Page 29 - Physico-Chemical Niche Conditions for Bone Cells

stimulation or signals [17]. The MuSC niche is a typical example of a “storage niche”. Normally MuSCs are quiescent, but upon stimulation during injury or mechanical overload, they are activated, proliferate, and generate committed muscle precursors (myoblasts) or self- renewing daughters. Committed myoblasts differentiate and fuse together to restore damaged myofiber segments or fuse with the host myofiber and add new myonuclei to the myofiber, whereas self-renewing cells return to quiescence and repopulate the niche between the basal lamina and the sarcolemma.
Cellular Niche Conditions for (Early)-Osteocytes and Myoblasts
An overview of the bone and MuSC niches is provided in Figure 1.
Cell types of bone
Bone contains osteogenic cells, periosteal fibroblasts, osteoblasts, pre-osteocytes, osteocytes, lining cells, and osteoclasts, which all interact [21]. Osteogenic cells can divide, proliferate, and differentiate into osteoblasts. Osteocytes are more responsive to mechanical loading than osteoblasts and periosteal fibroblasts, and affect osteoblast proliferation and differentiation [21]. Osteocytes subjected to mechanical loading inhibit osteoclast formation and resorption by means of soluble factors [21]. The more deeply embedded osteocytes are only connected via their cell processes to neighboring osteocytes, but the most recently incorporated osteocytes (early osteocytes) are connected to neighboring osteocytes and to cells lining the bone surface. Moreover, some osteocytes processes oriented towards the bone surface appear to pass through the layer of lining cells, thereby establishing a direct contact between the osteocyte syncytium and blood vessels, or the extraosseus space. This latter intriguing observation by Kamioka et al. [22] suggests the existence of a signalling system between the bone marrow compartment and osteocytes. Bidirectional signalling between osteoclasts and osteocytes may exist through interaction between ephrinB2 on osteoclasts, and EphB4 present on osteocytes [23]. Though purely hypothetical, osteoclast-to-osteocyte communication could be biologically relevant. In addition, the communication between osteoblasts and osteocytes has been investigated in vitro using the MLO-Y4 cell model under mechanical loading [24]. However, one limitation of the MLO-Y4 cells is that they only secrete low amounts of sclerostin. IDG-SW3 or OCY454 osteocytes have been employed as a model for early osteocytes. Moreover OCY454 treated by mechanical loading do secrete a.o. sclerostin, suggesting that OCY454 cells are representing an early osteocyte model with the potential to sense mechanical loading [24,25].
Chapter 2
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