Sequential changes in cellular properties accompanying amniote somite formation.

Area of Science:

• Developmental Biology
• Cell Biology
• Embryology

Background:

• Somites are segmented blocks of cells crucial for embryonic development.
• Somitogenesis involves mesenchyme-to-epithelial transition (MET) and cell polarization.
• Existing models do not fully explain the dynamics of MET during somitogenesis.

Purpose of the Study:

• To investigate the spatiotemporal progression of MET and cell polarization in the pre-somitic mesoderm (PSM).
• To clarify the role of core versus surface cells during somite formation.
• To challenge and refine current models of somitogenesis.

Main Methods:

• Scanning electron microscopy (SEM) for morphological analysis.
• Immunocytochemistry and confocal microscopy for visualizing cell polarity markers.
• Three-dimensional imaging to map marker distribution.

Main Results:

• PSM epithelialization initiates earlier than previously thought, starting dorsally, then medially, and finally ventrally/laterally.
• Internal core cells of the PSM and somites do not undergo MET.
• Cell polarization patterns are marker-specific and surface-dependent, not directly correlating with cell elongation progression.

Conclusions:

• The timing and pattern of MET in PSM differ from current assumptions.
• Core cell behavior suggests a 'trapping' mechanism within the somitocoel.
• These findings necessitate revisions to existing models of somite formation.