Fertilisation and blastocyst 3

• What happens once sperm enters perivitelline space


  • sperm in perivitelline space is surrounded by plasma membrane microvill, cortical granules in oocyte cytoplasm
  • Sperm equatorial segment fuses with oocyte plasma membrane - sperm is engulfed in oocyte cytoplasm. Cortical granules released into perivitelline space
  • Zona pellucida still there to block polyspermy
  • once sperm fully engulfed - sperm nucleus within oocyte
  • Sperm nucleus decondenses and forms male pronucleus inside the oocyte
• cortical granules function


  • released into perivitelline space to prevent polyspermy
• how is polyspermy prevented


  • by zona pellucida
  • and cortical granules once 1 sperm enters perivitelline space
• fertilisation - genetic content


  • gametes are haploid - 1N
  • haploid cells have single copies of each chromosome
  • at fertilisation the oocyte has a female pronucleus -1N
  • the sperm contributes a male pronucleus 1N
  • following fertilisation the male and female pronuclei fuse to produce a diploid 2N zygote.
• formation of the blastocyst 


  • cells form 2 distinct layers
  • inner ball of cells - inner cell mass - embryo
  • cells at the periphery - trophoblast cells - placenta
  • together known as the blastocyst
  • cells in the outer cell mass pump sodium into the blastocyst creating fluid accumulation forming the blastocoele.

  
• what does the Inner cell mass become 


  • the embryo
• what do the trophoblast cells become


  • the placenta
• what is the blastocoele


  • the accumulation of fluid in the blastocyst
  • sodium pumped into cell by cells in the outer cell mass
• blastocyst hatching 


  • the blastocyst continues to move down the oviduct towards the uterus
  • implantation is prevented by the zona pellucida.
  • at the uterus the blastocyst hatches by digesting a hole in the zona pellucida.
  • blastocyst becomes a free floating embryo -
  • dependent on uterus (maternal circulation) for survival
  • implants into lining of uterus
• early embryo development 


  • at fertilisation oocyte and sperm contribute male and female pronucleus which fuse to produce a diploid 2N zygote.
  • zygote undergoes mitosis cleavage
  • first mitosis cleavage division creates a two cell embryo
  • cleaves multiple times to form 4, 8, 16 cell embryo
  • each of the cells in the early embryo are called a blastomere (undifferentiated)
  • each blastomere is genetically identical
  • during these stages the zona pellucida is still intact
  • embryo doesn’t get bigger
• Blastomere what is it?


  • an undifferentiated cell in a developing embryo
  • Each blastomere is genetically identical so during these early dev stages pre implantation genetic diagnosis can be done by removing 1 of the blastomere cells and genetically analysing it
• pre implantation genetic diagnosis on embryo during early stages 


  • can be done during 4,8,16 cell stage
  • go into embryo with pipette and remove 1 of the blastomere cells and do genetic analysis on it