Small-square (SS) net exists in trout muscle.

The small-square-net form (with the large-square-net form) was clearly observed in brown trout muscle in cross-square-net-section. Fresh adductor mandibularis and dorsal muscle strips, isometrically clamped to prevent contraction, were prefixed in 3% glutaraldehyde, 0.1 M cacodylate, pH 7.4, and post-fixed in 1% osmium tetroxide in veronal acetate buffer at 2 degrees C. The samples were dehydrated, embedded in epon-araldite, cross-sectioned, and stained with uranyl acetate and lead citrate for electron microscopic observation. The sections provided the first clear evidence of the small-square-net form, coexisting with the basket-weave pattern, in fish muscle. In previous experiments, we could not demonstrate the small-square-net pattern convincingly with guppy tissue (Yamaguchi et al., 1985a) which may be more susceptible to chemical or physical stress than is trout tissue. The existence of the diverse structural patterns in cross-sections of some, if not all, vertebrate striated muscle must be derived from a basic unit which although it may vary in size among species is structurally identical. The findings agreed with our unified model of narrow and wide Z-line structure (Yamaguchi et al., 1980; Yamaguchi et al., 1985a; Yamaguchi et al., 1985b).