Age-related changes in primary and secondary immune organs of the mouse.

The present work describes the murine immune tissue evolution with age with special emphasis on the bone marrow. To that effect we monitored the weights of the thymus, spleen and axillary lymph nodes over the first year of life in C57BL/6 male and female mice. In addition, we monitored the relative proportions of erythroid, lymphoid and myeloid cells in the bone marrow, and performed in vitro migration assays of bone marrow cells to thymic supernatants, with the aim of determining whether the migration of such cells or the thymic attractive capacity are affected by age. Before puberty, a remarkable decline in the relative weight of the thymus, spleen and lymph nodes was observed; after that stage, however, only the thymus showed an involution. The proportion of myeloid cells in the bone marrow showed an increase with age. Furthermore, the migration of myeloid cells to thymic supernatants increased with age and paralleled the time-course of the myeloid cell increase found in the bone marrow. More interestingly, the proportion of lymphoid cells to total bone marrow cells showed a clear decline with age. The time-course of this decline closely paralleled that of thymus weight, suggesting that the involution of the thymus may be related to changes in the cell composition of the bone marrow.