Bone marrow

A distinction is made between red bone marrow (medulla ossium rubra), which in adults is located in the cells of the spongy substance of flat and short bones, the epiphyses of long (tubular) bones, and yellow bone marrow (medulla ossium flava), which fills the bone marrow cavities of the diaphyses of long bones. The total mass of bone marrow in an adult is approximately 2.5-3.0 kg (4.5-4.7% of body weight), with red bone marrow accounting for about half. Red bone marrow consists of myeloid tissue, including reticular tissue and hemocytopoietic elements. It contains hematopoietic stem cells - the precursors of all blood cells and the immune system (lymphoid series). In the red bone marrow, blood capillaries with a diameter of 6-20 µm and wide capillaries with a diameter of up to 500 µm branch out - sinusoids, through the walls of which mature formed elements (cells) of the blood and the immune system (B-lymphocytes) migrate into the bloodstream.

Yellow bone marrow is mainly represented by fatty tissue, which has replaced myeloid and lymphoid tissue. The presence of yellow fatty inclusions in the degenerated reticular cells gave this part of the bone marrow its name. Hematopoietic elements are absent in yellow bone marrow. With large blood losses, red bone marrow may appear again in place of yellow bone marrow.

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Functions of the bone marrow

Bone marrow is the main organ of hematopoiesis, which is located inside the bones. It has several important functions:

1. Hematopoiesis (blood formation): Bone marrow is the site of formation of all types of hematopoietic cells, such as erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (thrombocytes). These cells play an important role in the blood, performing functions such as transporting oxygen, fighting infections, and participating in the blood clotting process.
2. Stem cell storage: Bone marrow contains stem cells that can differentiate into different types of blood-forming cells. These stem cells may be important in treating various blood and immune system disorders.
3. Immune system function: Bone marrow plays a role in the formation and maturity of certain types of immune cells, such as lymphocytes. These cells are then carried into the lymphatic system and participate in the body's immune responses.
4. Involvement in mineral metabolism: Bone marrow can also serve as a storage reservoir for minerals such as calcium and phosphorus and is involved in the metabolism of these important elements in the body.
5. Regulation of blood metabolism: Bone marrow can regulate the amount of blood-forming cells in the blood in response to the body's needs. For example, when there is blood loss or an infection, the bone marrow can be activated to increase the production of blood-forming cells.

Bone marrow performs these functions due to its active participation in the processes of hematopoiesis and maintaining the blood in a normal state.

Development and age-related features of bone marrow

In the embryonic period, hematopoiesis occurs in the blood islands of the yolk sac (from the 19th day to the beginning of the 4th month of intrauterine life). From the 6th week, hematopoiesis is observed in the liver.

Bone marrow begins to form in the bones of the embryo at the end of the 2nd month. From the 12th week, blood vessels, including sinusoids, develop in the bone marrow. Reticular tissue appears around the blood vessels, and the first islets of hematopoiesis are formed. From this time on, the bone marrow begins to function as a hematopoietic organ. Starting from the 20th week of development, the mass of bone marrow increases rapidly, it spreads towards the epiphyses. In the diaphyses of tubular bones, the bone crossbars are resorbed, and a bone marrow cavity is formed in them. In a newborn, red bone marrow occupies all the bone marrow cavities. Fat cells in the red bone marrow first appear after birth (1-6 months), and by the age of 20-25, yellow bone marrow completely fills the bone marrow cavities of the diaphyses of long (tubular) bones. In old people, the bone marrow acquires a mucus-like consistency (gelatinous bone marrow). In the epiphases of tubular bones, in flat bones, part of the red bone marrow also turns into yellow bone marrow.

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Bone marrow diseases

Some of the more common bone marrow diseases include:

1. Leukemia: This is a cancer in which abnormal white blood cells begin to multiply uncontrollably in the bone marrow, competing with healthy cells. Leukemia includes several subtypes, such as lymphocytic leukemia and myeloid leukemia.
2. Myelodysplastic syndrome (MDS): This is a group of rare bone marrow disorders characterized by impaired function of the red bone marrow and insufficient production of blood cells.
3. Myeloproliferative disorders: These diseases are characterized by excessive production of blood cells, such as red blood cells (erythrocytes), platelets (thrombocytes), and neutrophils (white blood cells). Examples of these disorders include polycythemia vera, myelofibrosis, and chronic myelogenous leukemia.
4. Aplastic anemia: This is a condition in which the bone marrow does not produce enough blood cells, including red blood cells, white blood cells, and platelets.
5. Multiple myeloma: This is a cancer that affects the plasma cells responsible for producing antibodies. It can lead to weakened bones and other complications.

Symptoms and treatment of bone marrow diseases can vary greatly depending on the type and stage of the disease. Diagnosis and treatment of these conditions are best done under the supervision of a hematologist or oncologist.