fertilisation and contraception Flashcards
what are the requirements for fertilisation?
- sperm (that has matured and capacitated)
- an egg (arrested at metaphase 1)
- same place and same time
- synchronised with receptive endometrium
what Is contained within ejaculate?
- approx. 3ml
- sperm + seminal plasma = semen
- secreted by the accessory sex glands, seminal vesicles and prostate
- roles in transport, nutrition, buffering and antioxidants
- contains STIs
what does the bulbourethral gland?
- pre-ejaculate
- lubrication
- neutralisation (pH) –> urethra transports urine too which is acidic
why can’t sperm leaving the testis recognise the egg?
-need to undergo maturation in the male tract and capacitation in the female tract first
what is capacitation?
- undergoes physiological changes
- before capable of entering egg
- flagellar beat is hyper activated
- sperm gain ability to undergo acrosome reaction
what happens to sperm as they travel through the female tract?
- numbers decrease rapidly
- 200 million deposited In upper vagina –> 100,000 sperm enter uterus
1000 sperm enter each uterine tube -> 50% chance of choosing each uterine tube
- high viscosity
where does fertilisation occur?
- in the ampulla region of uterine tube
- sperm head for this region
- egg doesn’t travel far
how long do sperm and egg remain viable for?
- sperm–> 5 days within female tract
- oocyte –> 24hrs
list the steps of sperm interaction with oocyte vestments ?
- remote detection of oocyte-cumulus complex –> chemotaxis may be involved?
- penetration of cumulus cells outside egg
- zona binding
- acrosome reaction
- zona pentatlon
what is the cumulus?
- made of approx. 3000 cells embedded in a gelatinous matrix containing hyaluronic acid
- closely apposed cells form a tight, organised layer
- other less organised
- sperm penetrate and can disperse the cumulus
why is the zona pelludica important?
- EC matrix surrounds all mammalian eggs (4 ZP proteins)
- important for sperm-egg binding and induction of the AR
- persists post-fertilisation
- binding to the ZP induces the AR
why is the AR so important?
- vital
- permits zona penetration
- exposes the new inner membrane for oocyte fusion –> contains receptor crucial for activating the oocyte membrane
why cant all sperm undergo the AR?
- only capacitated sperm can that have hyper activated motility and have the ability to undergo the AR
what Is the process of fusion?
- sperm penetrates the ZP and occupies the perevitelline spaceEquatorial 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+]i – sweeps across egg from point of sperm fusion–> absolutely key for inducing next stages of development
what is Izumo?
- sperm membrane receptors for fusion
- only detectable on sperm surface after AR has occurred
- KO of this protein abolishes fusion
its partner protein present on the oocyte PM is Juno
what occurs during the oocyte activation process?
- 1/3 mins–> large rise in I Ca which sweeps across the egg from point of sperm entry
- followed by ca oscillations every 3-15 mins -> several hours –> important for triggering next stage
what protein triggers oocyte activation via increased ca?
- PLC zeta –> found in sperm only
what mechanisms are in place to block polyspermy?
- fast block –> electrical –> membrane depolarisation-> mins
- slow block–> cortical reaction-> triggered by increase In [ca] 1 hr +
what is the cortical reaction?
- cortical granules contain mixtures of enzymes (proteases) which diffuse and fuse within the ZP following exocytosis from egg
- induces the ZR
what is the zona reaction ?
- leads to an alteration in the structure of ZP catalysed by proteases form cortical granules
- cleavage of ZP2 by ovastacin protease meaning sperm can no longer bind or penetrate the ZP layer
what else prevents polyspermy?
- loss of Izumi binding partner on egg- Juno
- is shed from the oocyte PM with the cortical granules
- is undetectable within 40 mins of fusion
- no further sperm can fuse
what does the sperm contribute to F?
- haploid male genome
- sex of baby determined
- centrioles -> oocyte has none, this forming spindle for cell division
what does the oocyte contribute to F?
- haploid female genome
- cytoplasm
- all organelles
- mitochondria (maternally inherited)
what is the zygotic / pronucleate stage?
- decondensation of sperm DNA (protamine and histone exchange)
- male and female pronuclei replicate their DNA
- pronuclei migrate towards one another
- guided by sperm aster–>MT radiating from sperm centrosome
what is syngamy? and what does it mark?
- occurs at 18-24 hrs post F
- pronuclear membranes break down
- chromatin intermix
- NE reforms around Z new nucleus
- cleavage begins marking the end of F and the start of embryogenesis
what are the cleavage stages?
- zygote cleaves to form 2 blastomeres
- 4 cell –> 8 cell stages–> morula (16-32)
- ZP present
- no cytoplasmic synthesis so each blastomere size decreases with each division
what days does the blastocyst form?
late day 4/5
- blastocoel cavity
- distinct ICM and single layered trophoblast layer
2 poles formed (embryonic and abevbryonic poles)
when does hatching occur?
late day 6 onwards
- blastocysts expands out of hole in ZP-> at the abembryonic pole
- implantation occurs
outline the 3 key layers of the endometrium?
- outer muscular myometrial layer (c)
- inner uterine lining –> 2 layers
- basalis stratum (B) + functional layer (A)
what is the folicular phase ?
- proliferative phase
- post menstruation the endometrium is very thin and consists of a few layers of cells (basal)
- first every 14 days of M cycle the endometrial cells proliferate in presence of oestrogen
what is the luteal phase?
- post ovulation
- ovaries produce progesterone
- known as the secretory phase
- P stimulates synthesis of secretory material by glands rich in glycogen and gps/ AAs all providing nutrition for the blastocysts
- this endometrium is now receptive and is ready for implantation of the blastocyst
what is ovulation?
- release of egg from ovaries
