Evolution of vertebrate forebrain development: how many different mechanisms?

Area of Science:

• Developmental biology
• Neuroscience
• Comparative embryology

Background:

• Two main models explain nervous system induction: Nieuwkoop's 2-signal model and the multiple-organizer model.
• The anterior visceral endoderm (AVE) in mice is proposed as a 'head organizer' for forebrain induction, separate from Hensen's node.
• Identifying avian equivalents to the AVE has been challenging, suggesting potential mammalian-specific head patterning mechanisms.

Purpose of the Study:

• To investigate the role of the chick embryo's hypoblast in nervous system development.
• To determine if the hypoblast is functionally and embryologically equivalent to the mouse AVE.
• To reconcile competing models of early neural patterning.

Main Methods:

• Comparative analysis of chick and mouse embryonic development.
• Examination of gene expression markers for neural and forebrain induction.
• Observation of cell movements in the epiblast.

Main Results:

• The chick hypoblast induces pre-neural and pre-forebrain markers transiently, but does not act as a primary organizer.
• Hypoblast-directed epiblast cell movements shield the developing forebrain from caudalizing signals from Hensen's node.
• These findings align with mouse embryo data, suggesting conserved mechanisms.

Conclusions:

• The hypoblast is functionally and embryologically similar to the mouse AVE, playing a role in head development.
• The mechanisms for forebrain and spinal cord patterning are likely conserved across higher vertebrates.
• The study reconciles the 2-signal and multiple-organizer models, favoring Nieuwkoop's original concepts.