*3.3.1 Hematopoietic and mesenchymal niche*

HSC niches are present in various (prenatal) tissues, like the aorta-gonadmesonephros region and the yolk sac, followed by the placenta, fetal liver, spleen, and bone marrow. Postnatally, the bone marrow is the primary site of HSC presence [22]. The model of the HSC niche was first described by Schofield in 1978 [23], later confirmed by others, to describe the physiologically limited microenvironment in which HSCs, MSCs, and their progenitors reside in the bone cavity where they are enfolded by BM stromal cells [24], covered by the bony structure of the BM cavity. The stem cell niches in bone have been extensively described by Yin and Li, providing insights into the actions of osteoblastic and vascular niches, revealing central roles for numerous signaling and adhesion molecules [25]. A significant portion of these hematopoietic cells is found next to the endosteal bone surface, designating a clear role for osteoblasts in the regulation of HSCs and thus hematopoiesis [26]. Based on flow cytometry research by Kiel et al., HSCs are more likely than other hematopoietic cells to be immediately adjacent to a sinusoid, in the trabecular region of the BM [27]. This location suggests that HSCs and their niche may be directly, or indirectly, regulated by factors present near the bone planes. The HSC niche is comprised of many different niche constituents including osteoclasts, endothelial cells, fibroblasts, adipocytes, and the HSC progenitor cells [28].

#### *3.3.2 Perivascular niche*

The BM is highly vascularized, with large central arteries branching into progressively smaller microvessels like arterioles and transitioning into venous sinusoids near the bone (endosteal) surface. Therefore, it has been suggested that HSCs are maintained in a perivascular niche by endothelial or perivascular cells, as they are frequently located adjacent to the blood vessels [29]. These occurrences resulted in the expression of various perivascular mesenchymal cell makers CD146, stromal cell-derived factor-1 (SDF-1) also referred to as CXCL12, and Nestin-GFP, defining the heterogenous BM stroma cell composition [9], including the MSCs that surround the blood vessels [30]. The more perivascular nature of MSC niches was validated by Shi and Gronthos, demonstrating the expression of α-smooth muscle actin (αSMA) at perivascular sites, with the immunohistochemical localization of specific CD marker cells [31]. Mores studies confirmed the presence of MSCs in BM, expressing a Nestin-GFP transgene, localized and attached around the BM blood vessels and part of the perivascular HSC niche [32]. Kunisaki et al. indicated that most HSCs do not only have a perivascular presence, but they are preferentially located in the BM endosteal regions. The endosteal regions contain a complex network of stromal cells as well that have been implicated in HSC maintenance, including arteriolar and venous endothelial cells, pericytes, and chemokine (C-X-C) ligand 12 (CXCL12) reticular cells. Their study also suggested that quiescent HSCs localize preferentially to small arterioles near the endosteum, suggesting that distinct niches may exist for both quiescent and proliferating HSCs [33]. From all these findings, it can be concluded that pericytes are in fact MSCs, because they can differentiate in osteoblasts, chondrocytes, and adipocytes [34].

#### *3.3.3 Megakaryocyte niche*

Megakaryocytes (MK) are the precursor cells of blood platelets. BM hematopoietic cells are responsible for platelet production. MK may regulate HSCs indirectly as they are closely associated with BM sinusoidal endothelium, extending cytoplasmic protrusions into the sinusoids to produce platelets. A direct regulation of HSC by MK through signaling of transforming growth factor beta 1 was established, with activation of quiescent HSCs and increased proliferation rate. In the event of a sudden response to systemic stress signaling, fibroblast growth factor-1 as part of the MK growth factor pool will start signaling HSCs and will overshadow the TGF-b1 signaling in order to stimulate high volumes of HSC expansion [35].

