Comparison of functional and total capillary densities in fast and slow muscles of the chicken.

The anterior latissimus dorsi (ALD) and posterior latissimus dorsi (PLD) of the domestic chicken were used to study the possible differences in functional capillary density of slow and fast skeletal muscle. Frozen sections of each muscle were prepared for histochemical determination of fiber homogeneity, capillary/fiber ratios, and capillary densities. The functional capillary bed (percent open capillaries) was determined by comparing the number of ink-filled capillaries, after a systemic injection of india ink (for 90-120 s), with the total number of capillaries/mm2. ALD is a slow-tonic muscle consisting of two fiber types, with an average of 1.34 +/- 0.03 capillaries/fiber, a density of 699 +/- 20 capillaries/mm2, and a functional capillary bed representing 29% (ratio of open to total number of capillaries/mm2 was 0.286 +/- 0.022) of the total bed. PLD is a fast-twitch muscle also having two fiber types, with an average of 1.08 +/- 0.04 capillaries/fiber, a density of 561 +/- 26 capillaries/mm2, and a functional bed representing 11% (ratio of open capillaries to total is 0.111 +/- 0.014) of the total. These findings of increased capillarity (24% more capillaries/fiber and 25% more capillaries/mm2) and higher functional capillary density in an avian pure slow muscle (ALD), corroborate previous findings of increased capillarity around slow fibers in mixed muscles of several mammalian species. The values given represent the means +/- SEM.