Distribution of marrow within the skeleton has been determined in man, rabbits, and rats by in vivo labeling of the marrow compartment with radioiron and either assaying each bone separately for radioactivity or obtaining a gamma-ray image of the distribution of the marrow by whole body scanner or positron scintillation camera. In man, extension of marrow into unusual sites was demonstrated after prolonged and severe stimulation, excessive blood loss, or hemolysis for a long period. A rabbit made severely anemic by administration of phenylhydrazine for 7 days showed extension of marrow into the distal portion of the humeri and femora. In rats in which erythropoiesis was stimulated by erythropoietin administration there was no significant increase in total marrow volume and no change in distribution of marrow within the skeleton.

The positron scintillation camera provides an excellent method for qualitative evaluation of the marrow distribution. It has sufficient resolving power to give a good picture of the distribution of marrow with Fe52 in a skeleton as small as that of the rat. The distribution apparent from the positron pictures has been confirmed by complete skeletal analysis of individual bones.

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