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Fig. 3. Muscle cells exhibit ultrastructural defects in twister mutant
embryos. Electron micrographs of cross-sections (A,C,E) and longitudinal
sections (B,D,F) of somitic muscle taken from comparable rostro-caudal levels
of 26 hpf wild-type and twister mutant embryos. (A) Cross-section of
wild-type muscle with mature, ordered myofibrils. Polyhedral-shaped myofibrils
(open arrowheads) are surrounded by an organized membrane system of tubules
and sarcoplasmic reticulum cisternae (black arrow). Myofibrils form a
characteristic double-hexagonal array of thick and thin filaments at the A
band (open arrowheads and inset), and a tetragonal arrangement of thin
filaments in proximity of the Z line (black arrowhead). (B) Longitudinal
section of wild-type muscle. Myofibrils are organized into sarcomeres with
prominent A (A) and I (I) bands. Triads (3 arrows) are associated with the Z
(z) lines. (C) Cross-section of heterozygous twister mutant muscle in
which thick filaments are frequently surrounded by more than six thin
filaments (open arrowhead and inset). Similarly, the surrounding membrane
system (T tubules and sarcoplasmic reticulum) is less organized (black
arrows). (D) Longitudinal sections of heterozygous mutant muscle show
ill-defined A (A) and I (I) bands and Z (z) lines. (E) Cross-section of
homozygous twister mutant muscle. Myofibrils are in disarray and
extend in various orientations (double arrows), and the membrane system is
poorly developed. (F) In longitudinal sections, myofibril bundles are thin and
misaligned. A primitive T-tubule is present (3 arrows), but the typical triad
organization of SR cisternae and T-tubules is absent.
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