Page 176 - Physico-Chemical Niche Conditions for Bone Cells

General summary
GENERAL SUMMARY
Bone homeostasis is regulated and maintained by a tunable balance between bone-forming cells (osteoblasts) and bone-resorbing cells (osteoclasts). Under healthy physiological conditions, bone has several functions, including (a) protection of the central nervous system, (b) release of Ca2+ or other ions, (c) mechanical support for soft tissues, and (d) housing and support for hemopoiesis. At the macro and microscopic levels, the structure and mass of bone are regulated by the genetic blueprint, physical factors, and (bio)chemical factors which regulate bone homeostasis.
Bone and muscle communicate via secreted osteokines, myokines, and physical forces which are generated by exercise, gravity, locomotion, and external devices. Bone and muscle cells sense those forces via focal adhesions and translate them into biochemical responses, modulating the cell behavior, e.g. spreading, proliferation, differentiation, maturation, and tissue formation. This might cause changes in focal adhesion expression, extracellular matrix constituents, and cytoskeletal elements, as well as in the initiation of bone formation and muscle atrophy or hypertrophy. Bone and muscle mass, resistance to mechanical loading, and the stiffness of cells, matrix, tissues are increased, which affects bone fracture resistance and muscle power. This generates a continuous and dynamic reciprocity of physico-chemical interaction in bone and muscle. This process induces adaptation of the respective cell secretome (e.g. exosomes), which maintains an adequate homeostasis in bone and muscle. Lessons from disuse, microgravity, and unloading indicate that gratuitous tissue should be reorganized or removed, while inflammation and immobility cause bone and muscle fatty infiltration, as well as degenerative diseases, e.g. osteoporosis and sarcopenia. Therefore, it is important to maintain homeostasis in the bone and muscle cell microenvironment or niche. In chapter 2, we reviewed the physicochemical niche conditions and mechanosensing by osteocytes and myocytes. This chapter provides an overview of the bone and muscle cell physicochemical microenvironment, and how these are sensed and determine bone and muscle cell fate and function. In addition, we discussed how state-of-the art imaging techniques may help to increase our understanding of physicochemical conditions of cells and the mechanisms involved in the cellular sensing of these conditions.
Bone cell fate and behavior are affected by various physical stimuli via the regulation of intracellular signaling pathways. Direct sensing of mechanical loading or lack thereof by bone cells (e.g. osteocytes and osteoblasts) plays an important role in shaping of bone tissue. After sensing mechanical signals, those cells need to translate the mechanical signal into a biological response, resulting in changes in bone cell activity. Therefore, osteoblast cell and nucleus morphology and volume modulation by mechanical loading were investigated in
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