Developmental Biology


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Bog'liq
The Spemann experiment

Vocabulary - Axes

  • Animal-Vegetal
  • Dorsal
  • Ventral
  • Anterior
  • Dorsal-Ventral
  • Anterior Posterior
  • animal
  • vegetal

First Stages of Embryonic Development

  • Fertilization/Oogenesis
  • Cleavage
  • Gastrulation
  • neurulation

Fertilization/Oogenesis

  • Egg activation
    • Formation of the zygotic DNA
    • Initiation of the developmental processes
    • Symmetry breaking event

Cleavage

  • Embryo divides in an extraordinary fast rate – mitosis every 30-40 minutes
  • Virtually no growth in size
  • Controlled by pre-existing maternal proteins/mRNA: no time for transcription and translation new zygote genes
  • Creation of the blastocoel - cleavage cavity and blastula - sphere of cells surrounding it
  • movie

Animal pole (top) view

  • Blastocoel
  • Blastula

Gastrulation

  • Formation of three germ layers:
    • Ectoderm (outer layer) - skin
    • Mesoderm (middle layer) – muscles, bones
    • Endodern (inner layer) – digestive track
  • Formation of embryonic axes
  • Activation of zygote genes
  • Considerable movement of cells - without growth
  • movie
  • Vegetal (bottom) view
  • Vegetal (bottom) view
  • Blastopore lip
  • Blastopore
  • animal
  • vegetal
  • Blastopore lip
  • side view

Gastrulation

  • Movie

Gastrulation

  • Ventral
  • Posterior
  • Dorsal
  • Anterior
  • Anus
  • Tail
  • Trunk
  • Head
  • Skin (ectoderm)
  • Gut (endoderm)

Fate Map

Neurulation

  • Elongation of the embryo
  • Formation of the neural tube and notochord (in chordates), somites, and early organ predecessors: kidney, heart

Dorsal view

  • Neural Tube

Neurulation

  • movie

Spemann Experiment

  • Main observation: A graft of a specific tissue (the organizer) to a specific location can induce Siamese twins connected at the belly.
  • Conclusion: The embryo’s cells are not committed to a certain fate.

Spemann Experiment

  • movie
  • Experimental details

Summary of Results

  • “A piece taken from the upper blastopore lip of a gastrulating amphibian embryo exerts an organizing effect on its environment ... Such a piece can therefore be designated as an organizer
  • Orginial blastopore lip
  • Graft
  • Vegetal view

Summary of Results

  • “These secondary embryonic primordia are always of mixed origin.”
  • “…an organizer of another species is used for induction, then the chimeric composition can be established with certainty and great accuracy

Controls? Statistics?

  • Control:
    • The authors do not present a control experiment: grafting to other locations, at other times, etc.
    • They do state however, that development is impeded after the grafting procedure
  • Statistics
    • From H. Mangold’s lab notebooks we can learn that only 15% of the embryos survived the graft
    • Spontaneous Siamese twin may occur “naturally” at a lower rate

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