Page 27 - Physico-Chemical Niche Conditions for Bone Cells

INTRODUCTION
Development and maintenance of bone and muscle mass are fundamental processes in mammalian physiology, since the musculoskeletal system allows movement and provides support and protection for vital organs [1]. The musculoskeletal system does not only have an essential mechanical function, muscle and bone also provide a systemic supply of metabolites and signaling molecules. Bone serves as an ion pool for maintaining serum levels of Ca2+, Mg2+, and other physiological key elements, and produces active endocrine products such as FGF23 and osteocalcin [2-4]. Muscle, as a consumer and storage site for glucose and amino acids, secretes various myokines (myostatin, IL-6, IL-7, IL-8, CXCL-1, LIF, IL-15, Akt1, FGF- 21, BDNF, calprotectin, erythropoietin, and IL-4) thereby affecting metabolism in other tissues such as bone [5,6]. In terms of the far-reaching functions of these tissues in human health, a detailed understanding of the conditions that influence bone and muscle health is important, particularly those conditions that significantly decrease bone and muscle mass during prenatal or postnatal development or in adults as a consequence of ageing, as well as those conditions that prevent loss of bone and muscle mass. Such knowledge will help to maintain physical mobility, decrease the risk of human injury and metabolic diseases, and ultimately improve the quality of life and life expectancy [1].
Bone and muscle mass are influenced by several factors, including nutrition, hormones, genetics, growth factors, and mechanical stimuli [7,8]. Muscle and bone are not the only tissues sensitive to mechanical loading. Moreover, bone and muscle cells have few similarities and reside in very different niche conditions. However, close communication and/or interaction between bone and muscle is very important. In this review we focus on mechanical stimuli as influential factor, since changes in mechanical stimuli are incredibly potent regulators of both human bone and muscle mass. It is well known that increased mechanical stimulation of skeletal muscle leads to an increase in muscle fiber cross-sectional area (i.e. hypertrophy), while reduced mechanical stimulation results in rapid muscle atrophy [9,10]. At the critical stages of bone development and bone formation in response to injury, mechanical stimuli increase bone mass and/or reinforcement of bone tissue, bone mineral accrual, and bone strength. In contrast, reduced mechanical stimulation of mature bone and loss of mechanical stimuli at critical stages of bone growth, cause loss of bone mass, bone mineral accrual, and bone strength [11,12]. Importantly, the maintenance and development of bone mass is affected by skeletal muscle-derived mechanical stimuli [13]. Physical exercise might thus provide a safe and relatively cheap therapeutic intervention to maintain or enhance bone and muscle mass, and thereby overall health and fitness, at least in healthy individuals. Unfortunately, real progress is hampered because the knowledge on the exact cellular processes activated by physical stimuli seems fragmented. There is relatively little knowledge
Chapter 2
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