[Histological and ultrastructural characteristics of jaw-closing muscles. A review].

The aim of this work is giving, through a wide literature review, a detailed analysis of the histological and ultrastructural characteristics that distinguish masseter and temporal muscles from the other skeletal muscles. Furthermore we'll explain the functional meanings of these differences. We developed the following points: fibre type composition and relative frequency of the various fibre types, fibre size, myosin composition, capillarization and age-related changes. With standard staining method for the myofibrillar ATPase, besides the two main fibre types, I and II, in the masticatory muscles a moderate share of IM fibres with intermediate stainability, which usually don't appear in adult skeletal muscles, are shown. The relative frequency of the various fibre types is also peculiar, with a prevalence of type I fibres in almost every portion of the masseter and temporal muscles, which therefore are functionally slow muscles. Another unusual characteristic is also the mean diameter of type I fibres, that are commonly larger than type II fibres. This finding suggests that masticatory muscles are adapted to carry out specially prolonged and fatiguing tasks. The findings about contractile protein patterns and the changes in myosin heavy chain composition during ageing are also relevant. The deep differences between jaw-closing and limb and trunk muscles are reviewed on the basis of their special functional activities.