The vascular supply of three fibre types in the parietal trunk muscle of the Atlantic hagfish ( Myxine glutinosa, L) : A light microscopic quantitative analysis and an evaluation of various methods to express capillary density relative to fibre types

Abstract

Each segment of the parietal trunk muscle of Myxine is organized in myomeric units that contain a ventral and lateral layer of red muscle fibres, two or three dorsal layers of white fibres, and some intermediate fibres scattered in between. India ink-injected samples reveal that each myomer is supplied and drained by its own arterial and venous branch of the segmental vessels and, as might be expected from the histochemical and electrophysiological properties of the fibres, that 97% of the capillaries are located next to the red fibres and 53% of the white fibres lack contact with capillaries. Although the mean numbers of capillaries around the three fibre types are 2.76 for red, 1.95 for intermediate, and 0.69 for white fibres, the different sizes of the fibres make the surface fractions directly covered with capillaries 31, 14, and 0.4% for red, intermediate, and white fibres, respectively. Likewise, the mean portions of the fibre circumference served by one capillary are 53, 103, and 362 μm, respectively; the mean fibre volumes served by a 1-μm length of capillary are 580, 1385, and 6812 μm 3, respectively; and the mean vascularized surface areas per unit volume of fibre are 0.029, 0.011, and 0.0002 μm 2/μm 3, respectively. Based on these and other results, it is concluded that the vascular supply to distinct fibre types in a mixed muscle is adequately vizualized by some new counting methods whereby “pure” fibre contacts (i.e., capillaries found between fibres of the same type) are selected or whereby the probable “active” partner in any capillary-mixed fibre type contact (i.e., the fibre type to which the capillary owes its presence) may be judged. The results obtained by these methods, however, are difficult to relate in a quantitative way to other structural or functional parameters. For such purposes, the rather laborious morphometrical measurements mentioned above are better to use.