Scanning electron microscopy of high-pressure-frozen sea urchin embryos

Area of Science:

• Developmental Biology
• Microscopy Techniques
• Electron Microscopy

Background:

• Cryo-fixation is crucial for preserving biological ultrastructure.
• Traditional freezing methods often lead to ice crystal artifacts.
• Scanning electron microscopy (SEM) requires specific sample preparation for high-resolution imaging.

Purpose of the Study:

• To evaluate preparation protocols for high-pressure-frozen sea urchin embryos in SEM.
• To compare different dehydration and imaging methods for cryo-fixed samples.
• To identify optimal methods for preserving three-dimensional morphology.

Main Methods:

• High-pressure freezing (HPF) for rapid cryo-fixation of sea urchin embryos.
• Freeze-substitution with fixatives and critical-point drying.
• Direct imaging of frozen-hydrated samples on a scanning electron microscope cold stage.
• Partial freeze-drying of frozen-hydrated samples.

Main Results:

• High-pressure freezing preserved embryo shape and minimized ice crystal formation compared to other methods.
• Freeze-substitution introduced shrinking and extraction artifacts.
• Imaging in the frozen-hydrated state avoided dehydration artifacts but resulted in beam sensitivity and obscured structures.
• Partial freeze-drying revealed more structures but introduced drying artifacts.

Conclusions:

• High-pressure freezing is superior for preserving the three-dimensional morphology of sea urchin embryos.
• Imaging in the frozen-hydrated state is the preferred method for morphometric studies despite beam sensitivity.
• Further optimization is needed to balance artifact reduction and visibility in cryo-fixed samples.