fertilisation and blastocyst 2

• Sperm movement phases (transport in female)


  • rapid transport phase
  • Sustained transport phase
• rapid transport phase - sperm transport in female


  • Sperm swept up by fluid currents in uterus
  • sperm reaches oviduct within minutes,
  • Unable to fertilise the oocyte as they have not matured
• sustained transport phase - sperm transport in female 


  • sperm undergo maturation changes by interacting with epithelial cells of the female reproductive tract.
  • need to spend time in tract to mature
  • changes they undergo =
  • capacitation - capacity to fertilise
  • Hyperactivation - change quality of motility to deal with movement from fluid filled uterus to viscous oviduct
  • acrosome reaction - under go this once they reach oocytes
• capacitation of sperm 


  • freshly ejaculated sperm cannot fertilise the oocyte so must undergo capacitation
  • essentially reversing everything done to stabilise the sperm in the epididymis
  • progressive destabilisation of plasma membrane
  • glycoprotein molecules (decapacitation molecules) coating sperm head removed.
  • exposure of zona pellucida binding proteins
  • allows sperm to bind to the oocyte at fertilisation
  • surface charge altered - may attract sperm to oocyte
  • membrane fluidity increased to aid in breakdown of acrosome
• hyperactivation of sperm


  • capacitated sperm exhibit hyperactivated motility.
  • strong wide amplitude - whiplashing tail beats - travel further distance in the same time frame not straight line -
  • increases of intracellular calcium leads to elevated cAMP - causes whiplashing tail beats
  • increase force required to swim through viscous environment within the oviduct
  • In vitro hyperactivation leads to more head movement and less linearity.
• sperm changes


  • in epididymis surface molecules are added - (proteins and carbohydrates)
  • during ejaculation surface molecules coated with seminal plasma proteins - decapacitation factors
  • in female tract - capacitation occurs - female tract strips glycoproteins leaving exposed areas for sperm binding- equatorial segment
• Acrosome reaction 


  • capacitation exposes zone pellucida binding proteins on sperm plasma membrane (ZP3)
  • Sperm binds to zona pellucida via ZP3
  • Binding initiates sperm acrosome reaction =
  • Fusion of sperm plasma membrane and outer acrosome membrane
  • Release of enzymes to digest zona pellucida
  • Exposure of equatorial segment for oocyte fusion
  • Head will be only coated in inner acrosomal membrane.
• why do we have acrosome reaction


  • as sperm needs to get into perivitelline space to bind plasma membrane of oocyte.
• zona pellucida digestion 


  • acrosomal enzymes digest small hole in zona pellucida
  • penetration of zona pellucida is a rapid process.
  • sperm moves into perivitelline space between zona pellucida and oocyte plasma membrane
  • allows for fusion of sperm to membrane
• acrosome reaction images


  1. before acrosome reaction - all membranes intact
  2. During acrosome reaction - plasma membrane fuses with outer acrosome membrane
  3. Fusion causes vesiculation producing pores, release of hyaluronidase and acrosin.
  4. After - vesicles lost, inner acrosome membrane and equatorial segment exposed

  
• what are the digestive enzymes that break down the zona pellucida 


  • hyaluronidase
  • acrosin