Inactive bone marrow is called yellow bone marrow. It is the chief form of bone marrow in the medullary cavity of bones in the adult that are no longer hemopoietically active, such as the long bones of the arms, legs, fingers, and toes. In these bones, the red bone marrow has been replaced completely by fat. Even in hemopoietically active bone marrow in adult humans—such as that in the ribs, vertebrae, pelvis, and shoulder girdle—about half of the bone marrow space is occupied by adipose tissue and half by hemopoietic tissue. The yellow bone marrow retains its hemopoietic potential, however, and when necessary, as after severe loss of blood, it can revert to red bone marrow, both by extension of the hemopoietic tissue into the yellow bone marrow and by repopulation of the yellow bone marrow by circulating stem cells.
 FOLDER 10.6 Clinical Correlation: Cellularity of the Bone Marrow
Bone marrow cellularity is one of the most important factors in evaluating the function of the bone marrow. The assessment of bone marrow cellularity is semiquantita-tive and represents the ratio of hemopoietic cells to adipocytes. The most reliable evaluation of cellularity is ob-tained from the microscopic examination of a bone marrow biopsy that preserves the organization of the marrow. Smear preparations are not accurate preparations with which to assess bone marrow cellularity. Bone cellularity changes with age. Normal bone marrow cellularity for a specifc age can be calculated by subtract-ing an individual’s age from 100 and adding 10%. Thus, a 30-year-old individual’s bone marrow contains between 60% and 80% of active bone-producing cells (100 – 30 70 10%); in contrast, a 70-year-old individual’s marrow is in the range of 20% to 40% (100 70 30 10%).
As can be seen from this calculation, the number of hemopoietic cells decreases with age. Bone marrow with a normal age–specifc index is called normocellular bone marrow. Deviation from age-specifc normal indices indicates a pathologic change in the marrow.
In hypocellular bone marrow, which occurs in aplastic anemia or after chemotherapy, only a small number of blood-forming cells can be found in a marrow biopsy (Fig. F10.6.1a). Thus, a 50-year-old individual with this condition might have a bone cellularity index of 10% to 20%. In the same-aged individual with acute myelogenous leukemia, the bone cellularity index might be 80% to 90%. Hypercellular bone marrow is characteristic of bone marrow affected by tumors originating from hemopoietic cells (Fig. F10.6.1b).
FIGURE F10.6.1 • Cellularity of the bone marrow. a. This is an example of hypocellular bone marrow from an individual with aplastic anemia. The bone marrow consists largely of adipose cells and lacks normal hemopoietic activity. 120. b. This photomicrograph of bone marrow section from an individual with acute myelogenous leukemia shows hypercellular bone marrow. Note that the entire field of view next to the bony trabecula is filled with tightly packed myeloblasts. Only a few adipose cells are visible on this image. 280. (Reprinted with permission from Rubin E, Gorstein F, Schwarting R, Strayer DS. Rubin’s Pathology, 4th ed. Baltimore: Lippincott Williams & Wilkins, 2004, Fig. 20-12, Fig. 20-54.)
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