What Is Bone Marrow?

Bones are vital structures that protect the bodies of living beings, enable movement, and consist of bone, bone marrow, and surrounding connective tissue. One of the most important functions of bones is blood production. So, what is bone marrow, which plays a significant role in blood formation? What are the types of bone marrow? You can continue reading to learn more about bone marrow.

What Is Bone Marrow?

Bones, an essential part of the musculoskeletal system, are classified into types such as long, short, flat, irregular, and pneumatic bones. In the skeletal system, bones like the femur, hip, and sternum are longer than they are wide. The structure of long bones includes a long part called the diaphysis, a cavity, and a hard, compact structure surrounding the cavity. The nutrient-rich, spongy tissue inside these cavities is known as bone marrow. The primary function of bone marrow is to produce blood cells and release them into circulation. Stem cells in the bone marrow actively participate in blood production. Depending on the body's needs, these cells produce red blood cells (erythrocytes), white blood cells (leukocytes), and platelets. Stem cells have the ability to replicate and copy themselves. The copied stem cells produce mature blood cells, which, once fully developed, detach from the bone marrow and enter the bloodstream. There are three main types of cells in the bone marrow: stem cells, progenitor cells that give rise to various cell lines, and mature blood cells that join the bloodstream.

What Are the Types of Bone Marrow?

Bone marrow is divided into two subtypes: yellow bone marrow and red bone marrow. Both yellow and red bone marrow contain blood vessels, but there are some differences between the two. Here are some key distinctions between yellow and red bone marrow:

• The main reason for classifying bone marrow into yellow and red is the difference in their components. Red bone marrow gets its name from the erythrocytes and erythrocyte lines it contains. Yellow bone marrow, on the other hand, contains a large number of fat cells, giving it a yellow color.
• In newborns, only red bone marrow is present in the body. Around ages 5–6, red bone marrow begins to transform into yellow bone marrow. In adults, approximately half of the bone marrow is red, and the other half is yellow.
• One of the most significant differences between red and yellow bone marrow is related to blood production. While blood cells are produced in red bone marrow, no blood cell production occurs in yellow bone marrow.
• Red bone marrow plays a role in the breakdown of erythrocytes and the storage of iron released during their breakdown. Iron is also stored in organs like the liver and spleen, in addition to red bone marrow. Yellow bone marrow, however, serves as a fat storage.
• In cases of excessive blood loss, yellow bone marrow can transform into red bone marrow to support the body's blood production process.
• The production of undifferentiated T and B lymphocytes, which are transported to central lymphoid organs via the bloodstream, takes place in red bone marrow. Lymphocyte production in the bone marrow primarily occurs when there is damage or disease in one of the lymphoid system organs.

Bone Marrow Diseases

Since bone marrow plays an active role in the production of blood cells, various blood-related diseases may occur if it does not function properly. The main bone marrow diseases include:

• Multiple Myeloma: Multiple myeloma, or simply myeloma, is a type of cancer that originates from plasma cells, which are a type of white blood cell. In patients with myeloma, the DNA of plasma cells is damaged. These damaged cells are called myeloma cells. Unlike other types of cancer, there are no tumors in myeloma cancer. Myeloma cells multiply uncontrollably within the bone marrow.
• Sickle Cell Anemia: Red blood cells contain hemoglobin, which is responsible for carrying oxygen. Due to various reasons, an anomaly develops in the erythrocytes containing hemoglobin, causing these cells to take on a sickle shape.
• Mediterranean Anemia (Thalassemia): Mediterranean anemia is a blood disorder commonly seen worldwide, especially in people living in the Mediterranean region. Hemoglobin has a structure called a globin chain. A disorder in the structure of the globin chain causes less or no production of globin, leading to Mediterranean anemia, also known as thalassemia.
• Lymphoma: Lymph cancer, also known as lymphoma, is a type of cancer that occurs due to the uncontrolled proliferation of lymphocytes found in organs like the bone marrow, spleen, and liver. Lymphoma can be treated with therapies such as chemotherapy and radiotherapy, and in some cases, bone marrow transplantation can be life-saving.
• Leukemia: Leukemia, also known as blood cancer or bone marrow cancer, is a type of cancer that occurs due to the uncontrolled proliferation of cells called myeloid and lymphoid cells in the white blood cells produced in the bone marrow.

What Is Bone Marrow Transplantation and How Is It Performed?

Bone marrow transplantation, also known as stem cell transplantation, is a treatment method used for conditions such as Mediterranean anemia, lymphoma, and leukemia. Previously, only bone marrow was used to collect stem cells, making the terms “bone marrow transplantation” and “stem cell transplantation” interchangeable. Nowadays, the patient's own cells can also serve as the stem cell source, a procedure known as autologous stem cell transplantation. Typically, a healthy donor is needed for bone marrow transplantation. The procedure involves transferring bone marrow collected from a healthy donor to the patient via the bloodstream. If the main stem cells from the donor are compatible with the patient, a new blood production process begins in the patient. General anesthesia is used during bone marrow transplantation, where specialized doctors insert special needles into the donor's bone in a sterile operating room and extract the bone marrow into a syringe. The main stem cells taken from the donor are placed in a special bag and immediately transferred to the patient via the bloodstream. If the transplant is successful, healthy blood production begins in the patient within approximately three weeks. To increase the compatibility of the bone marrow with the recipient, the patient is closely monitored with various treatments for about six months.

Who Can Be a Bone Marrow Stem Cell Donor?

The success of a bone marrow transplant is directly related to the compatibility of the donor. Therefore, there are specific criteria for becoming a donor. Donors can include siblings with compatible tissue types, occasionally other blood relatives, non-relatives with compatible tissue types, the patient themselves, or their previously collected and stored healthy bone marrow. In some cases, stem cells may also be collected from the umbilical cord blood of a newborn sibling.

Are There Any Side Effects of Bone Marrow Transplantation?

Bone marrow transplantation has minimal side effects for the donor, including fatigue, soreness in the spleen area, fever, and cold-like symptoms. For the recipient, however, there may be serious complications before and after the transplant, with the primary complication being the body's rejection of the stem cells. The immune cells produced by the bone marrow may start to attack the patient's own tissues and organs, viewing them as foreign agents.

What Precautions Should Be Taken After Bone Marrow Transplantation?

After bone marrow transplantation, patients should regularly attend medical check-ups. Additionally, they should maintain a healthy, balanced diet, engage in light exercises, and avoid harmful habits such as tobacco use.