Development of chequerboard pattern in human foetal skeletal muscle.

The study was undertaken to race the histochemical and electron microscopic development patterns of human myogenesis from the 9th to the 26th week of foetal life. Particular attention was paid to the possibility of appearance of metabolic or structural differences between individual skeletal muscle fibres in early periods of myogenesis. The 9th week of foetal life is the period when primitive myotubes are formed. Irregular distribution of the ATPase (pH 9-4) activity observed at this time is due to differences in the structure of fusing myoblasts. The early myotubes show a high activity of the oxidative enzymes and lack of phosphorylase. Conversion of immature muscle cells into structurally mature fibres begins between the 20th and the 24th week. The latter fibres exhibit a uniform activity of all the studied enzymes and thus resemble the intermediate type of fibres of mature muscles. From about the 26th week on the typical mosaic pattern of the enzymatic activity is observed. All the differences in enzymatic activity which appear in fibers prior to their full morphologicaal maturity result from differences in developmental stages of the fibres at the given moment. The present study also suggests that there are no morphological or metabolic differences between individual humanskeletal muscle fibres prior to their metabolic differentiation into types which occurs after their innervation.