The main function of BM is to provide circulating blood with an optimal supply of erythrocytes, leukocytes, and platelets. In addition to this, BM supplies hematopoietic stem cells (HSCs), endothelial cells, MSCs and other precursor cells. The human skeleton possesses red BM which is hematopoietically active, and yellow, which is hematopoietically inactive.
Red and yellow BMs have different cellular and molecular content: Yellow BM comprises 95% fat cells, whereas the red BM comprises 60% hematopoietic cells. The whole skeleton is filled with red BM at birth, however, during childhood a physiological conversion of red BM into yellow BM occurs. The conversion of red to yellow marrow and progresses to the axial skeleton, and this entire process may be completed by the age of 25 years.
BM is a potent source of stem and progenitor cells, and this characteristic has gained attention for cell-based therapies in orthopedics[11-13]. Given the diversity in stem cell lineages and phenotypes in the marrow, BM represents a functional organ in which distinct types of cells function cooperatively. Specifically, HSCs play a critical role in the formation of the hematopoietic microenvironment, whereas MSCs support hematopoiesis and both MSCs and/or skeletal stem cells are responsible for the development and maintenance of skeletal tissues[14,15].
MSCs are non-hematopoietic stromal cells that are composed of a small fraction (0.001%–0.01%) of the stem cell content in BM. MSCs are found in other tissues, such as adipose tissue, placenta, and umbilical cord, and although they differ in their differentiation potential, they possess common features associated with those from the BM, which might imply that MSC-like populations share a similar ontogeny[17,18].
MSCs exhibit the potential ability to differentiate into mesodermal linage cells (e.g., cartilage, bone, fat, muscle, meniscus and tendon), which is fundamental for the regeneration process. Moreover, these cells have paracrine effects, thus are able to alter their local microenvironment.
Given the varying MSC markers that laboratories may use to characterize these cells, there is a lack in standard phenotypic criteria. This heterogeneity is also due to the fact that MSCs are able to express a range of cell-lineage specific antigens that may differ depending on the culture preparation, culture duration, or plating density[21,22]. However, the Mesenchymal and Tissue Stem Cell Committee of the International Society for Cellular Therapy have proposed minimal criteria to characterize MSCs, which comprise the following attributes: Must be plastic-adherent when maintained in culture; must be able to differentiate in vitro into chondroblasts, adipocytes and osteoblasts; and must express CD105, CD73 and CD90, and lack expression of CD45, CD11b, CD34 or CD14, CD79α or CD19 and HLA-DR surface molecules.
MSCs lack significant immunogenicity and can be easily isolated, which allows allogenic transplantation. In allogenic circumstances these cells should be considered immune evasive. However, the effects of MSCs in cellular-based therapies depends on the ability of these cells to home and engraft (long-term) into the target tissue. One theory suggests that MSCs have a rather short life span and are phagocytized by monocytes and subsequently stimulate the production of T-reg cells which may very well contribute to the overall clinical improvement.
Cells from injured tissue release chemokines responsible for MSC recruitment. Once in the target tissue, MSCs are able to modulate wound-healing responses by reducing apoptosis and fibrosis, attenuate the inflammatory process and stimulate cell proliferation and differentiation via paracrine and autocrine pathways. These properties are attributed to the ability of MSCs to release key agents, such as vasculoendothelial growth factor, transforming growth factor beta (TGF-β), stromal-derived factor 1, and stem cell factor, among others. Also, they induce a downregulation of pro-inflammatory cytokines, including interleukin 1 (IL-1), IL-6, interferon-γ, and tumor necrosis factor α[16,27,28]. MSCs also possess immunomo-dulatory properties as they are able to inhibit the activation of type 1 macrophages, natural killer cells, and both B and T lymphocytes.
HSCs also known for expressing CD34+, are located at the top of the hematopoietic hierarchy. They are responsible for the daily supply of more than 100 billion mature blood cells, including erythrocytes, leukocytes, and platelets. This process, called hematopoiesis, is of extreme importance in the maintenance and regulation of the immune system, especially for the cells from myeloid lineage, such as granulocytes, monocytes and dendritic cells, due to their short half-life[31,32].
Past studies have reported that the hematopoietic and stromal environments are related and overlapped. For example, Simons et al observed generations of fibroblasts colony-forming unit (CFU-F) from CD34+ human BM cells. Also, it has been reported that the number of osteoblast progenitor cells is higher in sorted CD34+ cells (1/5000 approximately) than in CD34 - populations (1/33000), and when these sorted cells were cultured in a long-term marrow system, the generation of a heterogeneous population that included smooth muscle cells, adipocytes, fibroblast and macrophages was observed. This possible relation was then supported by Mehrotra et al who reported that HSC give rise to osteocytes and chondrocytes in an experimental study.
Immune cells – Leukocytes have a common origin from the hematopoietic stem cell and develop along distinct differentiation pathways in response to external and internal stimuli. In order to promote regeneration, leukocytes circulate through the blood and lymphatic system and are recruited to specific regions of the body when damage occurs.
The mononuclear phagocyte system represents a subset of leukocytes that was originally described as BM-derived myeloid cells. Monocytes are immune effector cells that, although they circulate in the blood, BM and spleen, they do not proliferate in a steady state[36,37]. They are equipped with chemokine receptors that mediate migration from blood to the injured sites and produce inflammatory cytokines. During inflammation, the monocytes differentiate into dendritic cells (DC) or macrophages, and this process is likely determined by the inflammatory environment and pathogen-associated pattern-recognition receptors.
Macrophages are phagocytic cells that reside in lymphoid and nonlymphoid tissues. Given the broad range of pathogen-recognition receptors that macrophages possess, they are known as an efficient tool at maintaining tissue homeostasis as they provide clearance of apoptotic cells and remodeling of the extracellular matrix[39,40]. Macrophages play a key role in recruiting and inducing the proliferation of osteoblasts, stem and progenitor cells as they secrete bone morphogenetic proteins, IL-1β, TGF-β, platelet derived growth factor and insulin-like growth factors, in areas of infection or injury in different tissues in the body. Extrinsic stimuli that induce an inflammatory process, such as infection or injury, promote changes in gene transcription that classify macrophages as type 1 (M-1) and type 2 (M-2). The M-2 type offers a healing function, while M-1 promotes the host defense. After injury, M2 can switch into M1, and this change is modulated by the cytokines such as interferon-γ, and M2 type by IL-4.
Neutrophils belong to a polymorphonuclear family and are known for being the main cell type response to bacterial infections. It was reported that neutrophils are highly plastic cells influenced by environmental cues that result in a site-specific neutrophil transcriptome as they migrate from BM to sites of inflammation. As a granulocyte, which includes eosinophils and basophils, neutrophils are able to secrete a variety of cytokines, such as TGF-β, vasculoendothelial growth factor and platelet derived growth factor, playing an important role in angiogenesis and vasculogenesis. Neutrophils undergo spontaneous apoptosis to regulate the resolution of inflammation.