Bone: the organ - bone marrow
Two types of bone marrow occur in bony spaces: red and yellow varieties. Red bone marrow is composed primarily of hemopoietic tissue and sinusoids, where red blood cells, white blood cells and megakaryocytes differentiate. Yellow bone marrow is composed of 80-90% adipocytes and 10-20% hemopoietic tissue. The lipid in the adipocytes causes yellow bone marrow to appear yellow to the naked eye. 200x
Yellow bone marrow >
Yellow bone marrow functions primarily in fat storage, however small islands of hemopoietic tissue may be present. Under physiological stress, yellow marrow increases its ability to produce blood cells through the differentiation of mesenchymal stem cells. Adipocytes do not differentiate into blood-forming cells.
-Hemopoietic tissue in yellow marrow
Yellow bone marrow functions primarily in fat storage, however small islands of hemopoietic tissue may be present. Under physiological stress, yellow marrow increases its ability to produce blood cells through the differentiation of mesenchymal stem cells. Adipocytes do not differentiate into blood-forming cells.
Red bone marrow >
Red bone marrow is composed mostly of hemopoietic tissue rather than adipocytes. As individuals age, adipocytes replace hemopoietic tissue, resulting in a gradual increase in yellow bone marrow to about 70% or more in the elderly.
- Sinusoids >
Sinusoids are large diameter, thin walled capillaries (discontinuous capillaries). After maturing in the hemopoietic tissue, red and white blood cells migrate into the sinusoids for dispersal throughout the body.
- Megakaryocytes >
Megakaryocytes are large cells in the marrow that produce platelets, which are essential for blood clotting. Platelets are cell fragments shed from megakaryocytes.
Bone spicules >
Bone spicules form the spongy bone in which most hemopoiesis occurs in adults.
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