Bone Marrow: Where Blood Cells Are Made

A full blood count can look like a fairly ordinary set of numbers until several cell lines begin to fall at once. A patient may be exhausted and breathless because their haemoglobin is low, developing infections because their neutrophil count has dropped, and bruising more easily because they have too few platelets. Those problems look very different at the bedside, but they can share the same starting point: the bone marrow.

Bone marrow is not simply empty space inside bone. It is a highly organised, vascular tissue where haematopoiesis, the production of blood cells, takes place. In healthy adults it continually supplies erythrocytes, leucocytes and platelets while retaining a small population of haematopoietic stem cells capable of renewing the system. When marrow function changes, the consequences eventually appear in the circulation and, often, in the patient in front of you.

Bone marrow is living tissue inside the skeleton

Bone marrow occupies spaces within bones, particularly the trabecular spaces of cancellous bone and the medullary cavities of long bones. It contains developing blood cells, blood vessels, stromal cells, adipocytes, nerves and supporting cells that together create the environment needed for blood-cell production.

Marrow is commonly described as red or yellow. Red marrow is haematopoietically active and contains large numbers of developing blood cells. Yellow marrow contains a greater proportion of adipose tissue and is less active in blood-cell production. The balance changes through life. In childhood, active red marrow is distributed much more widely through the skeleton. With age, much of the marrow in the limbs becomes fatty yellow marrow, while adult haematopoiesis is concentrated mainly within the axial skeleton and proximal ends of some long bones.

That distinction is useful, but it can make marrow sound more static than it really is. Haematopoiesis is responsive. The marrow adjusts blood-cell production as physiological demand changes, and under severe haematopoietic stress fatty marrow can become more haematopoietically active.

One stem-cell population can maintain every blood-cell lineage

At the top of blood-cell production are haematopoietic stem cells (HSCs). These are rare cells with two crucial abilities: they can self-renew, preserving the stem-cell population, and they can produce progenitor cells that progressively commit to different blood-cell lineages. In adulthood, HSCs reside primarily in the bone marrow. Current research describes them not as isolated cells working independently, but as cells whose behaviour is strongly influenced by the surrounding marrow microenvironment or stem-cell niche. Cain, Derecka and McKinney-Freeman (2025) describe this niche as a key regulator of HSC maintenance and development.

As progenitor cells differentiate, they ultimately give rise to the major cellular components seen in peripheral blood. Erythroid precursors mature into erythrocytes, which transport oxygen using haemoglobin. Megakaryocytes develop within the marrow and extend processes from which platelets are released. Other progenitors produce the different leucocyte populations involved in innate and adaptive immunity.

You will often see haematopoiesis simplified into a neat branching diagram, traditionally divided into myeloid and lymphoid pathways. That is useful for learning the broad relationships between cell types, but real human haematopoiesis is more complex and flexible than a perfectly fixed two-branch tree. The important principle for clinical practice is that mature circulating cells are the end result of multiple stages of proliferation, differentiation and maturation within a tightly regulated system.

The marrow does not produce every cell at the same rate

Blood cells have different functions and lifespans, so their production has to respond to different signals. When tissue oxygen delivery is reduced, the kidneys can increase secretion of erythropoietin, stimulating erythroid production in the marrow. Platelet production is strongly regulated by thrombopoietin, produced mainly by the liver. Cytokines and colony-stimulating factors influence leucocyte production, particularly when infection or inflammation increases demand.

This ability to change output becomes particularly obvious during acute stress. Infection can drive increased production of innate immune cells, a process sometimes described as emergency myelopoiesis. Modern research shows that inflammatory and systemic signals can alter the activity of haematopoietic stem and progenitor cells rather than blood-cell production simply continuing at a constant background rate. Swann, Olson and Passegué (2024) review how acute insults can remodel haematopoiesis to increase production of cells needed during the response.

The marrow environment helps coordinate this. HSCs interact with vascular and perivascular cells and other components of the niche, which provide signals influencing whether stem cells remain relatively quiescent, self-renew or enter differentiation pathways. This protects a long-term stem-cell reserve while still allowing production to increase when the body needs it. It also explains why bone marrow should be thought of as an organ-like microenvironment, not merely a container full of immature blood cells. Carpenter and Maryanovich (2024) describe the interaction between systemic demand and local marrow regulation in maintaining haematopoietic homeostasis.

That regulation becomes clinically visible when marrow reserve is challenged. Cytotoxic chemotherapy, for example, can suppress haematopoiesis because rapidly proliferating marrow cells are vulnerable to treatment. A falling neutrophil or platelet count is therefore not just a laboratory change: it can alter a patient's immediate risk of infection or bleeding.

