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Bone marrow
Last reviewed: 23.04.2024
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The red bone marrow (medulla ossium rubra) is located in the cells of the spongy substance of the flat and short bones, the epiphyses of the long (tubular) bones, and the yellow bone marrow (medulla ossium flava) filling the bone marrow cavities of the diaphysis of the 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 about half of the red bone marrow. The red bone marrow consists of a myeloid tissue, which includes the reticular tissue and hemocytopoietic elements. It contains stem hemopoietic cells - the precursors of all blood cells and the immune system (lymphoid series). In the red bone marrow, the blood capillaries supplying it with a diameter of 6-20 μm and wide capillaries with a diameter of up to 500 μm branch into sinusoids, the mature blood cells and immune system (B-lymphocytes) migrate to the bloodstream through the walls of the blood.
The yellow bone marrow is represented mainly by the fatty tissue that replaced the myeloid and lymphoid tissues. The presence of yellow color of fatty inclusions in degenerated reticular cells gave the name to this part of the bone marrow. There are no blood-forming elements in the yellow bone marrow. With large blood loss, the red marrow may reappear on the site of the yellow bone marrow.
[Development and age specific features of the 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 of hematopoiesis observed in the liver.
The bone marrow begins to form in the bones of the embryo at the end of the second month. From the 12th week in the bone marrow blood vessels, including sinusoids, develop. Around the blood vessels appears reticular tissue, forming the first islets of hematopoiesis. From this time, the bone marrow begins to function as a hemopoietic organ. Beginning with the 20th week of development, the mass of the bone marrow increases rapidly, it spreads toward the epiphyses. In the diaphysis of the tubular bones, the bone bars are resorbed, forming a bone marrow cavity. In the newborn, the 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 the yellow bone marrow completely fills the bone marrow cavities of the diaphysis of long (tubular) bones. In old people, the bone marrow acquires a mucus-like consistence (gelatinous bone marrow). In epiphases of tubular bones, in flat bones, a portion of the red bone marrow also turns into a yellow bone marrow.