Red bone marrow

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Histological features of the bone marrow.

Red bone marrow is a jelly-like substance that represents the hematopoietically active unit of bone marrow. It consists of reticular connective tissue stroma, specialized blood vessels called sinusoidal capillaries, and a network of hematopoietic cells called hematopoietic cords or islands. In contrast, yellow bone marrow represents the hematopoietically inactive unit of the bone marrow, consisting mainly of adipose tissue.

Red bone marrow is abundant in all bones during intrauterine life and early childhood, when it occupies the medullary cavity of long bones and the small cavities of spongy bone. After the 5th year of life, most of the red bone marrow gradually transforms into yellow bone marrow. In adults, red bone marrow only persists in the axial flat bones (cranial bones, clavicle, sternum, ribs, scapula, vertebrae, and pelvis) and the proximal ends of the humerus and femur.

Red bone marrow houses pluripotent hematopoietic cells that give rise to all types of blood cells, including erythrocytes, lymphocytes, granulocytes, monocytes and platelets. The production of blood cells in the bone marrow is adjusted to the body’s needs, influenced by different endogenous and exogenous factors. Under certain conditions, such as severe bleeding or hypoxia, yellow bone marrow can revert back into red bone marrow to accommodate the increased need. In addition, the red bone marrow also contains stem cells that can produce other tissues besides blood cells.