BPS2026 - Label-free quantitative phase microscopy of skinned muscle fiber

The sarcomere is the fundamental force-generating unit of muscle, composed of thick and thin sliding filaments, regulatory proteins, and structural proteins that maintain its organization. The extent of overlap between thin and thick filaments is a critical parameter that determines sarcomere force production. Variation in protein density across the sarcomere produces the characteristic striated pattern of muscle, which can be observed with conventional phase-contrast microscopy. The striations arise from the thick filaments forming the A-band, while thin filaments contribute little contrast. We employ quantitative phase microscopy to accurately measure the relative phase shift of light transmitted through myofibrils. Because the phase shift can be used to estimate protein concentration, phase maps provide detailed information about protein density within the sarcomere. We ask whether analysis of the transmitted light phase can reveal the extent of thin and thick filaments overlap, which is not detectable with Zernike phase-contrast or Nomarski differential interference contrast microscopy. Although spatial resolution is limited by the Abbe diffraction limit, analysis of relative phase distribution may enable measurement of filament overlap length in unstained myofibrils.