How bone marrow makes blood, and what happens when it fails

Why marrow matters more than its size suggests

Most people never think about their bone marrow until a blood test raises a question. Yet the red cells, platelets and most white cells in the blood all begin their lives in this soft tissue. When it falls short, the effects reach almost every part of the body.

This page explains what bone marrow is, how it produces blood, and what the terms marrow failure, myelosuppression and bone marrow depression mean. It sits alongside the overview of aplastic anaemia, which covers the condition in more depth. In the US the condition is usually spelled aplastic anemia.

Where bone marrow sits in adults and children

Bone marrow is the soft, spongy tissue found in the hollow spaces inside bones. In an adult weighing 65 kg, it weighs about 2.6 kg, roughly 4 per cent of body weight (Medscape, Bone Marrow Anatomy). Its distribution changes with age. At birth, all marrow is red and active, and as people get older a large share turns yellow and fatty, although in adults about half remains red.

In adults, the active red marrow sits mainly in the flat bones: the pelvis, breastbone, skull, ribs, spine and shoulder blades. It also fills the ends of long bones such as the thigh bone. In children, the upper shin bone (tibia) still holds a substantial amount of red marrow, which is why it can give a good sample. For adults, doctors usually take a sample from the hip bone or the breastbone.

Stem cells: the starting point for every blood cell

All blood cells come from a single kind of starting point, the haematopoietic stem cell. These immature cells persist throughout life, and they divide to produce more stem cells while also developing into specialised cells (Medscape). The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) describes them as the source of the red cells, white cells and platelets carried in the blood.

Stem cells divide along two main lines. The myeloid line produces red cells, platelets and most white cells. The lymphoid line produces lymphocytes, which make up much of the immune system. Because of this branching, the marrow always holds blood cells at many stages of development at once.

The three blood lineages and what each one does

Red blood cells carry oxygen. They contain haemoglobin, the protein that moves oxygen from the lungs to every part of the body. The marrow makes a very large number of them: about three million every second, according to Great Ormond Street Hospital (GOSH). A red cell typically lasts around 120 days (MSK Cancer Center).

White blood cells defend the body against infection, and they are the most varied of the three lineages. Neutrophils help fight bacterial and fungal infections. Lymphocytes make up much of the immune system. Their lifespans range widely, from hours to years. In children, GOSH gives a neutrophil reference range of 1.5 to 8.5 x10^9 per litre, and it treats a count below 0.5 as neutropenia.

Platelets are small fragments of cells whose job is to stop bleeding. They last about 9 to 12 days (MSK Cancer Center). A child's platelet count is normally between 150 and 400 x10^9 per litre (GOSH).

What marrow failure means

Marrow failure is an umbrella term for a marrow that cannot make enough healthy blood cells to meet the body's needs. NIDDK places aplastic anaemia and myelodysplastic syndromes (MDS) under this heading. In both, the marrow is injured and at least two of the three cell types usually run low.

The problem can start in three broad ways. Each one leads to fewer working blood cells, but the route is different.

• Too few stem cells. In aplastic anaemia, many stem cells are injured or destroyed. The cells that are made are healthy, but there are not enough of them.
• Faulty stem cells. In MDS, some stem cells are damaged and produce blood cells that do not work properly.
• Crowding out. Abnormal cells multiply and take up room that healthy cells need. In leukaemia, for example, overproduced abnormal white cells crowd out healthy ones (Medscape).

The distinction matters in practice, because it shapes which tests are ordered and how the condition is treated. The diagnosis page explains how the team works out which route is involved. Some people have an inherited form that began early in life, and the difference between acquired and inherited forms is worth understanding early on.

What it feels like: the first signs

Symptoms follow the missing cells. Low red cells cause tiredness, paleness and breathlessness after exercise. Low white cells mean infections that come more often or last longer than usual. Low platelets lead to easy bruising, gums that bleed after brushing, nosebleeds, and small red pinprick spots under the skin called petechiae (GOSH).

Some people notice these changes slowly, over weeks. Others notice them more suddenly, since marrow failure can start acutely or develop over time (GOSH). Low counts are usually corrected first with transfusions, which are covered on the transfusions page. A fever in someone with a low neutrophil count needs prompt medical attention, so it should never be left to wait.

Myelosuppression and bone marrow depression

Myelosuppression is a word that appears often in cancer care. The National Cancer Institute defines it as a condition in which bone marrow activity is decreased, resulting in fewer red blood cells, white blood cells and platelets (NCI). It is a side effect of some cancer treatments, and when it is severe it is called myeloablation.

You may also see the phrase bone marrow depression, or bone marrow suppression. These terms describe the same drop in marrow output. What differs is the cause. Myelosuppression names an effect, often one linked to a treatment. Aplastic anaemia names a disease, and in many cases the immune system attacks the marrow (GOSH). Whether a low count comes from a drug, an infection, an inherited condition or an immune attack is the question the diagnostic work-up answers.

Low blood counts also reshape daily life, from how much energy is available to how far ahead a person can plan work and appointments. Those practical effects deserve as much attention as the numbers, and they are worth raising with the people caring for you.

Reading a blood count as a production line

Think of bone marrow as a production line that must keep several products moving at once. A blood count is a snapshot of how well each line is running. When one line slows, the symptoms follow the missing cells, and when the marrow itself is injured, more than one line tends to slow together.

Understanding these basics makes the rest of your results easier to follow. If a number on a report surprises you, ask your haematology team to explain what it means for your red cells, white cells or platelets specifically.