part i

• Fertilization:

  • Stimulates the egg to complete the second meiotic division
  • Restores the normal diploid chromosome number
  • Sex of the future embryo is determined by the chromosomal complement of spermatozoon
  • Zygote is genetically unique due to mingling of maternal and paternal chromosomes
  • Causes metabolic activation of the egg
• 4. Prevention of Polyspermy:

  A. Fast Block to Polyspermy:

  • Rapid electrical depolarization of the egg's plasma membrane
  • Resting membrane potential changes from -70 mV to +10 mV within 2 to 3 seconds after fusion
  • Prevents other spermatozoa from adhering to the egg plasma membrane

  B. Slow Block to Polyspermy:

  • Propagation of Ca ions from the site of sperm-egg fusion
  • Ca wave acts on cortical granules causing them to fuse with plasma membrane and release content into perivitelline space
• Steps in Fertilization:

  1. Penetration of corona radiata:

  • Corona radiata is a highly cellular layer with an intercellular matrix consisting of proteins and high concentration of carbohydrates (hyaluronic acid)
  • Active swimming movements of spermatozoa play a significant role in penetration of the corona radiata
• Steps in Fertilization:

  5. Metabolic Activation of the Egg:

  • Penetration of egg by sperm results in rapid intensification of the egg’s respiration and metabolism
• Cleavage:

  • Refers to the early mitotic division of the zygote into a number of cells called blastomeres
• Steps in Fertilization:

  2. Attachment to and penetration of Zona Pellucida:

  • Zona Pellucida is 13 microns thick in humans
  • Composed principally of sulfated glycoprotein
• Steps in Fertilization

  3. Fusion of Sperm and Egg:

  • Content of sperm sink into the egg
  • Plasma membrane of the sperm becomes incorporated with the plasma membrane of the egg
• Steps in Fertilization:

  6. Decondensation of Sperm Nucleus:

  • Chromatin spreads out within the nucleus as it moves closer to the nuclear materials of the egg
• Steps in Fertilization:

  7. Completion of meiosis and Development of pronuclei in the Egg:

  • After penetration of the egg by the sperm, the nucleus of the egg completes the last division, releasing second polar body into the perivitelline space
• Early frog blastula

  A) Animal Pole

  B) micromeres

  C) macromeres

  D) vegetal pole

  
• starfish cleavage

  A) blastocoel

  B) blastomeres

  C) Animal Pole

  D) vegetal pole

  
• Blastula

  • hollow ball of cells

  A) Animal Pole

  B) micromeres

  C) blastocoel

  D) macromeres

  E) vegetal pole

  
• Types of Cleavage:

  • Holoblastic cleavage
  • Meroblastic Cleavage
  • Superficial Cleavage
• Superficial Cleavage: During the early stages, cleavage is incomplete and only the nucleus divides. Eventually, the daughter nuclei migrate into the peripheral cytoplasm, forming a single layer of cells at the surface

  
• Meroblastic Cleavage: The partitioning of the cell is incomplete and confined to a small disc of yolk-free cytoplasm

  A) cephalopod molluscs

  B) fish, reptiles, birds

  C) most insects

  
• • Holoblastic cleavage – the cleavage furrow extends through the entire cell

  A) echinoderms, amphioxus

  B) annelids, molluscs, flatworms

  C) tunicates

  D) mammals, nematodes

  E) amphibians