Accurate Placement of Ultrathin Sections on Grids; Control by Sol-Gel Phases of a Gelatin Flotation Fluid

A 3% solution of gelatin in a petri dish at 25-30 C. provides a liquid, viscous surface upon which ultrathin sections can be floated. On cooling, the gelled substratum immobilizes the sections, allowing grids to be placed on them with any desired grid-to-section orientation. When the gelatin is remelted, the sections remain attached to the grids. After draining, traces of gelatin adhering to the grids are removed by flotation (section side down) for 30 min on 2% acetic acid at 60 C. This is followed by flotation for 3-5 min on Tris buffer, pH 7.1, and then on distilled water for 30 min—both treatments at 60 C. The technique is particularly useful for mounting serial sections.