#### **3.4 Extracellular matrix**

The role and function of the extracellular matrix (ECM) can be defined as key structural-functional components of cell niches, including soluble factors, cell-cell contacts, and cell-matrix adhesions present in these microenvironments. ECM components include fibrillar proteins, with, among others, collagen fibers, fibronectin, and other filamentous network components. The ECM's mechanical stability is provided by collagen [36]. Other significant ECM components supporting the BM niches are glycosaminoglycans and mainly hyaluronic acid via its receptor CD44. The surface marker is also expressed by MSCs and HSCs [37]. Intracellular signaling in the ECM occurs through cytokine and growth factor membrane receptors, similar to the MSC niche. These cytokines and receptor activities contribute to cross

**7**

*The Rationale of Autologously Prepared Bone Marrow Aspirate Concentrate for use…*

talk between ECM components and MSC niches, provoking cell differentiation. For instance, Djouad et al. demonstrated that the induction of MSC differentiation towards chondrocytes in articular cartilage was induced and/or influenced by molecules from both the MSC niche and the ECM components of this microenvironment, leading to chondrogenic differentiation of MSCs [38]. Other studies suggested that ECM deposited by microvascular endothelial cells enhances MSC endotheliogenesis [39]. In general, no specific ECM components are identified that maintain MSCs in their immature state, as a niche matrix would do. However, it has become clear that the ECM can regulate MSC differentiation on a solitary basis, indicating potential applications for regenerative medicine applications and tissue

Exploiting BM preparations at POC seeks to overcome the limitations of ex vivo MSC culturing. Clinicians utilizing regenerative medicine applications have a growing interest in using the concentrated bone marrow products, since it is well acknowledged that BM is a plentiful source of MSCs, progenitors, and other cells residing in the trabecular part of flat and long bones, acquired via appropriately performed BMA procedures [40, 41]. The regenerative medicine market is rapidly growing, with many procedures performed in musculoskeletal, orthopedic, and spinal disorders, wound care management including critical limb ischemia, and tissue engineering [42–45]. Several groups have mentioned some considerations when performing BM harvesting procedures, addressing a variety of factors that have an impact on patient comfort and the quality of the harvested BM. Emphasis was given to procedural safety issues when using harvesting needle systems, level of experience of the operator, the choice for concentration technology and centrifugation devices, and pain management [46]. Autologous regenerative medicine BM-MSC applications may range from a harvesting a low volume of BM and direct, unprocessed, tissue injection to the use of centrifugation protocols to concentrate and filter the BMA prior to inject-

Various bone marrow needle harvesting systems are available on the market, each with their own proprietary design characteristics and thus marrow aspiration dynamics when transferring marrow cavity cells through a needle system into collection syringes. In **Figure 2**, three different needle systems are shown. Potentially, different needle design features might affect the quality and viability of the harvested BM tissue, as well as the cellular yields. Therefore, BM needle system features and harvesting dynamics are important considerations when considering BMA procedures. Physicians have been using a variety of harvesting needles during the last decades, including the traditional Jamshidi™ harvesting needle (Ranfac Corporation, Avon, MA, USA). Based on design differences, not every BMA is born equal, and cellular yields, composition, and viability might vary among harvesting devices. For interpretation purposes, some of the cellular difference between two newly developed BMA needle harvesting systems, the Aspire Bone Marrow Harvesting System™ and the Marrow Cellution Bone Marrow Aspiration Device™ (EmCyte Corporation, Fort Myers, FL, USA, and Ranfac Corporation, Avon, MA, USA, respectively) is shown. A significant difference between the two harvesting

*DOI: http://dx.doi.org/10.5772/intechopen.91310*

**4. Bone marrow aspiration procedures**

**4.1 Bone marrow harvesting needle systems**

engineering.

ing it in patients [47].

*The Rationale of Autologously Prepared Bone Marrow Aspirate Concentrate for use… DOI: http://dx.doi.org/10.5772/intechopen.91310*

talk between ECM components and MSC niches, provoking cell differentiation. For instance, Djouad et al. demonstrated that the induction of MSC differentiation towards chondrocytes in articular cartilage was induced and/or influenced by molecules from both the MSC niche and the ECM components of this microenvironment, leading to chondrogenic differentiation of MSCs [38]. Other studies suggested that ECM deposited by microvascular endothelial cells enhances MSC endotheliogenesis [39]. In general, no specific ECM components are identified that maintain MSCs in their immature state, as a niche matrix would do. However, it has become clear that the ECM can regulate MSC differentiation on a solitary basis, indicating potential applications for regenerative medicine applications and tissue engineering.
