Safety considerations regarding the use of propane and other liquefied gases as coolants for rapid freezing purposes

Liquid propane and similar coolants are used in the rapid freezing of biological specimens. These coolants form explosive gas mixtures with air, with a 14,000-fold increase in volume over that of the liquid. The liquefied gases have high vapour pressures and, unless they are maintained below their flashpoint, the vapour above them will reach ignitable concentrations. The flashpoint of liquid propane is -104 degrees C. Ethane has a higher vapour pressure, and vapour mixed with air above liquid ethane can be ignited at a coolant temperature of -130 degrees C. The danger is minimized if the coolant is maintained near its freezing point and under a nitrogen atmosphere, in a fume cupboard. Liquid nitrogen evaporates to a 690-fold increase in volume at room temperature. It is important to ventilate the working area, especially when cryo-sectioning in a small room, otherwise there is a possibility of asphyxiation.

Temporal coincidence between synaptic vesicle fusion and quantal secretion of acetylcholine

We applied the quick-freezing technique to investigate the precise temporal coincidence between the onset of quantal secretion and the appearance of fusions of synaptic vesicles with the prejunctional membrane. Frog cutaneous pectoris nerve-muscle preparations were soaked in modified Ringer's solution with 1 mM 4-aminopyridine, 10 mM Ca2+, and 10(-4) M d-Tubocurarine and quick-frozen 1-10 ms after a single supramaximal shock. The frozen muscles were then either freeze-fractured or cryosubstituted in acetone with 13% OsO4 and processed for thin section electron microscopy. Temporal resolution of less than 1 ms can be achieved using a quick-freeze device that increases the rate of freezing of the muscle after it strikes the chilled copper block (15 degrees K) and that minimizes the precooling of the muscle during its descent toward the block. We minimized variations in transmission time by examining thin sections taken only from the medial edge of the muscle, which was at a fixed distance from the point of stimulation of the nerve. The ultrastructure of the cryosubstituted preparations was well preserved to a depth of 5 - 10 micron, and within this narrow band vesicles were found fused with the axolemma after a minimum delay of 2.5 ms after stimulation of the nerve. Since the total transmission time to this edge of the muscle was approximately 3 ms, these results indicate that the vesicles fuse with the axolemma precisely at the same time the quanta are released. Freeze-fracture does not seem to be an adequate experimental technique for this work because in the well-preserved band of the muscle the fracture plane crosses, but does not cleave, the inner hydrophobic domain of the plasmalemma. Fracture faces may form in deeper regions of the muscle where tissue preservation is unsatisfactory and freezing is delayed.

Cryopreservation of human lymphocytes: a brief review and evaluation of an automated liquid nitrogen freezer

Successful cryopreservation of human lymphocytes has been previously described. Cryopreserved lymphocytes are useful for a variety of in vitro immunologic studies. This study was performed to determine the applicability and/or advantages of using a programmable freezing system, and compares glycerol versus dimethyl sulfoxide (DMSO) at varying concentrations on post-thaw viability, E-rosetting and immunoglobulin fluorescence. Prefreeze T and B lymphocyte percentages were determined. Cells were then frozen in varying concentrations of glycerol and DMSO. Optimum cryoprotectant type and concentration was determined. Lymphocytes from seven individuals were frozen by the batch method in a mechanical freezer and with the automated liquid nitrogen injection system. Data on post-thaw T and B percentages and viability revealed 10% DMSO and liquid nitrogen control freezing method at 1 C/minute as the best conditions for lymphocyte preservation as reflected by post-thaw in vitro testing.