Fibre type grouping in porcine masseter and soleus muscles assessed by the enclosed fibre type concept. A statistical and computational analysis.

Abstract

Muscle function is determined not only by the frequency of slow (type I) and fast (type II) fibres but also the spatial fibre type organization. During ageing as well as in muscle disorders, divergencies from the normal pattern may occur and are expressed as 'fibre-type grouping'. To elucidate whether intramuscular differences in spatial fibre type arrangements may underlie the functional heterogeneity of the porcine masseter, the arrangements of intrafascicular type I and type II fibres were assessed in terms of the number of enclosed fibres in whole fascicles. The intrafascicular proportions of edge- and centrally located type I and type II fibres were investigated. Two hundred and forty-two porcine masseter fascicles (six masseters) and 63 pig soleus fascicles (five soleus muscles) were investigated by ATP-ase histochemistry. All fascicles were from 11 domestic pigs (1 year, 70-90 kg body weight, all female). Sixty-nine to 90% of the total fibres were type II fibres in the porcine masseter (P < 0.01). In four of five soleus muscles the type II fibre population was dominant. No enclosed type I fibres could be identified in the porcine masseter muscles. This was in contrast to the finding of 1270 enclosed type II fibres. Five enclosed type I fibres (two soleus) and 361 enclosed type II fibres were identified in the pig soleus. The spatial analysis indicated that the fibre type arrangements in the porcine masseter muscles were not homogeneous. No biopsies with random type II fibre organization were identified. Furthermore, half the number of biopsies showed type I fibre segregation. These data suggest that the porcine masseter cannot be considered as having one homogeneous structural identity.