Developmental Biology

Vocabulary - Axes

• Animal-Vegetal

• Dorsal

• Ventral

• Anterior

• Posterior

• 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

• Outline: Graft a tissue from one embryo into another embryo, and see what happens – Cut and Paste

• 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