Transitional stages in the histochemical development of muscle fibres during post-natal growth.

Serial frozen sections of longissimus dorsi muscles from seven pigs at different live weights (13 to 127 kg) were reacted for ATPase by the calcium method at an alkaline pH and for NADH oxidative activity. One hundred muscle fibres from each animal were identified individually in serial sections and their staining intensity was measured with a microscope photometer at 600 nm. For each section, staining intensity of fibres (% tranmission) was measured and converted to the nearest one-tenth unit of the range from the darkest to the lightest staining fibres. Frequency of occurrence of fibre types was plotted on a 10 X 10 grid using the range co-ordinates for NADH oxidative activity (vertical) and ATPase activity (horizontal). The commonly recognized histochemical fibre types in this muscle appeared as crowded areas in the grid but, in many cases, these areas were part of a continuous 'L' shaped range, a continuous but skewed distribution with regard to NADH oxidative activity was detected. In fibres with NADH oxidative activity of 0.6 to 1.0 units of the range, a continuous but irregular distribution with regard to ATPase activity was detected. Within this range, there was some evidence of a growth-related shift towards weaker ATPase activity.