Lymphocyte populations in mouse bone marrow: quantitative kinetic studies in young, pubertal and adult C3H mice.

Continuous 3-H-thymidine infusion was used to characterize two kinetic subpopulations of small lymphocytes in mouse bone marrow during normal growth and development. Young (4 wk), pubertal (8 wk) and mature (16 wk) C3H mice were infused subcutaneously with 3-H-thymidine for periods up to 10 days. Femoral marrow was then examined in radioautographic smears. During the first 3 days the proportion of marrow small lymphocytes labelled by 3-H-thymidine showed a rapid exponential increase to 93%, 81%, and 72% in 4 wk, 8 wk and 16 wk mice respectively. The rate of appearance of labelled small lymphocytes then declined markedly but remained higher in younger than in older animals. The labelling curves were found to represent the summation of two exponential curves from which the proportions and renewal rate of corresponding cell populations were calculated. Most marrow small lymphocytes comprised a rapidly renewing population but in mice of increasing age the relative incidence of these cells fell (93-3% at 4 wk; 88-0% at 8 wk; 78-5% at 16 wk) and their half-renewal time (T1/2) lengthened (14 hr at 4 wk; 18 hr at 8 wk; 24 hr at 16 wk). The remaining small lymphocytes were slowly renewing with mean T1/2 OF 4, 7 and 14 days in 4, 8 AND 16 wk mice, RESPECTIVELY. Some heavily labelled small lymphocytes persisted in the marrow up to 10 wk after fourteen daily 3-H-thymidine injections in 10-12 wk mice. The numbers of rapidly renewing cells decreased from 604 times 10-3 to 228 times 10-3 per mm-3 of marrow from 4 wk to 16 wk, respectively, while slowly renewing cells increased from 44 times 10-3 to 61 times 10-3 per mm-3. The total number of nucleated marrow cells per femur increased from 4 wk to 16 wk but the rapidly renewing small lymphocytes per femur fell in numbers by 36% and in renewal rate by 63%. The results demonstrate a selective change in bone marrow small lymphocytes with age; rapidly renewing cells decline in number and renewal rate while the number of slowly renewing cells increases. The concept of bone marrow as a primary lymphoid organ is discussed.