Structural organization of BM tissues. (A) BM stromal microarchitecture. The microarchitecture of the vascular system has been studied in detail in the mouse femoral cavity. A central artery (ca) penetrates the cavity through the nutrient canal and splits into ascending and descending branches, which run longitudinally, arborizing into smaller radial arteries that migrate toward endosteal regions. In the proximity of endosteum, arteries give rise to a dense plexus of arterioles that travel along the cortical bone area and eventually develop into venous vessels, of fenestrated endothelium, termed sinusoidal. Sinusoids form a labyrinth that extends inward and merges in a big central collecting sinus (cs) that drains into peripheral circulation (see also Figure 1). Thin periosteal arteries also penetrate the bone and merge with arteriolar vessels in endosteal regions, connecting BM and bone circulation. In trabecular bone areas, multiple smaller arteries enter the marrow cavities and give rise to sinusoidal networks along the endosteal surface of trabeculae. CXCL12-abundant fibroblastic reticular cells (CARc) extend throughout the entire cavity in the form of dense networks. Nestin-GFP^hi NG2⁺ (nes) elongated cells run adjacent to arteries and arterioles and bundles of nonmyelinating Schwann cells (nmsc), which ensheath adrenergic nerves. Mature osteoblasts (ob) line endosteal surfaces and are mostly derived from reticular progenitors of osetoadipogenic potential. The emergence of adipocytes (adip) takes place gradually during aging and can be abrupt and prominent in certain pathological conditions. (B) Schematic representation of cell or developmental stage-specific niches. (i) HSC niches: a number of studies have determined that HSCs are found scattered throughout BM tissues adjoining the extraluminal surface of sinusoidal endothelial cells and in contact with stem cell factor–producing, LepR⁺ CAR cells^13,14,50,90  and, in some cases, to megakaryocytes.^103,104  In addition, a minor fraction enriched in quiescent HSCs has been reported to localize in a protective niche adjacent to nonpermeable arterioles, under the regulatory influence of neighboring Nestin-GFP^hi, NG2⁺ mesenchymal cells (nes) and nonmyelinating Schwann cells (in yellow).^32,55,65  (ii) B-cell progenitors have been suggested to sequentially migrate along different niches as they progress through maturation. Early stage pre-pro B cells are mostly found adjacent to the cell bodies of CAR cells and migrate toward IL-7–expressing CAR cells as they enter the pro B cell stage. Mature B220⁺IgM⁺ B cells, in turn, move away from both stromal cell types.¹⁰⁵  The latest stages of B-cell maturation occur intravascularly, within sinusoids, where the dynamic behavior of immature B cells has been visualized in vivo.¹⁰⁶  In addition, early pro B cells (B220⁺CD43⁺) have been shown to accumulate in endosteal zones and gradually decrease in frequency toward bone-distal marrow regions.⁶⁸  (iii) Megakaryocytes (Mk) are found in close adjacency to the endothelial surface of sinusoidal vessel wall, which they traverse in the form of protrusions from which proplatelets are continuously shedded into the venous circulatory system.¹⁰⁷  Red blood cell development takes place in so-called erythoblastic islands (Ei), where erythroid precursors proliferate, enucleate, and terminally differentiate into reticulocytes around a central macrophage.¹⁰⁸  Plasma B cells are long-lived antibody-secreting cells that have been found in physical association to CAR cells, megakaryoctes, and eosinophils (eos).^109-111  (iv) Although a significant fraction (30%) of early lymphoid progenitors (Lin⁻IL7-rα⁺) has been shown to lie proximal to mature, bone-lining osteoblasts,⁴⁸  the vast majority of phenotypically defined CLPs are in contact with IL-7–expressing CAR cells.⁵⁰  Quiescent CD4⁺ memory T cells are found scattered throughout the BM in contact with perisinusoidal IL-7–secreting stromal cells.¹¹²  Regulatory T cells (T_regs) have been reported to lodge in close proximity to the endosteum of trabecular bone.¹¹³