[3] FROG DEVELOPMENT

• SPERM

  • Structure: small, elongated, streamlined shape
  • Size: smaller than frog eggs
  • Requires a microscope to be seen
  • Mobility: highly motile
  • Flagellum – for propulsion
• SPERM

  • Function: fertilization of frog eggs
  • Composition: 
  • DNA – genetic material
  • Acrosome – enzymes to aid in penetration of egg’s protective jelly
• SPERM

  • Production:
  • Testes → released during breeding season → enters the female cloaca → amplexus
  • Amplexus – mating embrace
• EGG

  • Size: relatively large; visible to naked eye
  • Protection: encased in a jelly-like substance
• EGG

  • Composition: 
  • Cytoplasm – contains the nutrients and organelles
  • Function: non-motile serves as female gametes → waiting to be fertilized
• Animal Pole

  Location: Upper hemisphere
• Animal Pole

  • High concentration of cytoplasm
  • Metabolically active
  • Lesser Yolk
• Ectoderm (outer tissues e.g. skin, nervous system) is the associated embryonic layer of the animal pole
• Vegetal Pole

  • Located in the Lower hemisphere
• Vegetal Pole

  • Lower Concentration of cytoplasm
  • Metabolically less active
  • More yolk
• Amplexus – distinctive behavior in frog mating where the male frog grasps the female from behind in a tight embrace
• Amplexus

  • Helps position the male’s cloaca (reproductive opening close to the female’s cloaca for efficient sperm transfer
• • Nuptial Pads – projection in the thumbs of male frogs for gripping on to the female
• External Fertilization – males release sperm into the water
• CLEAVAGE

  • Series of mitotic divisions
  • zygote volume remains the same
• • Blastomeres - individual cells formed during cleavage
• Processes after fertilization (enumerate):

  • First cleavage
  • Grey Crescent Formation
• First cleavage – two equal-sized blastomeres
• Grey Crescent Formation – marks the future location of the notochord and nervous system
• Grey Crescent Formation -Located between the animal and vegetal pole
• CLEAVAGE

  • Undergoes rapid cleavage with successive cell divisions
• Holoblastic – the entire egg divides into small blastomeres
• • cleavage furrow: a groove in the cell wall that splits the cell in half
• Mesolecithal - yolk is moderately distributed throughout the egg
• Photomicrograph is of early cleavage stage

  
• Photomicrograph is of late cleavage stage

  
• a morula looks like the late cleavage stage but with more crowding

  
• MORULATION: The development of an ovum into a morula.
• Morula – Compacted mass of blastomeres
• Morula is Mulberry shaped
• Morula lacks a fluid filled cavity (i.e. blastocoel which is yet to appear at blastula stage)
• Gray Crescent – key development marker in morulation
• Gray Crescent

  • Remains positioned on the dorsal side of the morula
  • For establishing the future dorsal-ventral axis
  • Specifying tissue differentiation during later development
• MORULATION

  • Unequal Size Distribution – i.e. there is the presence of micromeres and megameres
• Micromeres found in the animal pole
• Megameres concentrated on the vegetal pole
• BLASTULATION

  • Blastula – hollow ball of cells
• Morulation follows after the cleavage stage
• Gametogenesis involves the creation of sperm and egg cells.
• Fertilization is the process where a sperm cell penetrates an egg cell.