This can be carefully manipulated with forceps. If the cell pellet cannot be manipulated with forceps, small drops of the concentrated cell suspension may be placed directly into a small amount of cryo-protectant. After a suitable incubation time a drop of the cell suspension, in sucrose, is placed on a specimen stub and frozen by immersion in liquid nitrogen. However, this method of preparation may result in a low concentration of cells in the sections. An alternative way of keeping cell suspensions together is to embed them in gelatin.
Infiltration is done on ice to prevent the gelatin from warming.
Lower concentrations of gelatin may be used and fixed prior to cryo-protection. The PVP appears to act as a support, entering extracellular spaces, and greatly facilitates the sectioning process. It may require that the biological material be left in the solution for longer than for sucrose alone. Mounting Specimens and Freezing Ideally the pieces of tissue or cell pellets should be trimmed to a pyramidal shape so that when mounted onto the specimen stub the base is wider than the top. If possible, the block face should be flat and resemble a square or trapezoid.