An optimized approach using cryofixation for high-resolution 3D analysis by FIB-SEM.

Area of Science:

• Microscopy
• Cell Biology
• Materials Science

Background:

• Focused ion beam scanning electron microscopy (FIB-SEM) provides 3D nanoscale imaging of cellular substructures.
• Cryofixation via high-pressure freezing preserves near-native biological sample morphology.
• Cryofixed samples often exhibit poor image contrast and weak signals in FIB-SEM, limiting heavy metal staining.

Purpose of the Study:

• To compare FIB-SEM image quality of brain tissues prepared with common freeze-substitution media.
• To develop an optimized sample preparation protocol for high-pressure freezing and freeze-substitution to overcome FIB-SEM imaging limitations.
• To demonstrate the broad applicability of the optimized protocol for various biological samples.

Main Methods:

• Comparison of FIB-SEM image quality across different freeze-substitution media for brain tissues.
• Development of a novel staining protocol using osmium tetroxide, uranyl acetate, tannic acid, and potassium permanganate.
• Application of high-pressure freezing and freeze-substitution for sample preparation.

Main Results:

• The optimized protocol significantly improved FIB-SEM image contrast and signal quality for cryofixed samples.
• Perfectly smooth membrane morphology, including lipid bilayers, was achieved using the developed method.
• The protocol was successfully applied to diverse biological samples: brain, plant tissues, C. elegans, C. albicans, and chlorella.

Conclusions:

• The optimized sample preparation protocol enhances 3D ultrastructural analysis of biological samples using FIB-SEM.
• This approach effectively combines the benefits of cryofixation for large-volume 3D analysis with FIB-SEM's high resolution.
• The protocol's broad applicability makes it a valuable tool for detailed subcellular structure investigation across various organisms.