Foetal and/or neonatal MHC could be demonstrated biochemically only in a limited number of samples. IIB, neonatal, alpha-cardiac and other fibres was low. Even in biopsies that contained relatively large amounts of these last three fibre types, the amount of neonatal and/or alpha-cardiac MHC detected on SDS-PAGE was limited, suggesting that these MHCs are a minor component in the fibres in which they arc expressed. EMT inhibitor-2 = number of fibres analysed. 3.2. Masseter muscle Muscle fibre types Because myofibrillar ATPase staining was highly variable in a large portion of masseter fibres a classification scheme was developed using antibody and NOV ATPase staining together. Eight fibre types could be identified (Table 1). The type I, IIA and IIB fibres were found in addition to two other groups of fibres. The first group contained a mixture of IIA and I MHC which was divided into two fibre types C type IM and IIC C since this classification already exists in the literature (Ringqvist et al. 1982; Eriksson and Thornell 1983). Relative to type IIC the type IM fibres stained less intensely for the IIA and fast antibody and were less EMT inhibitor-2 reactive for ATPase after alkali preincubation. Both had similar positive reactivity for the type I antibody, but after acid preincubation type IM fibres were more reactive for ATPase. IM fibres were consistently more reactive than type I fibres and less reactive than type IIA and IIB fibres after alkali preincubation (Table 1). The second group contained the neonatal, cardiac and other fibre types identified almost solely on their antibody reactions. Type neonatal contained fibres that stained positively for the neonatal antibody, type cardiac contained fibres that stained positively for the alpha-cardiac antibody and type other contained fibres that stained positively for both neonatal and alpha-cardiac antibody. It was impossible to describe these final three fibre type categories further using immunohistochemical staining since type I and/or type IIA MHCs were often present, but in highly variable proportions (Fig. 2 and Table 1). Type IIM fibres, type II masticatory fibres that stain positively for the IIM antibody, were never found. Open in a separate window Fig. 2 Serial sections of masseter stained for reactivity to anti-fast (A), anti-IIA (B), anti-I (C) and anti-neonatal EMT inhibitor-2 (D) MHC antibodies; and for myofibrillar ATPase activity after preincubation in buffer of pH EMT inhibitor-2 10.4 (E), 10.2 (F). 4,6 (G) and 4.3 (H). Small arrow marks a type IIA fibre, large arrow a type IM fibre and arrow head a type IIC fibre. Magnification, 95. 3.3. Fibre morphology and distribution Average fibre diameters ranged from smallest to largest as follows: type IIA, neonatal, other. IIC, atrial, IIB, IM and type I (see Table 3). The percent composition of masseter muscle fibres was highly variable in comparison to the rectus muscle. For example, the percent of type I fibres varied by up to 50.4% in the masseter biopsies but only by 27.3% in the rectus biopsies (Table 4). This percent variability in masseter fibres was greatest for the EMT inhibitor-2 type IIB. neonatal, atrial and additional categories. Interestingly, only two biopsies in the original sample human population of 58 were found to contain type IIB myosin. One of these biopsies was included in the group of 28 for full analysis, the other is definitely offered in the SDS-PAGE results (Fig. 3). The distribution of fibre types was also highly unusual in the masseter. In many biopsies groups of 4 to 6 6 or more small diameter type IIA fibres could be found.