Mechanical influences on cells, tissues and organs - 'Mechanical Morphogenesis'.

Cells can sense changes in their mechanical environment and promote alterations and adaptations in tissue structure and function. Mechanical stimuli regulate such fundamental processes as cell division and differentiation and determine tissue form. The current editorial outlines the general scope of a subject area we have called 'mechanical morphogenesis'. We are promoting it as an area of special interest for future issues of the European Journal of Morphology. Clearly, mechanical loading is of pivotal importance to the development, function and repair of all tissues in the musculoskeletal system, including bone, ligament, tendon, skeletal muscle, intervertebral disc and meniscus. Bone in particular has attracted special interest and mechanical strain is central to both Wolff 's law and Frost's 'mechanostat' model of bone behaviour. But it is skeletal muscle that shows the most obvious and rapid response to altered load, with striated muscle fibres hypertrophing with strength-training programmes, and atrophing in the absence of adequate mechanical stimulation. Articular cartilage, together with tendons and ligaments is also responsive to changing exercise levels, and either abnormally high or low loads are detrimental. However, the influence of mechanical forces extends to many other organ systems, including the respiratory, cardiovascular, nervous and integumentary systems. The bronchial mucosa and the alveoli are subject to tensile and compressive loading during the volume changes that occur in respiration, and surface tension is also of paramount importance. The whole form of the cardiovascular system is driven by the haemodynamic influences of blood, and atherosclerosis has an underlying mechanical basis. The characteristic plaques tend to occur at sites of obvious mechanical significance - regions of arterial branching and curvature, where shear stress on the vessel wall may be low, but tensile stress high. Sensory perception by the nervous system has a well known mechanical basis and the cochlea is perhaps the most elaborate example of a site where sensory cells transduce mechanical forces and relay information to the brain. Mechanical force has also been proposed as a regulating factor in controlling axonal growth. Finally, the integumentary system has several structural adaptations that obviously relate to the influence of mechanical forces. The thickened layer of keratinised squames in the palms and soles is directly related to the high levels of shear at these locations.