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. 2016 Apr 13;17(4):553. doi: 10.3390/ijms17040553

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© 2016 by the authors; licensee MDPI, Basel, Switzerland.

This article is an open access article distributed under the terms and conditions of the Creative Commons by Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).

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Topobiology of a normal Hematopoietic Unit (HU) in humans. In a normal HU, mesenchymal stromal/stem cells (MSCs) are located in paratrabecular and perivascular areas, and to a lesser extent in the parenchyma. Osteoblast number decreases with age. Megakaryocytes (meg) are located adjacent to sinusoids, MSCs form an outside layer on the sinusoids. Arterioles and capillaries are covered sequentially by pericytes/vascular smooth muscle and a full and thick layer of MSCs, and are often rimmed by plasma cells. Macrophages (mQs) are as abundant as MSCs, and are scattered throughout the bone marrow in perivascular, paratrabecular and parenchymal areas. CD34+ hematopoietic progenitor/stem cells (HPSCs) are located in perivascular areas, with fewer at paratrabecular regions. CXCL12 is produced predominantly by endothelial cells and by osteoblasts.