Capillaries Are Embedded In The Sarcolemma Of Murine Slow Twitch Skeletal Muscle Fibers

The purpose of this study was to evaluate the in vivo , 3D spatial relationship between capillaries (CAP), mitochondria (MITO), and three fiber types within murine Tibialis anterior (TA) muscle. Two photon excitation microscopy was utilized for in vivo imaging of MITO (NADH autofluorescence), vasculature, and interstitial space using injected dyes. A custom program was used to trace fibers and determine contact and volume ratios between CAP and 127 muscle fibers. Fiber type (slow (ST), intermediate (IT), fast‐twitch (FT)) was determined from relative NADH intensity and fiber size. Microscopy revealed that half of ST fiber CAP (53.2 ± 4.0%) had at least 50% of their circumference embedded in a groove in the sarcolemma. Embedded CAP were tightly associated with dense MITO populations lateral to the CAP grooves but nearly absent below the groove where fibrils dominated. CAP embedding was significantly lower in IT (29.5 ± 4.2%) and FT (14.2 ± 1.8%). Electron microscopy confirmed ST embedded CAP and MITO localization. A specialized sarcolemma/CAP groove is present in murine ST fibers resulting in CAP embedding in a MITO rich region. This structure optimizes CAP O 2 delivery to highly aerobic ST fibers and selectively restricts O 2 delivery to FT fibers. Localized contact of CAP with ST plasma membranes suggests receptors or transporters for blood borne signaling or substrate molecules may also be concentrated in these regions.