Method to select mice in the steady state for biological studies.

Collection of small amounts of blood from the orbital sinus was found to be a satisfactory method for repeated sampling in mice, which means that these animals can be selected for further study on the basis of the leukocyte count. In biomedical research it is often necessary to have detailed information about the effect of injected material on the numerical course of circulating leukocytes. However, the present study has shown that 2 stress-producing procedures on 1 day disturb the steady state, and that this disturbance is expressed in changes in the number of leukocytes. Such stress could be avoided by alteration of the experimental design to include only 1 stressful situation each day. When blood was sampled in the orbital sinus on 1 day to determine the animals' condition and on the next day only the tail was punctured as sham injection, the number of blood leukocytes remained constant throughout the observation period. Comparative studies on the numbers of monocytes, lymphocytes, and granulocytes in blood from the tail, heart, and orbital sinus showed a systematic difference in the mean numbers of certain types of leukocytes. Statistically significant differences were found between the mean numbers of monocytes, lymphocytes, and granulocytes in orbital blood from normal mice of 5 specific pathogen-free strains, i.e., Cpb:SE (Swiss), CBA/Cpb, BALB/cCpb, C3H/Rij, and DBA/2Rij.