Myeloid blasts are immature white blood cells that originate in the bone marrow. As precursors to several types of specialized blood cells, they are not normally found circulating in the bloodstream. The presence of myeloid blasts in the blood, or a high concentration of them in the bone marrow, can signal a significant health issue. This is why their numbers are closely monitored by medical professionals.
Normal Production and Function of Myeloid Cells
All blood cells originate from hematopoietic stem cells in the bone marrow. These stem cells differentiate along two main paths: the lymphoid line and the myeloid line. The myeloid line produces:
- Red blood cells (erythrocytes)
- Platelets (thrombocytes)
- Granulocytes (a type of white blood cell)
- Monocytes (a type of white blood cell)
Myeloid stem cells develop into myeloblasts as an intermediate, immature stage.
Myeloblasts normally mature into functional white blood cells like neutrophils, eosinophils, and basophils, which are then released into the bloodstream. Neutrophils serve as a primary defense against bacterial infections, while monocytes mature into macrophages that consume debris and pathogens. This regulated production ensures the body has a steady supply of cells to fight infection, carry oxygen, and clot blood.
Significance of Increased Myeloid Blasts
An elevated count of myeloid blasts indicates a disruption in normal blood cell production. These immature cells are non-functional, and their accumulation shows they are not maturing properly into healthy white blood cells. This failure of maturation is a defining characteristic of several bone marrow disorders.
As these abnormal blasts multiply, they take up space within the bone marrow, crowding out healthy blood-forming cells. This leads to shortages of normal blood cells. A lack of red blood cells results in anemia, while a deficiency of functional white blood cells impairs the body’s ability to fight infections. A low platelet count leads to easy bruising, and a blast count over 5% in the bone marrow is considered abnormal.
Diseases Characterized by Abnormal Myeloid Blasts
Elevated myeloid blast counts are a defining feature of several medical conditions, primarily cancers of the blood and bone marrow. Acute Myeloid Leukemia (AML) is the most common acute leukemia in adults and involves the rapid proliferation of abnormal myeloblasts. In AML, the bone marrow contains 20% or more blasts, which also spill out into the bloodstream.
Myelodysplastic Syndromes (MDS) are cancers where the bone marrow fails to produce enough healthy blood cells. In some types of MDS, specifically “MDS with excess blasts,” the bone marrow contains between 5% and 19% myeloblasts. While lower than in AML, these conditions are serious and can progress to AML. A “blast crisis” in Chronic Myeloid Leukemia (CML) is when blast counts rise dramatically, often exceeding 20%, signaling a transformation to a more aggressive phase.
Detection and Diagnostic Evaluation
Identifying an abnormal number of myeloid blasts begins with routine blood tests. A complete blood count (CBC) can reveal unusual cell levels, prompting a peripheral blood smear. This test, where blood is examined under a microscope, allows technicians to see blasts that are not present in healthy blood.
A definitive diagnosis requires a bone marrow aspiration and biopsy, which involves taking a sample of bone marrow fluid and tissue. Pathologists examine the sample to count the percentage of blasts, helping to distinguish between conditions like MDS and AML. Flow cytometry is also used to analyze proteins on the surface of the blast cells, which confirms their myeloid origin and helps classify the disease subtype to guide treatment.