Fig. 6. Electron microscopy shows that wild-type (WT) embryonic myofibrils align to form a regular sarcomeric array that attaches obliquely to the myosepta (asterisks) (A). In sap homozygotes, fibres showing detached ends (arrows in B,C,G) and shortening of both the entire fibre and the sarcomeres, are visible. In these cells, the separation and regularity of sarcomeric banding is greatly reduced or collapsed compared with that in intact neighbouring cells, and absent in some places (B,C). Actin filaments (AF) run longitudinally from the terminal sarcomeres to the vertical myoseptum in both WT and intact sap mutant muscle fibres (D,E). Nuclear changes were also followed by electron microscopy in order to examine whether detachment precedes or follows cell death in sapje mutants. Nuclear condensations indicative of apoptosis were only present in detached mutant fibres (G), but were not observed in either intact mutant or WT (F) fibres, demonstrating that detachment is not a secondary process resulting from apoptosis of muscle fibres. Four days post-fertilisation, parasagittal sections. AS, absent sarcomeres; CS, collapsed sarcomeres; IS, intact sarcomeres; N, nucleus.

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