In Amphioxus, Serial Block-Face Scanning Electron Microscopy Reveals the Absence of Hatschek's Right and Left...

Area of Science:

• Developmental biology
• Comparative embryology
• Chordate evolution

Background:

• Hatschek's 19th-century description of amphioxus (lancelet) development proposed pharyngeal diverticula forming key embryonic structures.
• These proposed diverticula were considered precursors to the rostral coelom and preoral organ.

Purpose of the Study:

• To reexamine the formation of proposed pharyngeal diverticula in amphioxus embryogenesis.
• To clarify the cellular origins of the rostral coelom lining and preoral organ.

Main Methods:

• Serial block-face scanning electron microscopy (SBFSEM) was employed.
• Three-dimensional ultrastructural modeling of early amphioxus embryonic tissues.

Main Results:

• No evidence of Hatschek's proposed right and left pharyngeal diverticula was found.
• The anterior gut tip forms an irregular cell mass, the source of the rostral coelom peritoneum.
• A distinct cell cluster associated with the first left segment is identified as the likely source of the preoral organ.

Conclusions:

• Hatschek's model of pharyngeal diverticula in amphioxus development is inaccurate.
• The findings necessitate a reevaluation of proposed homologies for deuterostome head cavities based on developmental origins.