CIRCANNUAL VARIATION OF CELL-PROLIFERATION IN LYMPHOID ORGANS AND BONE-MARROW OF BDF1 MALE-MICE ON 3 LIGHTING REGIMENS

BDF1 male mice, which had been raised for several generations on a lig hting regimen of LD 12:12, were studied. Experiments were conducted ov er 24h spans during winter, spring, summer, and fall. For 3-4 weeks pr ior to each study, one-third of the animals were kept on a lighting re gimen of 8h of light alternating with 16h of darkness (LD 8:16), one-t hird was kept on a regimen of LD 12: 12, and one:-third was kept on a regimen of LD 16:8. Subgroups of mice on all three lighting regimens w ere killed at 4h intervals over a 24h span. At 20 minutes prior to sac rifice, the animals received 5 mu Ci of H-3-thymidine/0.2 ml/20 gm bod y weight intraperitoneally. The thymidine uptake in DNA (DPM[H-3]/mu g DNA) was studied as an index of cell proliferation in the thymus, ing uinal lymph node, spleen, femur, and a segment of the lumbar vertebral column. A circannual variation of H-3-thymidine uptake in DNA was fou nd in all organs irrespective of the lighting regimen under which the animals were kept. Tile timing of the circannual variation, however, v aried among the organs. In the thymus, the lowest thymidine uptake occ urred during summer, with higher thymidine uptake during the other thr ee seasons. In the inguinal lymph node, the peak in thymidine uptake w as in the spring, with lower values during the other three seasons, th e lowest during summer. In the spleen, the highest thymidine uptake oc curred in the mice on all three lighting regimens during fall, with lo wer uptake during winter, spring, and summer. In the bone marrow of bo th the femur and the vertebral column, the thymidine uptake was high i n winter and fall and low in spring and summer. Serum corticosterone m easurements were available in winter, spring, and fall, and they showe d statistically significant lower values in winter and fall than in sp ring. The conclusion was drawn that circannual rhythms of H-3-thymidin e uptake in the DNA of the thymus, spleen, lymph nodes, and bone marro w are found in mice reared for generations under a LD 12:12 lighting r egimen and persist if the animals are kept under a regimen of LD 8:16 or LD 16:8 for 3-4 weeks prior to sacrifice.