Light microscopy and image analysis of thin filament lengths utilizing dual probes on beef, chicken, and rabbit myofibrils1

Image analysis procedures for immunofluorescence microscopy were developed to measure muscle thin filament lengths of beef, rabbit, and chicken myofibrils. Strips of beef cutaneous trunci, rectus abdominis, psoas, and masseter; chicken pectoralis; and rabbit psoas muscles were excised 5 to 30 min postmortem. Fluorescein phalloidin and rhodamine myosin subfragment-1 (S1) were used to probe the myofibril structure. Digital images were recorded with a cooled charge-coupled device controlled with IPLab Spectrum software (Signal Analytics Corp.) on a Macintosh operating system. The camera was attached to an inverted microscope, using both the phase-contrast and fluorescence illumination modes. Unfixed myofibrils incubated with fluorescein phalloidin showed fluorescence primarily at the Z-line and the tips of the thin filaments in the overlap region. Images were processed using IPLab and the National Institutes of Health's Image software. A region of interest was selected and scaled by a factor of 18.18, which enlarged the image from 11 pixels/microm to approximately 200 pixels/microm. An X-Y plot was exported to Spectrum 1.1 (Academic Software Development Group), where the signal was processed with a second derivative routine, so a cursor function could be used to measure length. Fixation before phalloidin incubation resulted in greatest intensity at the Z lines but a more-uniform staining over the remainder of the thin filament zone. High-resolution image capture and processing showed that thin filament lengths were significantly different (P < 0.01) among beef, rabbit, and chicken, with lengths of 1.28 to 1.32 microm, 1.16 microm, and 1.05 microm, respectively. Measurements using the S1 signal confirmed the phalloidin results. Fluorescent probes may be useful to study sarcomere structure and help explain species and muscle differences in meat texture.