When marrow production fails, the blood count begins to tell the story

Now return to the patient with fatigue, infections and bruising. If erythrocyte production is inadequate, anaemia can reduce oxygen-carrying capacity and contribute to pallor, fatigue, breathlessness or tachycardia. If neutrophils are reduced, susceptibility to infection may increase. If platelet production is impaired, petechiae, bruising or bleeding may occur. When all three major cell lines are reduced, the laboratory pattern is described as pancytopenia. That finding is not a diagnosis by itself. It is a prompt to work out why the circulating cell populations are low.

Pancytopenia does not automatically mean bone marrow failure. Cell numbers may be low because production within the marrow is reduced or ineffective, because the marrow has been infiltrated or replaced, or because cells are being destroyed, consumed or sequestered after they enter the circulation. Nutritional deficiencies, medicines and systemic disease can also contribute, and some conditions involve more than one mechanism. This is why a full blood count has to be interpreted alongside the clinical picture, blood film and other investigations rather than being treated as a direct measurement of marrow function.

Marrow can be disrupted in very different ways. In aplastic anaemia, haematopoietic production is markedly reduced. In leukaemia, abnormal haematopoietic cells can proliferate and interfere with normal blood-cell production. In myeloma, clonal plasma cells accumulate in the marrow and can suppress normal haematopoiesis as disease burden increases. Cytotoxic chemotherapy can also suppress rapidly dividing marrow cells, which is why falling neutrophils, platelets and haemoglobin are clinically important during cancer treatment.

For suspected haematological malignancy, symptoms and blood results have to be acted on in context. NICE NG12 advises a very urgent full blood count in adults with features that can suggest leukaemia, including persistent fatigue, pallor, recurrent or persistent infection, unexplained bruising, bleeding or petechiae. Those are familiar nursing observations, but understanding marrow physiology shows why they may appear together.

Sometimes clinicians need to look directly at the marrow

Peripheral blood gives important clues, but it cannot answer every question about what is happening inside the marrow. A bone marrow aspirate removes liquid marrow so individual cells can be examined. A trephine biopsy removes a small core of marrow tissue, preserving its architecture and allowing assessment of cellularity and the relationship between cells within the marrow. The tests provide different but complementary information and may be performed together, depending on the clinical question.

For a student nurse, the procedure also has a very human side. Bone marrow samples are commonly taken from the posterior iliac crest under local anaesthetic, and patients may still experience pressure or brief sharp pain during aspiration. Nursing involvement will depend on the setting and local policy, but may include helping the patient understand what to expect, providing reassurance, assessing pain and checking the biopsy site and dressing for bleeding afterwards.

For example, NICE NG35 recommends morphology to determine plasma-cell percentage and flow cytometry to determine plasma-cell phenotype when bone marrow aspirate and trephine biopsy are used to confirm myeloma. The marrow sample is therefore not simply checked for whether cells are present. Their number, appearance, phenotype and organisation can help establish what process is occurring.

The same physiology underpins haematopoietic stem-cell transplantation. The aim is to restore healthy blood-cell production when the patient's haematopoietic system is severely diseased or has been deliberately ablated as part of treatment. The NHS describes stem-cell or bone marrow transplantation as replacing damaged blood cells with healthy blood-forming stem cells so that new healthy blood cells can be produced. During the period before engraftment and adequate cell production, infection, bleeding and anaemia remain major clinical concerns.

So when you look at haemoglobin, neutrophils and platelets on a patient's blood results, you are seeing the circulating output of a living tissue hidden inside the skeleton. Bone marrow has to preserve stem cells for the future, respond to today's physiological demands and continually release mature cells in the right numbers. When that system falters, the blood count changes, but the patient often tells you first through fatigue, infection, bruising, bleeding or breathlessness.

References

Cain, T.L., Derecka, M. and McKinney-Freeman, S. (2025) ‘The role of the haematopoietic stem cell niche in development and ageing’, Nature Reviews Molecular Cell Biology, 26, pp. 32–50. https://www.nature.com/articles/s41580-024-00770-8

Carpenter, R.S. and Maryanovich, M. (2024) ‘Systemic and local regulation of hematopoietic homeostasis in health and disease’, Nature Cardiovascular Research, 3, pp. 651–665. https://www.nature.com/articles/s44161-024-00482-4

National Health Service (2026) What is a stem cell or bone marrow transplant? https://www.nhs.uk/tests-and-treatments/stem-cell-or-bone-marrow-transplants/what-it-is/

National Institute for Health and Care Excellence (NICE) (2026) Suspected cancer: recognition and referral (NG12). https://www.nice.org.uk/guidance/ng12

National Institute for Health and Care Excellence (NICE) (2016, updated) Myeloma: diagnosis and management (NG35). https://www.nice.org.uk/guidance/ng35

Swann, J.W., Olson, O.C. and Passegué, E. (2024) ‘Made to order: emergency myelopoiesis and demand-adapted innate immune cell production’, Nature Reviews Immunology, 24, pp. 596–613. https://www.nature.com/articles/s41577-024-00998-7