The effects of knee immobilization on marrow adipocyte hyperplasia and hypertrophy at the proximal rat tibia epiphysis

Marrow adipose deposition is observed during aging and in association with extended periods of immobility. The objective of this study was to determine the contribution of adipocyte hypertrophy and hyperplasia to bone marrow fat deposition induced by immobilization of the rat knee joint for 2, 4, 16 or 32 weeks. Histomorphometric analyses compared immobilized to sham-operated proximal tibia from age and gender matched rats to assess the contribution of aging and duration of immobilization on the number and size of marrow adipocytes. Results indicated that marrow adipose tissue increased with the duration of immobilization and was significant larger at 16 weeks compared to the sham-operated group (0.09956 ± 0.13276 mm2 vs 0.01990 ± 0.01100 mm2, p = 0.047). The marrow adipose tissue was characterized by hyperplasia of adipocytes with a smaller average size after 2 and 4 weeks of immobilization (at 2 weeks hyperplasia: 68.86 ± 33.62 vs 43.57 ± 24.47 adipocytes/mm2, p = 0.048; at 4 weeks hypotrophy: 0.00036 ± 0.00019 vs 0.00046 ± 0.00023 mm2, p = 0.027), and by adipocyte hypertrophy after 16 weeks of immobilization (0.00083 ± 0.00049 vs 0.00046 ± 0.00028 mm2, p = 0.027) compared to sham-operated. Both immobilized and sham-operated groups showed marrow adipose conversion with age; immobilized (p = 0.008; sham: p = 0.003). Overall, fat deposition in the bone marrow of the proximal rat tibia epiphysis and induced by knee joint immobilization was characterized by hyperplasia of small adipocytes in the early phase and by adipocyte hypertrophy in the later phase. Mediators of marrow fat deposition after immobilization and preventive countermeasures need to be investigated.