The presence of in vitro colony-forming units (CFUc) was demonstrated in leukocyte suspensions collected by continuous flow leukaphereses performed serially in dogs. The CFUc yield of one leukapheresis was usually between 10⁴ and 10⁶. Higher yields were obtained in the 2nd to 5th leukaphereses than in the first one in the same dog. The number of CFUc collected during one leukapheresis was about four times the number of CFUc present in the circulating blood at the beginning of the leukapheresis. After a diminution during leukapheresis, the circulating CFUc pool was restored in 2 or 3 days with an increase in the ratio of CFUc to mononuclear cells. These results indicate that CFUc entered the circulating pool during and after leukapheresis and that the population size regulation of canine-circulating CFUc is different from that of the bulk of the mononuclear leukocytes.

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.

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.