Hormonal influences on the muscle-bone feedback system: a perspective.

Hormones, muscle and bone tissues have co-existed virtually during the whole evolution of vertebrates, and it is obvious that they constitute a complex system able to cope with needs and challenges arising from a variety of physiological and locomotive needs. All body movements are produced by co-ordinated contractions of skeletal muscles, while consequent dynamic muscle work provides the fundamental source of mechanical loading to the skeleton. Mechanical competence of the skeleton is principally maintained by a mechanosensory feedback system that senses the loading-induced deformations within the bones and maintains the skeletal rigidity through structural adaptation. In contrast to the prevalent view suggesting a modulatory effect of hormones on the sensitivity of the mechanosensory system, a new conceptual scheme is proposed. In particular, it is argued that the mechanical and hormonal functions in the skeleton are fundamentally independent but can be seemingly interactive through hormonally-induced modifications in the bone structure, those basically forming a mineral reservoir for maintenance of physiological homeostasis. Whenever needed, utilization of this strategically placed reservoir would not essentially compromise the mechanical competence and locomotive capability of the skeleton. Although plausible, the present view is necessarily speculative and awaits corroborative experimental evidence.