The myeloid stem cell lineage gives rise to red blood cells, platelets, and most white blood cells. Specifically, myeloid stem cells produce erythrocytes (red blood cells), megakaryocytes (which fragment into platelets), neutrophils, eosinophils, basophils, monocytes (which mature into macrophages), and mast cells. The only formed elements that do not come from the myeloid line are lymphocytes: T cells, B cells, and natural killer cells, which arise from lymphoid stem cells instead.
The Myeloid Family Tree
All blood cells trace back to a single type of multipotent hematopoietic stem cell in the bone marrow. That stem cell divides into two major branches: the myeloid line and the lymphoid line. The myeloid branch is the larger of the two, responsible for producing the vast majority of formed elements circulating in your blood at any given moment.
From the myeloid stem cell, an intermediate cell called the common myeloid progenitor (CMP) emerges. The CMP then splits into two further progenitors, each committed to a different set of mature cells:
- Megakaryocyte-erythrocyte progenitor (MEP): produces red blood cells and megakaryocytes (the giant cells that shed platelets into the bloodstream)
- Granulocyte-monocyte progenitor (GMP): produces neutrophils, eosinophils, basophils, monocytes, and mast cells
This hierarchy has been confirmed through experiments showing that the CMP gives rise to both the GMP and MEP, and that these two progenitors are mutually exclusive. MEPs cannot produce granulocytes, and GMPs cannot produce red blood cells or platelets.
Red Blood Cells and Platelets
Red blood cells are by far the most abundant formed element in the body, with normal counts ranging from 4.0 to 6.0 million per microliter of blood depending on sex. They develop from the MEP branch under the influence of erythropoietin, a hormone released primarily by the kidneys when oxygen levels drop. Early erythroid progenitors appear within about 8 to 9 days of initial differentiation in the bone marrow, though full maturation takes longer as the developing cell ejects its nucleus and fills with hemoglobin.
Platelets follow a unique path. Rather than maturing as individual cells, the MEP produces megakaryocytes, massive cells that extend long projections into bone marrow blood vessels. These projections fragment into thousands of tiny platelets that enter circulation. Thrombopoietin is the key signal driving megakaryocyte growth. Normal platelet counts range from 150,000 to 400,000 per microliter.
Granulocytes: Neutrophils, Eosinophils, and Basophils
The three granulocytes all descend from the GMP branch and are named for the granules packed inside them, which contain enzymes and signaling molecules used during immune responses.
Neutrophils are the most common white blood cell, making up 50 to 70 percent of all circulating white cells. They are the first responders to bacterial infections and tissue damage. Neutrophil maturation from the progenitor stage takes roughly 8 to 10 days in the bone marrow before the cells are released into the bloodstream.
Eosinophils typically account for only 1 to 3 percent of white blood cells. Their development depends heavily on the signaling molecule IL-5, which drives their differentiation, activation, survival, and recruitment to inflamed tissue. They play a central role in fighting parasitic infections and contribute to allergic reactions.
Basophils are the rarest granulocyte, making up less than 2 percent of circulating white cells, with counts often below 100 per microliter. Along with mast cells (their tissue-resident cousins), basophils are involved in allergic and inflammatory responses. Mast cell development depends on stem cell factor and IL-3.
Monocytes and Macrophages
Monocytes develop from the GMP under the direction of a growth factor called M-CSF. They circulate in the blood for a day or two, comprising about 3.5 to 9 percent of white blood cells, before migrating into tissues and maturing into macrophages. Macrophages are versatile cells that engulf bacteria and dead cells, present pieces of invaders to other immune cells, and help regulate inflammation. Depending on where they settle, macrophages take on specialized roles: Kupffer cells in the liver, microglia in the brain, and alveolar macrophages in the lungs are all tissue-specific forms.
Dendritic Cells: A Dual-Origin Exception
Dendritic cells are the body’s most important antigen-presenting cells, responsible for activating the adaptive immune system. They complicate the neat myeloid-versus-lymphoid divide because they can arise from both lineages. Both the common myeloid progenitor and the common lymphoid progenitor can generate functional dendritic cells with similar efficiency on a per-cell basis. However, because myeloid progenitors are roughly 10 times more numerous than lymphoid progenitors in the bone marrow, the majority of dendritic cells in the body are myeloid in origin.
Where Myeloid Differentiation Happens
In adults, nearly all myeloid differentiation takes place in the red bone marrow of flat and irregular bones: the pelvis, sternum, ribs, vertebrae, and the ends of long bones like the femur. During fetal development, the picture is different. Blood cell production begins in the yolk sac, shifts to the fetal liver (where the erythroid lineage dominates early on), and then transitions to the bone marrow around mid-gestation. By 3 to 4 weeks after birth, the bone marrow niche has fully taken over, and hematopoietic stem cells settle into their characteristic quiescent state, dividing only as needed to replenish the blood supply for life.
Why the Myeloid-Lymphoid Distinction Matters
Understanding which cells come from the myeloid lineage is not just an academic exercise. Cancers of the blood are classified based on the lineage they affect. Acute myeloid leukemia involves uncontrolled growth of myeloid progenitors, producing abnormal white blood cells, red blood cells, or platelets. Acute lymphoid leukemia targets the lymphoid line instead, producing abnormal lymphocytes. The two diseases require different treatment approaches, and distinguishing between them relies on identifying the lineage of the cancerous cells through specialized staining and surface marker testing. A clear grasp of which formed elements belong to each branch is the foundation for understanding these diagnoses.