Vascularization in the tail musculature, which contains red and white muscle fibers, of the prometamorphic anuran tadpole was analyzed quantitatively by scanning electron microscopy (SEM). The sample was fixed in glutaraldehyde and osmium tetroxide; this was followed by freeze-fracturing in liquid nitrogen. As good ultrastructural preservation and reliable identification of capillaries were given by this technique, various morphometric parmeters, cross-sectional capillary area in particular, could be measured exactly.

In red muscle fiber that had a small cross-sectional area (2,060.1 μm2), high capillary density (1,283.5 capillaries/mm2) and a large cross-sectional capillary area (96.4 μm2) was found. Although white fiber (9,372.2 μm2) was 4.6 times greater than red fiber in cross-sectional fiber area, capillary density (95.8 cappilaries/mm2) and cross-sectional fiber area, capillary area (29.5 μm2) were 13.4 times and 3.3 times smaller than those of red fiber, respectively. From these morphometric values the following parameters were evaluated; (1) capillary/muscle fiber number ratio of red muscle fiber (2.64) was 3.0 times greater than that of white fiber (0.89); and (2) total cross-sectional capillary araa per crosssectional area of one muscle fiber was 44.0 times greater for red fiber (1.235.4 μm2/104 μm2) than for white fiber (28.1 üm2/104 μm2). Comparison of the latter parameter between the different fiber types may reflect the differences of real blood supply to them; i.e., red fiber was supplied a 44.0-times richer blood flow than white fiber.

Advantages of morphometric study by SEM, and the relationship between obtained parameters for vascularization and blood supply to the different muscle fiber types, are discussed.