fertilisation and development

• sperm

  labels

  A) flagellum

  B) mitochondrion

  C) centriole

  D) nucleus

  E) acrosome

  
• sea urchins

  • external fertilisation
  • large eggs
  • transparent embryos
• steps of fertilisation (sea urchin)

  • contact
  • acrosomal reaction
  • contact and fusion of sperm and egg membrane
  • cortical reaction
  • entry of sperm nucleus
• contact

  • sperm contacts eggs jelly layer, trigger exocytosis of acrosome from sperm
• acrosomal reaction

  • hydrolytic enzymes released
  • hole in jelly coat
  • actin filaments form
  • protrude from head into jelly coat
  • bindin receptor on surface of vitelline membrane bind to bindin
  • specificity of receptors
• fusion causes fast block to polyspermy, by depolarisation of membrane
• fusion causes

  • ion channel to open
  • Na+ influx from sea water
  • membrane depolarised
  • is transient
• slow block

  • cortical reaction
  • granules in egg fuse with plasma membrane
  • clips off sperm binding receptors
  • cause fertilisation envelope to form
• signal transduction triggering release of Ca2+, causing wave, stimulates egg activation
• cortical granules cause

  • break down adhesion of vitelline layer and membrane
  • increase osmotic pressure
  • snip off sperm receptors
  • harden fertilisation envelope
• protein synthesis and metabolic rate increase after 6 minutes since sperm addition
• female pronucleus guided by microtubules from male pronucleus centrosome, fuse to form zygote, DNA synthesis begins
• ovulation releases secondary oocyte and first polar body
• human fertilisation

  • sperm contact corona radiata
  • acrosomal reaction to digest zona pellicuda
  • fusion of membranes and sperm enters
  • fast and slow blocks to polyspermy
• sperm contacts corona radiata

  • hyaluronidase on sperm
  • granulosa cells embedded in hyaluronic acid
• block of polyspermy in mammalian fertilisation doesn't involve depolarisation, only calcium wave
• molecular interactions of fertilisation

  • zona pellucida ZP3 binds to beta-1,4-galactosyltransferase (acrosomal contents released)
  • acrosin and beta-N-acetylglucosaminidase digest zona pellucida proteins
  • fertilin on sperm head binds to integrin-like protein and CD9 in secondary oocyte (allows membrane fusion)
  • release Ca2+ to harden zona pellucida
• pronucleus

  • meiosis II resumes as secondary oocyte splits (ovum and second polar body)
  • female pronucleus develops
  • male pronucleus also develops and spindle fibres from centrosome (prepare for first cleavage division)
• cleavage

  • chromosomes line up to divide
• cleavage is cell division in early embryo
• gastrulation is cell movements which produce gut and 3 primary germ layers
• cleavage from fertilised egg to 16 cell stage
• morula to blastula
• embryonic cell cycle has no G1 and G2, because need to divide quickly
• cell cycle in embryo has similar rate from 2 cell to 16 cell for cleavage, because no G1 and G2, as M and S cannot be varied much in length
• blastomeres become smaller each division because

  undergo holoblastic cleavage, half cell each time (divide entire cell)
• amphibian cleavage

  • unequal holoblastic division after 4 cells
  • smaller in animal pole than vegetal pole, due to presence of yolk in vegetal hemisphere
  • animal pore divides more rapidly
• chick development

  • cleavage doesn't bisect yolk (Meroblastic cleavage)
  • blastula (epiblast top, hypoblast bottom)
• 3 primary germ layers

  • ectoderm
  • mesoderm
  • endoderm
• ectoderm

  • epidermis of skin
  • nervous
  • pituitary
  • adrenal medulla
  • teeth
  • germ cells
• mesoderm

  • notochord
  • muscular
  • skeletal
  • circulatory
  • lymphatic
  • excretory + reproductive
  • dermis of skin
• endoderm

  • epithelial lining of gut
  • lining of respiratory/excretory and reproductive tracts
• gastrulation occurs after blastula formation
• epiblast of chick will develop into the 3 primitive layers, hypoblast will develop into extra-embryonic layers
• primitive streak

  • cell movement through this and displace hypoblast cells
• first cleavage (humans) 24-30 hours after 


  fertilisation
• cleavage in humans as move down fallopian tubes
• 32 cell stage is a 


  blastocyst
• hatching (day 5)

  • digestion by blastocyst of zona pellicuda
  • required before implantation
• trophoblast forms part of the placenta