Structure and Function
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Bone marrow is in the medullary canals of long bones and in the small cavities of cancellous or spongy bone.1
It is a crucial component of the body's hematopoietic system, responsible for producing blood cells.
The marrow is supplied by nutrient arteries, which divide and eventually give rise to venous sinusoids that are lined by endothelial cells and adventitial cells.
Sinusoids have thin walls that allow newly formed blood cells to enter circulation easily. They play a crucial role in the overall hematopoiesis process, supporting the formation and release of blood cells.
The frond-like tissue between the sinusoids contains a mixture of
🔹 Functions
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Blood Cell Production (Hematopoiesis)
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Erythrocytes (RBCs) → transport oxygen (via hemoglobin).
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Leukocytes (WBCs) → immune defense (neutrophils, lymphocytes, monocytes, etc.).
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Thrombocytes (platelets) → clotting.
🔹 Regulation
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Erythropoietin (EPO) from the kidney stimulates RBC production.
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Colony-stimulating factors (CSFs) and interleukins regulate WBC formation.
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Thrombopoietin regulates platelet production.
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- Stem Cell Reservoir
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Hematopoietic stem cells (HSCs) → give rise to all blood cell lineages.
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Mesenchymal stem cells (MSCs) → can form bone, cartilage, fat.
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- Immune System Role
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Produces lymphoid precursors that migrate to the thymus, spleen, and lymph nodes to mature.
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In the fetus, blood cells are also formed in the liver and spleen. In children, blood cells are actively produced in the marrow cavities of all the bones.
By age 20, the marrow in the cavities of the long bones, except for the upper humerus and femur, has become inactive.
75% of the cells in the marrow belong to the white blood cell–producing myeloid series and only 25% are maturing red cells, even though there are over 500 times as many red cells in the circulation than white cells. This difference reflects the fact that the average life span of white cells is short, whereas that of red cells is long.
Hematopoietic stem cells (HSCs) are bone marrow cells capable of producing all types of blood cells. They differentiate into committed stem cells (progenitor cells) which in turn form the various types of blood cells. There are separate progenitor cells for megakaryocytes, lymphocytes, erythrocytes, eosinophils, and basophils; neutrophils and monocytes arise from a common precursor. The bone marrow stem cells are also the source of osteoclasts, Kupffer cells , mast cells, dendritic cells, and Langerhans cells.
References