ch 7

• Structure of a mammalian sperm

  • Head- acrosome + nucleus, Midpiece- generates ATP for propulsion, Tail- axoneme; motor portion of flagella
• Fertilization generally consists of 6 major events: 1. Movement of gametes towards each other 2. Contact & recognition between sperm and egg 3. Fusion of egg and sperm cell membranes 4. Regulation of sperm entry into the egg 5. Fusion of genetic material 6. Activation of egg metabolism and start of development
• Fertilization accomplishes two things: 1. Combining genes from two parents 2. Generating a new organism
• Fertilization


  Mechanism whereby gametes (sperm & egg) meet and fuse to initiate the creation of a new organism
• Egg Anatomy

  • Materials needed for organism's growth & development must be stored in the egg, Oocyte accumulates materials during meiotic divisions: nutritive proteins, ribosomes & tRNA, messenger RNAs, morphogenetic factors, protective chemicals
• Egg Cell Membrane & Extracellular Envelope


  • Egg's membrane regulates ion flow during fertilization and must be capable of fusing with the sperm cell membrane, In mammalian eggs, the extracellular envelope is called the zona pellucida
• It is difficult to mimic the female reproductive tract environment in the lab
• Mammalian fertilization occurs internally within oviducts of the female
• Female reproductive tract


  • It is a highly specialized set of tissues that actively regulate the transport & maturity of both gametes
• Events leading to fertilization

  The acrosomal and cortical reactions
• Hyperactivation

  Towards the end of capacitation; swimming at higher velocities & generating greater force
• Capacitation


  The set of physiological changes by which mammalian sperm become capable of fertilizing an egg
• Translocation to the Oviduct


  1. Male & female gametes use a combination of small-scale biochemical reactions & large-scale physical propulsions to reach the ampulla (upper end of oviduct)

  2. ~200-300 million sperm enter the vagina; only ~1 million enter the oviducts; meaning ~200 reach the vicinity of the egg
• What directs sperm to the oocyte

  Thermal & chemical gradients (thermotaxis & chemotaxis); progesterone secreted by cumulus cells; progesterone also helps activate CatSper calcium pump
• Translocation to the Oviduct Involves Several Processes

  1. Sperm motility

  2. Uterine muscle contractions

  3. Sperm rheotaxis (movement of an organism towards or away from a current of water)
• Acrosomal Reaction & Fertilization

  1. Acrosome contains enzymes that allow the sperm to penetrate the egg

  2. Acrosomal reaction occurs in the acrosome of the sperm as it approaches the egg, releasing surface antigens necessary for binding to the egg's cell membrane, enzymes responsible for breaking through the egg's tough membrane
• Acrosomal reaction

  1. Reaction that occurs in the acrosome of the sperm as it approaches the egg

  2. Releases surface antigens necessary for binding to the egg's cell membrane, enzymes responsible for breaking through the egg's tough coating & allowing fertilization
• Acrosomal reaction is activated by progesterone
• Acrosome


  • Contains enzymes that allow the sperm to penetrate the egg
• Components of the human zona pellucida

  • ZP1
  • ZP2
  • ZP3
  • ZP4
• Activation of the Mammalian Egg

  1. Intracellular calcium is released when triggered by IP3

  2. IP3 is produced by phospholipase C, which also activates block to polyspermy (removes juno from egg cell membrane)
• Gamete Fusion and Sperm Entry

  1. Sperm-egg fusion involves several proteins

  2. Izumo protein and membrane fusion in mouse fertilization

  3. Zinc spark at fertilization

  4. Initiation of calcium pulses in the mammalian oocyte by soluble PLC-zeta from the sperm

  5. Pronuclear movements during human fertilization

  6. Fusion of Genetic Material
• Polyspermy occurs when the oocyte is penetrated by more than one sperm