TY - CHAP M1 - Book, Section TI - Structure of the Marrow and the Hematopoietic Microenvironment A1 - Davé, Utpal P. A1 - Koury, Mark J. A2 - Kaushansky, Kenneth A2 - Lichtman, Marshall A. A2 - Prchal, Josef T. A2 - Levi, Marcel M. A2 - Press, Oliver W. A2 - Burns, Linda J. A2 - Caligiuri, Michael PY - 2015 T2 - Williams Hematology, 9e AB - SUMMARYThe marrow, located in the medullary cavity of bone, is the site of hematopoiesis in humans.* The marrow produces approximately 6 billion cells per kilogram of body weight per day. Hematopoietically active (red) marrow regresses after birth until late adolescence, after which it is focused in the lower skull, vertebrae, shoulder and pelvic girdles, ribs, and sternum. Fat cells (yellow marrow) replace hematopoietic cells in the bones of the hands, feet, legs, and arms. Fat comprises approximately 50 percent of red marrow in the adult. Further fatty replacement of the red marrow continues slowly with aging, but hematopoiesis can be expanded when demand for blood cells is increased.The marrow stroma consists principally of a network of sinuses that originate at the endosteum from cortical capillaries and terminate in collecting vessels that enter the systemic venous circulation. The trilaminar sinus wall is composed of endothelial cells; a thin basement membrane; and adventitial reticular cells that are progenitors of chondrocytes, osteoblasts and adipocytes. Stem cells can leave and reenter marrow as part of their normal circulation.Hematopoiesis, the proliferation and differentiation of stem cells and their progeny in the intersinus spaces, is controlled by a complex array of stimulatory and inhibitory cytokines, cell–cell contacts, and interactions with the extracellular matrix. In this unique environment, lymphohematopoietic stem cells differentiate into all the blood cell lineages. Mature cells are produced and released to maintain steady-state blood cell levels. The hematopoietic system also can respond to meet increased demands for additional cells as a result of blood loss, hemolysis, inflammation, immune cytopenias, and other causes. SN - PB - McGraw-Hill Education CY - New York, NY Y2 - 2021/06/20 UR - hemonc.mhmedical.com/content.aspx?aid=1135461624 ER -