Circannual variation of cell proliferation in lymphoid organs and bone marrow of BDF1 male mice on three lighting regimens

BDF1 male mice, which had been raised for several generations on a lighting regimen of LD 12:12, were studied. Experiments were conducted over 24h spans during winter, spring, summer, and fall. For 3-4 weeks prior to each study, one-third of the animals were kept on a lighting regimen of 8h of light alternating with 16h of darkness (LD 8:16), one-third 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 were killed at 4h intervals over a 24h span. At 20 minutes prior to sacrifice, the animals received 5 mu Ci of H-3-thymidine/0.2 ml/20 gm body weight intraperitoneally. The thymidine uptake in DNA (DPM[H-3]/mu g DNA) was studied as an index of cell proliferation in the thymus, inguinal lymph node, spleen, femur, and a segment of the lumbar vertebral column. A circannual variation of H-3-thymidine uptake in DNA was found in all organs irrespective of the lighting regimen under which the animals were kept. The timing of the circannual variation, however, varied among the organs. In the thymus, the lowest thymidine uptake occurred during summer, with higher thymidine uptake during the other three seasons. In the inguinal lymph node, the peak in thymidine uptake was in the spring, with lower values during the other three seasons, the lowest during summer. In the spleen, the highest thymidine uptake occurred in the mice on all three lighting regimens during fall, with lower uptake during winter, spring, and summer. In the bone marrow of both the femur and the vertebral column, the thymidine uptake was high in winter and fall and low in spring and summer. Serum corticosterone measurements were available in winter, spring, and fall, and they showed statistically significant lower values in winter and fall than in spring. The conclusion was drawn that circannual rhythms of H-3-thymidine uptake in the DNA of the thymus, spleen, lymph nodes, and bone marrow are found in mice reared for generations under a LD 12:12 lighting regimen 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.