Bone marrow

Soft stuff lives inside your bones.

This is called bone marrow. It makes your blood cells. It makes many cells every day.

Some of these cells are red. Some are white. It also makes tiny pieces called platelets.

In adults, this stuff is in your ribs. It is in your hips too. Some of it is red. Some of it is yellow. The red parts make blood. The yellow parts are made of fat.

Your body can change the yellow parts back to red. This happens if you need more blood. It is a busy place inside you!

Bone marrow is a soft tissue found inside your bones.

It is a very busy place. In humans, it makes about 500 billion blood cells every single day.

These cells include red blood cells, white blood cells, and platelets.

There are two main types of marrow. Red marrow makes blood cells. Yellow marrow is made mostly of fat.

Newborn babies have only red marrow. As people get older, much of it turns into yellow marrow. However, your body can change yellow marrow back into red marrow. This happens if your body needs more blood cells.

In adults, red marrow stays in the center of the skeleton. You can find it in your ribs, your spine, and your pelvis. Bone marrow also holds special cells called stem cells. These stem cells can grow into many different types of tissue. They can become bone, fat, or even cartilage. Doctors can sometimes use bone marrow transplants to treat serious diseases like leukemia. This helps people who have sick bone marrow.

Bone marrow is a soft, semi-solid tissue found inside your bones.

It lives within the spongy parts of the bone. This tissue is very important for staying healthy. It acts as a main factory for making new blood cells. In humans, this tissue makes up about 5% of your total body mass. A person who weighs 73 kg will have around 3.7 kg of marrow.

How does this busy factory work? It uses special cells called hematopoietic stem cells to build what the body needs. These cells can turn into red blood cells, white blood cells, or platelets.

Every single day, human marrow produces about 500 billion blood cells. Once they are ready, these cells travel through tiny blood vessels called sinusoids. These vessels allow the new cells to join the rest of your blood. This constant work keeps your blood moving and working correctly.

Scientists have learned much about how marrow changes over time. Newborn babies have only red marrow, which is very active. As people grow older, much of this turns into yellow marrow. Yellow marrow is mostly made of fat cells.

However, the body can change yellow marrow back into red marrow. This happens if the body faces a hard job like chronic hypoxia. This is when the body does not have enough oxygen.

In adults, red marrow stays in specific parts of the skeleton. You can find it in the ribs, the spine, and the pelvis. It is also in the skull and the shoulder blades.

Some parts of long bones, like the femur, also have it. Scientists also study the stroma, which is the supportive tissue around the cells. This stroma includes fat cells and cells that build bone. It helps create the right home for the growing blood cells.

Understanding bone marrow helps doctors treat many different illnesses. Some diseases, like leukemia, can attack the cells in the marrow.

Doctors can sometimes perform a bone marrow transplant to help. This is a way to give a patient healthy new cells. They might also use special imaging like an MRI to look inside. An MRI can show the difference between fatty yellow marrow and red marrow. This helps doctors see if the marrow is healthy or sick.

Bone marrow is a semi-solid tissue located within the spongy, or cancellous, portions of bones. It serves as a vital biological hub for the human body. This tissue is the primary site for haematopoiesis, which is the production of new blood cells. In healthy adult humans, bone marrow makes up approximately 5% of total body mass. For a person weighing 73 kg, this equals about 3.7 kg of marrow.

The mechanism of blood production is a massive, continuous process. Within the medullary cavity, hematopoietic stem cells act as the foundation. These stem cells give rise to three main classes of blood cells. These include erythrocytes, which are red blood cells. They also include leukocytes, or white blood cells, and thrombocytes, known as platelets. Every day, human marrow produces approximately 500 billion blood cells. Once these cells reach maturity, they enter the systemic circulation through permeable blood vessels called sinusoids.

Bone marrow is organized into different types based on its cellular makeup. It is often categorized as either red marrow or yellow marrow. Red marrow is highly active in haematopoiesis and contains many hematopoietic cells. Yellow marrow consists mostly of marrow adipose tissue, or fat cells. A newborn baby's bones contain exclusively red marrow. As a person ages, there is a progressive conversion toward yellow marrow. However, the body can adapt to chronic hypoxia, a condition of low oxygen. In these cases, the body can convert yellow marrow back into red marrow to increase cell production.

The composition of the marrow is supported by a complex structure called the stroma. The stroma is the connective tissue that is not directly involved in blood production. Instead, stromal cells provide a specialized microenvironment that influences how hematopoietic cells function. This environment includes various cell types like fibroblasts and macrophages. Macrophages are particularly important because they deliver iron for hemoglobin synthesis. Other cells include adipocytes, which are fat cells, and osteoblasts, which synthesize bone. Endothelial cells form the sinusoids that allow mature cells to exit the marrow.

Research into the marrow's role in the immune system has grown recently. In 2003, scientists described the marrow as a priming site for T-cell responses. Mature T cells move into the marrow parenchyma to interact with dendritic cells. These dendritic cells can take up and process antigens. This interaction leads to T-cell activation and proliferation. Further studies in 2013 used two-photon dynamic imaging in mice to corroborate these findings. The bone marrow also acts as a sanctuary for plasma cells and a nest for memory T cells.

Clinical significance is found in how the marrow responds to disease. The architecture of the marrow can be damaged by infections like tuberculosis or malignancies. Certain cancers, such as leukemia, attack the hematologic progenitor cells directly. Diseases like aplastic anemia can also decrease the production of blood cells and platelets. Because radiation and chemotherapy kill rapidly dividing cells, they often damage the bone marrow. This damage can lead to a depressed immune system. To diagnose these issues, doctors may perform a bone marrow aspiration. This procedure usually involves using a hollow needle to collect a sample from the ilium.

Doctors use various medical imaging tools to assess bone marrow health. Plain film X-rays cannot visualize soft marrow directly, though they show bone changes. CT imaging has a slightly better capacity to assess the medullary cavity. MRI is the most sensitive and specific tool for assessing bone composition. MRI can distinguish between the high T1-relaxivity of fatty yellow marrow and the darker signal of red marrow. This helps clinicians identify pathologic lesions or changes in marrow density.