Understanding leukemia

Stem cells and blood formation

Every blood cell begins with a single “parent” cell. This topic explains how blood is made, what makes a stem cell special, and where along the way leukemia begins.

Last updated: 30 September 2026~7 min read

Translucent spongy bone tissue on a dark navy background; in one cavity a large teal glowing stem cell, with thin vessels and soft red cells around it. — **Figure 1.**Concept: a glowing stem cell in its bone-marrow “niche”, surrounded by support cells and tiny blood vessels.

01Short answer

A blood-forming (hematopoietic) stem cell is a special cell in the bone marrow that gives rise to every kind of blood cell: red blood cells, white blood cells and platelets. It has two unique abilities: it can copy itself (self-renewal) and it can turn into specialised cells. From the stem cell, two main lines branch off first — the myeloid and the lymphoid line. Cells mature step by step along these lines and move into the blood when they are ready. This process is called blood formation, or hematopoiesis. Leukemia begins at one of the young stages of this path, because of a change in DNA.

02Simple explanation

Picture blood formation as a large family tree. At its root sits the stem cell. On one hand, it makes new copies of itself so that the “family” never runs out; on the other, it gives rise to different “children” — the different kinds of blood cells.

Two large branches grow from the root. The myeloid branch produces red blood cells, platelets and some of the white blood cells (neutrophils, monocytes, eosinophils and basophils). The lymphoid branch produces the immune system’s “scouts” — B lymphocytes, T lymphocytes and NK cells.

Along each branch, cells grow up from “childhood” to “adulthood”. Cells at the very youngest stage are called blasts. Normally blasts mature quickly into working cells, which is why a healthy marrow contains only a few blasts and the blood usually contains none.

The body adjusts the speed of this “factory” to its needs. For example, when the blood is short of oxygen, the kidneys make more of the hormone erythropoietin, and the marrow produces more red cells; during an infection, neutrophil production speeds up. Every day the bone marrow makes billions of new blood cells.

In leukemia, the DNA of a cell on one of the young branches of the tree changes. Its “offspring” do not mature, but keep multiplying. Depending on the branch it starts in, the leukemia is called myeloid or lymphoid. The special power of stem cells matters for another reason too: stem cell transplantation relies on their ability to “re-seed” the bone marrow.

03Scientific explanation

Hematopoiesis is the process by which all types of blood cells arise from the hematopoietic stem cell (HSC). HSCs make up only a very small fraction of marrow cells. They are defined by two properties: self-renewal (keeping at least one stem-cell copy when they divide) and multipotency (the ability to give rise to every blood lineage). Many stem cells spend most of their time “resting” (not dividing), which both protects them and keeps a reserve.

HSCs give rise to multipotent progenitors and then to progenitors with narrower options: the myeloid stem (progenitor) cell becomes red cells, megakaryocytes (which produce platelets), granulocytes and monocytes; the lymphoid progenitor becomes B, T and NK cells. At each stage the cells multiply and gradually lose their ability to divide as they specialise — from the young blast stage to the mature cell.

The process is controlled by growth factors (cytokines): erythropoietin (made mainly by the kidneys) drives the red-cell line, thrombopoietin drives platelets, granulocyte colony-stimulating factor (G-CSF) drives neutrophils, and interleukins drive lymphoid cells. In addition, the marrow “niche” — stromal cells, blood vessels and the surrounding matrix — sends signals to stem cells and regulates whether they rest or start working. The final maturation of T lymphocytes takes place in the thymus.

Leukemias arise from acquired genetic changes at different levels of this hierarchy. In acute leukemias, differentiation is blocked and immature blasts accumulate; in chronic leukemias the cells can mature further but multiply out of control. In some leukemias the change arises at the level of the stem cell or a very early progenitor (for example, BCR::ABL1 in CML). The ability of normal HSCs to rebuild the marrow is the biological basis of allogeneic and autologous transplantation; stem cells can be collected from the bone marrow, from peripheral blood (after special medicines move them into the bloodstream) or from umbilical cord blood.

04What happens in the body?

The stem cell’s “home”

Stem cells live in special corners of the bone marrow called niches. They are surrounded by support cells, tiny blood vessels and signalling molecules. This environment “tells” the stem cell when to rest and when to divide. As leukemia cells multiply, they disturb this environment too.

Two branches, many offspring

The myeloid and lymphoid lines branch off from the stem cell. Within each line a cell passes through several stages: blast, then intermediate forms, and finally the mature cell. For example, a neutrophil starts as a myeloblast, while a red blood cell loses its nucleus at the end of maturation and spends about a week maturing in the marrow before it is released into the blood.

A glowing teal stem cell in the centre; cells spread from it along paths of light in two directions — red discs, tiny platelets and granular white cells on one side, small round blue lymphocytes on the other. — **Figure 2.**The blood-cell family tree: from the stem cell grow the myeloid branch (red cells, platelets, granulocytes, monocytes) and the lymphoid branch (B, T and NK cells).

Into the bloodstream

Mature cells leave the marrow for the blood and do their jobs: red cells carry oxygen, platelets stop bleeding and white cells fight infection. Their lifespans differ — some neutrophils live less than a day, red cells about 120 days — so production never stops.

A stream of cells passing from the edge of a translucent bone into a small vessel; red discs, pale white cells and tiny platelets travel along the glowing vessel against a dark background. — **Figure 3.**Mature cells move from the bone into the blood: red cells, white cells and platelets join the bloodstream.

Where along this path does leukemia begin?

Leukemia usually starts in a young blood cell or progenitor. If the change affects the myeloid line, a myeloid leukemia develops (AML, CML); if it affects the lymphoid line, a lymphoid leukemia develops (ALL, CLL). As leukemia cells fill the marrow, the normal stem cells and progenitors cannot make enough healthy cells.

