Variations in the histologic distribution of rat bone marrow cells with respect to age and anatomic site.

Bone marrow cellularity in untreated Fischer 344/N rats was subjectively evaluated in hematoxylin and eosin (H & E)-stained histologic sections from femur, tibia, humerus, sternum, lumbar vertebrae, ribs, pelvis, and skull of 2-, 4-, 7-, 16-, and 24-month-old males. Marrow cellularity varied with age of the rat and bone site sampled. Hematopoietic cellularity was consistently higher in rats less than 4 months of age and less consistently higher in 24-month-old rats versus intermediate age groups examined. The 24-month-old rats had the greatest animal-to-animal variation in cellularity. Site differences in bone marrow cellularity were present and similar at 4, 7, and 16 months. Mean percentage of marrow space occupied by hematopoietic cells ranged from 33-75%. Categories for histologic grading of bone marrow cellularity are presented. Sternum, femur, and humerus are recommended sites for histologic evaluation of bone marrow cellularity from conventional H & E-stained sections. Definitive evaluation and assessment of hematopoietic perturbations should not be solely based on subjective evaluation of routine histologic sections.