Localization of lipids in freeze-dried mouse brain sections by imaging TOF-SIMS

Imaging time-of-flight-secondary ion mass spectrometry (TOF-SIMS) was used to analyse the lateral distributions of lipids on the surface of freeze-dried mouse brain sections. Tissue sections (14 mm thick) were prepared by cryosectioning, placed on glass or Si substrates and desalinated by submersion in NH3HCOO solution. Immediately prior to analysis, the samples were freeze-dried by thawing the sample in vacuum. TOF-SIMS analysis was carried out using 25 keVAu3 + or Bi3 + primary ions, always keeping the accumulated ion dose below 4 1012 ions/cm2. Positive and negative ion images over the entire mouse brain section and of analysis areas down to 100 mm 100 mm show characteristic distributions of various lipids. The signals from cholesterol and sulfatides are primarily located to white matter regions, while the phosphocholine and phosphatidylinositol signals are strongest in grey matter regions. By using two different staining methods, structures observed in the TOFSIMS images could be identified as ribosome-rich regions and cell nuclei, respectively. Analysis of freeze-dried mouse brain sections at varying sample temperatures between 130 and 60 8C showed an abrupt increase in the cholesterol signal at T > 0 8C, indicating extensive migration of cholesterol to the tissue surface under vacuum conditions.