05From stem cell to bloodstream

Healthy blood formation follows four main steps and ends up covering the blood’s three main jobs.

1Stem cellA self-renewing “parent” cell in the bone marrow.

2BranchingCommitment to the myeloid or the lymphoid line.

3MaturationStages from blast to mature, working cell.

4Release into the bloodFinished cells enter the bloodstream.

Oxygen transportRed cells carry oxygen using hemoglobin.

DefenceWhite cells fight germs.

ClottingPlatelets help stop bleeding.

06The blood-cell “family tree”

Cell	Line	Main job	Where it matures
Red blood cell (erythrocyte)	Myeloid	Carries oxygen and carbon dioxide	Bone marrow
Platelet	Myeloid	Helps form blood clots	Breaks off a megakaryocyte in the marrow
Neutrophil	Myeloid	First line of defence against bacteria and fungi	Bone marrow
Eosinophil, basophil	Myeloid	Allergic reactions, defence against parasites	Bone marrow
Monocyte / macrophage	Myeloid	“Swallows” germs and dead cells, guides the immune response	Made in the marrow, becomes a macrophage in tissues
B lymphocyte	Lymphoid	Makes antibodies	Bone marrow, then lymph nodes
T lymphocyte	Lymphoid	Recognises infected cells, directs the immune response	Starts in the marrow, matures in the thymus
NK cell	Lymphoid	Kills virus-infected and some tumour cells	Bone marrow

* The scheme is simplified: the real hierarchy has more intermediate stages, and the science keeps being refined by new research.

07Key terms

Hematopoiesis: The process of making blood cells.
Hematopoietic stem cell: A cell that can renew itself and give rise to every type of blood cell.
Self-renewal: A stem cell keeping a copy of itself when it divides.
Progenitor cell: A young cell derived from a stem cell that can become fewer lineages.
Myeloid line: The path of development that gives red cells, platelets, granulocytes and monocytes.
Lymphoid line: The path of development that gives B, T and NK cells.
Blast: The youngest, immature stage of a blood cell.
Growth factor: A signalling molecule that stimulates cells to divide and mature (for example, erythropoietin).
Niche: The micro-environment in the marrow where stem cells live and are regulated.
Stem cell transplant: Restoring blood formation by transferring healthy stem cells.

08Key points

All blood cells begin with the hematopoietic stem cell in the bone marrow.
A stem cell can both renew itself and turn into different blood cells.
There are two main lines: myeloid (red cells, platelets, granulocytes, monocytes) and lymphoid (B, T, NK).
Cells mature step by step, starting at the blast stage, and then move into the blood.
The body regulates blood production with growth factors such as erythropoietin.
Leukemia begins at a young stage of this path with a DNA change; depending on the line, it is called myeloid or lymphoid.
The ability of stem cells to rebuild the marrow is the basis of transplantation.

09Frequently asked questions

Are blood stem cells the same as embryonic stem cells?

No. Hematopoietic stem cells are found in the bone marrow of adults and can only turn into blood cells. Embryonic stem cells can become almost any cell in the body. Leukemia treatment is about hematopoietic stem cells.

If leukemia starts in a stem cell, are all my stem cells diseased?

Usually not. Leukemia is a clone that comes from a change in a single cell. Normal stem cells remain in the marrow too, but leukemia cells crowd them out and weaken normal blood production. When treatment shrinks the leukemia clone, normal cells often start working again and blood counts recover.

Are there healthy blasts?

Yes. Blasts are the normal young stage of blood cells, and a healthy marrow always contains a small number of them. The problem is when there are far too many, when they fail to mature, or when many of them appear in the blood. Your doctor will explain what the numbers mean in your own results.

Can food or vitamins speed up blood cell production?

Iron, vitamin B12 and folic acid are needed for normal blood formation, and a shortage of them can cause anemia. But in leukemia, low blood counts are mainly due to the marrow being filled with leukemia cells or to the effects of treatment — they cannot be “fixed” with food. Check any vitamins or supplements with your doctor.

Does everyone with leukemia need a stem cell transplant?

No. A transplant is recommended only in certain situations, depending on the type of leukemia, its genetic features, the response to treatment and the person’s overall health. Many people are treated without a transplant. Your hematologist will make this decision together with you.

How are stem cells collected from the blood?

Stem cells live mainly in the bone marrow, but special medicines can move them into the bloodstream, where they can be collected by a procedure called apheresis. In some cases the marrow itself or umbilical cord blood is used instead. The transplant team decides which source to use.

10Questions for your doctor

Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.

Did my leukemia start in stem cells or in more mature cells?
Is a stem cell transplant being considered for me?
How long might it take my marrow to make blood again after treatment?
Which tests will we use to follow the recovery of blood formation?

My questions

12Sources

National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: hematopoietic stem cell
National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: myeloid stem cell
National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: lymphoid stem cell
National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: blast
National Cancer Institute (NCI). Adult Acute Myeloid Leukemia Treatment (PDQ®) — Patient Version
National Cancer Institute (NCI). Stem Cell Transplants in Cancer Treatment
American Cancer Society. What Is Acute Myeloid Leukemia (AML)? — normal bone marrow, blood and lymph tissue
American Society of Hematology (ASH). Blood Basics
Cleveland Clinic. Bone Marrow: What It Is & Function
MedlinePlus (U.S. National Library of Medicine). Blood
National Human Genome Research Institute (NHGRI). Talking Glossary: Stem Cell

Medical disclaimerThis information is for educational purposes only and does not replace medical advice.

Last updated: 30 September 2026← Back to the list

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