According to the Atlas of Bone Marrow Pathology, bone marrow cellularity refers to the volume ratio of haematopoietic cells (cells that make blood cells) and fat. In newborns, bone marrow cellularity is normally 100 percent, but it decreases with age. Normal bone marrow cellularity in an adult is between 30 and 70 percent. Thus, hypercellular marrow is that which contains more than 70 percent haematopoietic cells.
According to Medline Plus, bone marrow is a spongy tissue located inside some bones, such as those in the hip and thigh. Bone marrow contains stem cells, immature cells that have the ability to develop into different types of blood cells. These include red blood cells (RBCs), which carry oxygen to body tissues, white blood cells (WBCs), which fight infection, and platelets, which help the blood clot. Marrow diseases usually involve stem cells and result in marrow that has too many or too few haematopoietic cells.
Hypercellular bone marrow often is caused by a myeloproliferative disorder, according to the Merck Manual for Health Care Professionals. These disorders are characterized by an abnormal proliferation of stem cells, which show up as increased numbers of RBCs, WBCs or platelets in the blood. Myelodysplastic syndrome (MDS), sometimes called preleukemia, is another disease that results in hypercellular bone marrow. However, in MDS, the numbers of RBCs, WBCs and platelets in the bloodstream usually are decreased because the stem cells in the bone marrow are abnormal and cannot produce normal blood cells.
Yet another cause of hypercellular bone marrow is a malignancy, or cancer, according to Johns Hopkins Medicine.This includes different types of leukemia, lymphoma, AIDS-related lymphoma and multiple myeloma, as well as the myelodysplastic syndromes mentioned above. Various treatments are available for these disorders including bone marrow transplants.