bone marrow

Hematopoiesis (production of blood) occurs in the bone marrow, which is located within the hollow interior of bones, predominantly in flat bones and long bones. []Gray's, labeled photomicrograph, diagram []

The tissue within bone marrow is differentiated as 'red' marrow (erythropoiesis, thrombopoiesis, production of WBC) and 'yellow' marrow, which contains fat cells. The proportion of 'yellow marrow' increases with the accumulation of fat cells during the individual's life span, though reversion to red marrow is possible in response to blood loss.

normal [] bone marrow section [] bone marrow section 2 [] bone marrow smear []

In addition to hematopoietic stem cells (HSC), mesenchymal stem cells (hMSC) are located around the central sinus of the bone marrow and have the potential to differentiate into osteoblasts, chondrocytes, myocytes, and other cell types. Bone marrow stem cells have also been demonstrated to aid in the regeneration of adipocytes, hepatocytes, lung, alimentary canal, skin, and endothelial cells. Side population cells (SP's) are believed to be present in skeletal muscle, and thus scientists hypothesize that SP's express plasticity by contributing to hematopoiesis as well as skeletal muscle tissue.