Cellular and molecular aspects of muscle growth, adaptation and ageing

Although major advances have been made over the past few decades in prosthetic dentistry, deterioration in oral function and altered facial appearance are still common accompaniments of ageing. Molecular biology methods now allow us to understand these age-related changes at the level of gene expression. Muscle loss as well as bone loss still present major problems, the magnitude of which increases as the age profile of our society changes. Both muscle and bone tissue respond to mechanical signals for which bone depends on muscle and for muscle, stretch has been shown to be important as it induces protein synthesis and an increase in girth as well as length of the muscle fibres. The latter involves the production of more sarcomeres in series so that the jaw muscles adapt to a new functional length following changes in vertical dimension of occlusion. It also determines the postural position of the lower jaw. In our investigations into the control of muscle mass we have recently cloned a growth factor which is expressed in exercised and/or overloaded muscles. This comes in two forms: an autocrine or local form and a paracrine or systemic form. Both growth factors influence muscle growth markedly and it is probable that the systemic type is also involved in maintenance of bone. The discovery of these growth factors provides the mechanisms by which mechanical signals are transduced into chemical signals that in turn regulate gene expression and protein synthesis.