Bone marrow is particularly designed to support the proliferation, differentiation and maintenance of hematopoietic cells. Cells that constitute the bone marrow stroma are fibroblasts, microphages, osteoblasts, osteoclasts, adipocytes endothelial cells which form the sinusoids. Bone marrow cellularity varies with stress, site, and age and is subject to diurnal variation. Bone marrow evaluation is indicated when abnormalities observed in the peripheral blood suggest a pathologic condition of the marrow. Most toxicological studies require bone marrow smears to be taken routinely. The microscopic examination of a bone marrow smear also yields more information than does a histologic section through bone.