Regeneration of blood-forming organs after autologous leukocyte transfusion in lethally irradiated dogs. I. Distribution and cellularity of the bone marrow in normal dogs.

Marrow cellularity in adult beagles (1-2 yr old) is highest in centrally located bones, with values between 8000 and 12,000 nucleated cells per sq mm. It decreases gradually towards the peripheral parts of the body, reaching values below 1000 per sq mm in bones distal to the elbow and knee. The first tail segment always contains some active marrow. The fifth segment has only stromal elements. In spongy bones fat cells appear to be distributed at random among the blood-forming elements. In the middle part of the femur the fatty marrow predominates in the center of the cylinder, while the subendosteal area is very cellular. The proximal and distal ends of the femur are more cellular than the middle. The small standard deviation of the cellularity in the spongy bones of the trunk and in the proximal and middle part of the humerus makes these sites areas of choice for quantitative studies of marrow regeneration. The large variations in cellularity of the marrow in the radius and tibia of young adult dogs make these sites unsuitable for such studies. The distribution of active and fatty marrow in dogs is similar to that of humans. The differential count of active marrow is quite constant between different dogs and in the various sites of each animal.