A modified ATPase method for the simultaneous demonstration of capillaries and fiber types in skeletal muscle is presented. Muscle biopsies were obtained from mice, hamsters, rats, cats, and dogs, quick frozen, and sectioned at 8 microns in a cryostat. The frozen slides were fixed in a neutral formalin solution at 4 C for 5 min, and then incubated at 37 C for 1 hr in a medium containing ATP, Pb2+, and Ca2+ in a tris-maleate buffer (pH 7.2). Dilute (NH4)2S was used as a developer. To test the reliability of the proposed method, serial sections of each biopsy were stained separately for capillaries (amylase-PAS method) and for fiber types by a standard myosin ATPase (m-ATPase) method. Fiber type percent and capillary parameters were determined for each biopsy. No difference in results was observed for parameters determined using the modified ATPase method compared to the standard capillary and fiber type staining methods. This modified technique is therefore suitable for the simultaneous demonstration of capillaries and fiber types in skeletal muscle.

Striated muscles of hindlimb, jaw, or tongue in dogs, cats, rabbits, rats, and guinea pigs were perfused under high pressure with Microfil (a silicone elastomer of viscosity 20 cP approx.) to outline the vascular bed. When the material had set, the muscles were fixed in formalin. Exmination of histological sections, strained by a modified Gomori trichrome method, showed capillaries containing Microfil as well as some, unperfused, filled with red cells. From well-perfused bundles the numbers of capillaries surrounding each fiber were counted; in all muscles these ranged from 0 to 9 with mean values betweeen 3.2 and 4.0. This was true of both red and white muscle. The mean number of fibers sharing one capillary ranged from 2.0 to 3.2, the lower value being found when four vessels surrounded a fiber and the higher value when there was only three. The results indicate a continuum of transitional models from a square array of fibers with each capillary between two adjacent fibers to a hexagonal array with capillaries at alternate vertices.