3) ligase which targets cyclin B for destruction and once lost meiosis can continue
- as well as this, cohesion protein complexes (hold sister chromatids together - oppose pulling force of microtubules
- scc1 subunit of cohesin is cleaved by separase
- securin - inhibits separase activity until ubiquitinated by APC/C (caused by increased Ca levels)
- resumption of cell cycle in the oocyte and completion of meiosis II
14
Q
How does black to polyspermy occur?
A
Polyspermy = more than one sperm fertilises an egg Fast block - electrical - membrane depolarisation - takes minutes
Slow block
- cortical reaction = cortical granules fuse and release a mixture of enzymes (inc. proteases) which diffuse into zona pellucida => induces zona reaction
- triggered by increased [Ca2+]
- takes 1hr+
15
Q
What is the Zona reaction?
A
- the alteration in the structure of the zona pellucida catalysed by proteases from cortical granules
- cleavage of ZP2 by ovastacin protease
- sperm can no longer bind or penetrate
