Histochemical fibre types in human extraocular muscles, an investigation of inferior oblique muscle.

In order to test histochemical fibre types in human extraocular muscles, 2000 fibres from 5 inferior oblique muscles were examined in serial sections for myosin ATP-ase following preincubation at pH 4.3, menadione-linked alpha-GPD and NADHTR activity. In ATP-ase 4.3, +/- 15% of fibres in central and peripheral areas stained dark, the other fibres were slightly stained or unstained. Nearly all fibres with high activity in ATP-ase 4.3 had type I-like histochemical characteristics with high or very high NADHTR activity and low alpha-GPD activity. Fibres with low activity in ATP-ase 4.3 had a spectrum-wide variation in activity of NADHTR and alpha-GPD and could be divided into 3 subtypes with IIA, IIB and IIC-like characteristics. The histochemical profile of the fibres with type IIC-like characteristics differed from that of type IIC fibres in other skeletal muscles, by a lower ATP-ase 4.3 activity. In the central areas all 3 subtypes were more or less equally represented. In the peripheral areas most fibres with low ATP-ase 4.3 activity belonged to the IIC-like subgroup.