The ultrastructure and vascular supply of the different fibre types in the axial muscle of the sturgeon Acipenser stellatus, Pallas

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Abstract
The ultrastructure and vascular supply of the different fibre types in the lateral muscles of the sturgeon Acipenser stellatus were studied by light- and electron microscopy and morphometry. Three fibre types form separate layers without intermingling. The red fibres are superficial, the white fibres deep and the intermediate fibres between them. From morphometric analyses, the mitochondrial volume fraction in red fibres is 30%, in intermediate fibres 3.7% and in white fibres 0.7%. Z lines are most fuzzy in the red fibres. Triads of the sarcotubular system are always situated at the level of the Z discs. In red fibres the three elements are arranged in a series along the myofibrils, whereas in white fibres they are arranged transversely and in the intermediate fibres they are aligned obliquely. The number of capillaries surrounding each fibre is 2.3, 0.9 and 0.2 for the red, intermediate and white fibres, respectively. In red fibres 16% of the surface is directly covered by capillaries. The corresponding percentages for intermediate and white fibres are 5 and 1, respectively. Per unit volume of the fibre, the directly vascularised fibre surface in red fibres is about ten times larger than that of white fibres. The degree of vascularisation of the fibre types is directly related to the volume fraction of mitochondria, and thus to their aerobic capacities.

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