**3.3 Bone marrow niches**

*Regenerative Medicine*

**3.2 Bone marrow-specific regions**

red bone marrow is a rich source of bone marrow-derived cells and present in most skeletal system bones of the iliac crest, tibia, spine vertebrae, humerus, calcaneus, ribs, and near point of attachment of long bones of legs and arms. In this wellshielded environment, an estimate of 500 billion cells per day can be produced, in particular erythrocytes, granulocytes, and platelets [10]. Regenerative medicine applications have a focus on the use of the red bone marrow as it contains myeloid and lymphoid stem cells and MSCs. In contrast the yellow marrow consists primarily of fat cells with poor vascularity and is deprived of the multipotential MSCs [11].

The BM cavity in the trabecular bone is subdivided into four regions: endosteal, sub-endosteal, central, and perisinusoidal regions [12]. In **Figure 1**, the four regions, according to the model of Lambertsen and Weis, have been adopted and modified [13]. In general, the bone marrow consists of a hematopoietic component (parenchyma) and a vascular component (stroma). The parenchyma includes hematopoietic progenitor and hematopoietic stem cells (HSCs), which are localized close to the endosteum and around the blood vessels. BM stroma cells, including endothelial cells, are recognized as multipotential non-hematopoietic progenitor cells, capable of differentiating into various tissues of mesenchymal origin, including osteoblasts, chondrocytes, tenocytes, endothelial cells, myocytes, fibroblasts, nerves, and adipocytes, as verified in in vitro and partially in in vivo research [14, 15]. The bone marrow's microvasculature includes single layers of endothelium arising in sinusoids, where they also contribute in rolling extravasations of leukocytic cells into and out of the BM tissue structures. The function of the vasculature and BM-derived endothelial cells is that they provide a barrier between the BM compartment as a functional and spatial entity from the extra-lymphoid BM section and the peripheral circulation, as described by Kopp et al. [9]. The endothelial cells likewise contribute

*Bone marrow subdivisions. On the left side, the Aspire introducer (Aspire Bone Marrow Harvesting System™, EmCyte Corporation, Fort Myers, FL, USA) has passed the cortical bone entering the marrow cavity. The harvesting cannula is inserted through the introducer in the marrow cavity. On the right side, a representation of the subdivisions in the bone marrow cavity subdivisions is indicated, showing the endosteal, sub-endosteal, central, and perisinusoidal regions. The endosteal and sub-endosteal regions compose the endosteal niche, harboring the proliferative and quiescent HSC-MSC niches. The marrow tissue is extracted via the side holes of* 

*the harvesting cannula (adapted and modified from Lambertsen and Weis [13]).*

**4**

**Figure 1.**

A *niche* is defined by anatomy and function. Stem cell niches are defined as specific cellular and molecular microenvironments regulating stem cell and progenitor functions. A niche consists of signaling molecules, intercellular contact, and the interaction between stem cells and their neighboring extracellular matrix (ECM). This three-dimensional microenvironment is thought to control genes and properties that define "stemness," including the control and balance between quiescence, self-renewal, proliferation, and differentiation of diverse cell types. Additionally, the microenvironment provides stem cell autonomous signaling mechanisms [17, 18], and it engages in specific cascades to a stress response [19]. Acquired and prepared BM stem cells from one of the niches and subsequently injected into a totally different microenvironment can potentially differentiate into cell types of this new environment [20]. Zhao et al. used a rat stroke model in which BM-MSCs were transplanted into neural tissues. They demonstrated that MSCs originating from the BM-MSC niche differentiated into neuronal cells after transplantation into the neural microenvironment [21]. Their observation revealed the plasticity potential of BM-MSCs, as well as the possible influence of the recipient niche, as BM-MSCs were capable of dedifferentiation into cells from other cell lineages. Their finding has potentially significant clinical implications for regenerative medicine applications overall. Since autologously prepared MSCs originate from their specific and original BM niche but are used in other cellular tissue types to treat various pathologies, they can be successfully engaged in tissue repair and regeneration through regenerative medicine application techniques. This is a distinctly different approach in the physiological release of newly formed BM cells, because they are retained in the BM cavity until they mature and thereafter released in the vascular peripheral circulation [15].
