Gametes and fertilisation

• Structure of a spermatozoon

  Head - contains Acrosome, Nucleus, Centriole

  Neck - connects head to Middle piece

  Middle piece - contains mitochondrion (spiral shape)

  Flagellum
• Why study the fertilisation of sea urchine

  Large eggs

  transparent embryos

  external fertilisation
• Steps of sea urchin fertilisation

  1. Contact: sperm contacts the eggs jelly coat which triggers the exocytosis from sperms acrosome
  2. Acrosomal reaction: hydrolytic enzymes make a hole in jelly coat and actin filament form the sperm (acrosomal process) binds to receptors on the eggs plasma membrane.
  3. Contact and fusion of sperm and egg membrane: fusion triggers depolarization of membrane, FAST BLOCK POLYSPERMY
  4. Cortical reaction: Cortical granules in egg fuse with plasma membrane. These clip-off sperm-binding receptors and cause fertilization envelope to form. SLOW BLOCK POLYSPERMY
• Describe the role of bindin

  Bindin receptor on surface of vitelline membrane bind to bindin which triggers more enzymes to break down vitellin layer and plasma membrane
• Fast block to polyspermy

  • egg resting potential -70mV
  • Membrane potential present across sea urchin
  • Fusion of plasm membrane leads to:
  • Na+ influx from sea water, this leads to depolarisation
  • change in plasma membrane, so no more sperm can enter
• Slow block polyspermy (calcium, cortical reaction)

  • Binding of sperm to the egg, activates signal transduction pathway triggering release of Ca2+ into the cytosol from the ER
  • Happens in a wave centre (where the sperm is bound)
  • forms a fertilisation envelope or bubble
• Calcium wave of the cortical reaction

  Binding of sperm triggers a signal transduction pathway triggering the release of Ca2+ into the cytosol form the ER
• Slow block polyspermy

  Ca2+ wave increases cytosolic conc of Ca2+ where sperm fuses

  cortical granules fuse with plasma membrane and release enzymes:

  • break down adhesion between vitelline layer and membrane (perivitelline space)
  • Increase osmotic pressure causing water influx (due to nucleopolysaccharides)
  • Snip off sperm receptors
  • harden fertilisation envelope
  • Stimulates egg activation
• Egg activation leads to:

  increase in protein synthesis

  increase in metabolic rate
• Pronuclei fusion:

  Migration of female pronucleus guided by microtubles from male pronucleus centrosome

  Haploid set of chromosomes

  directly fuse to form a zygote

  DNA synthesis begins then cleavages
• Oocyte at ovulation:

  Releases a secondary oocyte and first polar body

  Surrounded by zona pellucida and corona radiata - granulosa cells (both layers must be penetrated by the sperm)
• Why are multiple sperm required to interact with egg

  for more hyaluronidase to penetrate zona pellucida
• stages of human fertilisation:

  1. Sperm contact with corona radiata
  2. Acrosomal reaction to digest zona pellicuda
  3. fusion of membranes and sperm enters
  4. fast and slow blocks to polyspermy
• Sperm-egg molecular interactions

  1. Zona pellucida glycoprotein ZP3 binds to beta 1-4-galactosyltransferase on sperm, Triggers acrosomal content release
  2. Acrosomal enzyme digest zona pellucida glycoproteins
  3. Fertilin on the sperm head binds to integrin like protein and CD9 in secondary oocyte in the plasma membrane
  4. Block polyspermy
• What enzymes are found in an acrosome

  acrosin and beta-N-acetylglucoaminidase
• Pronucleus formation:

  The entire spem enters and is absorbed into cytoplasm

  Meiosis II resumes

  • Secondary oocyte splits in 2 haploid of uenqual size (ovum and polar body)
  • Ovum contains pronucleus
• Spindle formation and cleavage preparation

  Male pronucleus develops

  Spindles fibres from the centrosome appear in preparation for 1st cleavage division
• Preparation for first mitotic division

  each pronucleus enters mitosis separately, DNA replicates separately

  Nuclear membrane breaks down and pronuclei fuse forming a zygote

  Chromosomes aline along the metaphase plate

  Chromosomes attach to common spindle
• Cleavage

  chromosomes line up along the metaphase plate and prepare for division