Bone marrow, a spongy tissue inside bones, produces all types of blood cells, including red blood cells, white blood cells, and platelets. This process is known as hematopoiesis. Cellularity refers to the proportion of blood-forming cells relative to fat cells within the bone marrow. While hypercellular bone marrow can cause concern, it does not automatically indicate a serious health problem.
Understanding Bone Marrow Cellularity
Bone marrow cellularity measures the percentage of hematopoietic cells compared to fat cells. This ratio is dynamic and changes throughout a person’s life. Newborns have nearly 100% cellularity. As individuals age, fat naturally increases, leading to a gradual decrease in cellularity. For healthy adults, normal cellularity typically ranges between 30% and 70%. Hypercellularity is defined when the cellular component exceeds these normal ranges, generally considered above 70% hematopoietic cells. This indicates an elevated number of blood-producing cells.
When Hypercellularity Is Not Concerning
Hypercellular bone marrow does not always signify a serious underlying illness; it can often be a temporary or reactive response. For instance, the bone marrow may become hypercellular to regenerate blood cells following treatments like chemotherapy or radiation, or in response to acute blood loss or conditions like hemolytic anemia. Infections or inflammatory conditions can also stimulate the bone marrow to produce more white blood cells, leading to hypercellularity. Certain medications or growth factors designed to boost blood cell production, like erythropoietin, can also result in increased marrow activity. In some instances, hypercellularity might even be an isolated finding with no clear clinical significance.
Conditions Associated with Hypercellularity
While often benign, hypercellular bone marrow can also be a significant finding in more serious medical conditions. Myelodysplastic Syndromes (MDS) are a group of disorders where the bone marrow appears hypercellular, yet it produces dysfunctional or immature blood cells. This ineffective blood cell production can lead to low blood counts despite the increased cellularity. MDS can involve one or more blood cell lineages and may progress to acute myeloid leukemia.
Myeloproliferative Neoplasms (MPNs) are another category of conditions characterized by hypercellular bone marrow due to the overproduction of specific blood cell types. Examples include Polycythemia Vera (excess red blood cells), Essential Thrombocythemia (excess platelets), and Primary Myelofibrosis (abnormal stem cells leading to scar tissue). Chronic Myeloid Leukemia, also an MPN, is characterized by hypercellular bone marrow with an increase in myeloid cells.
Leukemias, particularly acute leukemias, present with hypercellular bone marrow due to the uncontrolled proliferation of immature, abnormal white blood cells, known as blasts. These abnormal cells rapidly fill the marrow space, suppressing the production of healthy blood cells. Cancer originating from other parts of the body can also metastasize and spread to the bone marrow, causing hypercellularity.
Diagnosis and Follow-Up
Diagnosing hypercellular bone marrow and determining its cause involves a comprehensive evaluation. The primary method for assessing bone marrow cellularity and identifying abnormalities is a bone marrow biopsy and aspiration. This procedure collects samples of both liquid marrow and solid bone marrow tissue, usually from the hip bone, for microscopic examination. The biopsy allows for a detailed assessment of the marrow’s architecture and the types and proportions of cells present.
A complete medical history and physical examination provide context for the bone marrow findings. Peripheral blood counts are also crucial, providing information about the number and types of circulating blood cells, which correlate with marrow activity. To further characterize abnormalities, specialized laboratory tests such as cytogenetic studies (analyzing chromosomes for genetic changes) and molecular studies (looking for specific gene mutations) are performed on the bone marrow sample. These tests help identify specific conditions like myelodysplastic syndromes or myeloproliferative neoplasms. A hematologist or oncologist, a specialist in blood disorders and cancers, interprets all these findings to establish a diagnosis and recommend appropriate follow-up or treatment.