Capillary-to-fiber geometry and mitochondrial density in hummingbird flight muscle

Abstract

We investigated structural characteristics for high O 2 flux in hummingbird flight muscle, i.e. the most O 2 demanding skeletal muscle per unit tissue mass among vertebrates. Pectoralis and supracoracoideus muscles of 3–4 g hummingbirds ( Selaphorus rufus) were perfusion fixed in situ, processed for electron microscopy and analyzed by morphometry. Small fiber size ( group mean ± SE, 201 ± 14 μ m 2 at 2.1 μm sarcomere length), large capillary length per fiber volume ( 8947 ± 869 mm −2 ) and high mitochondrial volume density per volume of muscle fiber ( 34.5 ± 0.9%) were characteristic features of the muscles. Considering capillary supply and mitochondrial volume on an individual fiber basis showed that the size of the capillary-to-fiber interface ( i.e. capillary surface per fiber surface) was also high in the muscles. Comparison with mammalian hindlimb pointed to a major role of the size of the capillary-to-fiber interface in providing a great potential for O 2 flux rate from capillary to muscle fiber mitochondria in hummingbird flight muscle.