animal development

• Embryonic development


  Important stage in the animal lifecycle where tissue layers (gastrulation) and later rudimentary organs (organogenesis) are formed

  
• Embryonic development


  Starts with fertilization
• Fertilization


  The formation of a diploid zygote from a haploid sperm and egg cell

  
• Fertilization


  1. Molecular interactions occur between the sperm and egg cell

  2. Sperm releases enzymes from acrosome to digest egg's jelly coat

  3. Sperm binds to receptors on egg's plasma membrane and vitelline layer

  4. Fusion of sperm and egg cell membrane

  5. Cortical reaction lifts vitelline layer and removes sperm-binding receptors

  
• Cleavage


  1. Rapid cell division without cell growth

  2. Partitions cytoplasm of fertilized egg into smaller blastomeres

  3. Forms a hollow ball of cells called the blastula
• Blastocoel


  Fluid-filled space in the middle of the blastula
• Gastrulation


  Part of the blastula invaginates itself into the blastocoel to form germ layers
• Germ layers


  • Ectoderm (outer layer)
  • Endoderm (innermost layer)
  • Mesoderm (middle layer)

  
• Blastocyst


  • In humans, the equivalent of the blastula
  • Inner cell mass forms the developing embryo
  • Trophoblast forms the outer epithelial layer and initiates implantation
• Amniotes


  • Reptiles, birds, and monotremes with shelled eggs
  • Marsupial and eutherian mammals with uterus
• Extraembryonic membranes in amniotic egg


  • Chorion (gas exchange)
  • Amnion (encloses embryo in fluid)
  • Yolk sac (encloses nutrients)
  • Allantois (disposes waste, contributes to gas exchange)

  
• Organogenesis


  1. Cell communication results in induction, expression of select genes, and changes in cell shape and location

  2. Leads to cell determination and differentiation

  3. Cytoskeleton reorganization and cell migration mediated by adhesion molecules

  4. Apoptosis important for digit formation
• Neurulation


  Formation of the central nervous system
• Neurulation


  1. Notochord forms and induces ectoderm above to form neural plate

  2. Neural plate changes shape and detaches to form neural tube

  
• Body axes


  • Anterior-posterior, dorsal-ventral, proximal-distal
  • Determined by molecular cues coding for positional information and resulting in positionally dependent gene expression
• Caenorhabditis elegans is a model organism for animal development and the developmental lineage of all 959 cells present in the adult animal have been mapped (fate mapping)