Size and distribution of histochemical fibre types in masseter muscle of adults with different states of occlusion

Abstract

Histochemical staining methods were applied to biopsy specimens from the masseter muscles of 17 healthy subjects with mandibular prognathism and 4 subjects with benign parotid lesions. Fibre sizes and percentage fibre distributions were assessed in transverse sections stained for myofibrillar ATPase at pH 9.4. Size or percentage distribution of different fibre types was not significantly correlated to the state of occlusion. In subjects with unilateral chewing only, the frequency of Type I fibres (lightly stained) was higher ( P < 0.01) and the frequency of intermediate fibres (moderately stained) lower on the chewing side than on the non-chewing side ( P < 0.05). Type I fibres were invariably larger than Type II (strongly stained) fibres—mean value 33.2 μm versus 17.9 μm. There was no significant relationship between the sizes of Type I and Type II, but there was between Type I and intermediate fibres ( P < 0.05). Intermediate fibres were generally intermediate in size to Type I and Type II, mean value 23.8 μm. The range of percentage distribution of the 3 types of fibres was wide. Average values were Type I 29%, type II 57% and intermediate fibres 14%. There was an indication that fibres from different depths of the muscle differ in size and in percentage distribution. The size of Type I and its percentage distribution were significantly negatively correlated ( P < 0.01). The percentages of Type II and intermediate fibres were negatively correlated ( P < 0.001) but not those of Type I and intermediate fibres. It is suggested that ATPase-intermediate fibres may result from transformation of Type II fibres.