Staining of human <i>VL</i> muscle lipid and glycogen stores with an optimized protocol reveals the existence of four metabolically distinct fiber types.

<p>(A) Representative images of consecutive muscle crossections stained for lipid (top), glycogen and laminin (middle) and myosin heavy chain I (green) and II (red)(bottom) are presented. Bars; 20 µm. (B) ATPase staining of consecutive sections allowed for type IIa and IIx distinction. (C) Lipid (grey bars) and glycogen (black bars) stores were analyzed in <i>vastus lateteralis</i> muscle from 6 young healthy subjects and, the results are plotted as mean grey value ± SEM. Statistical differences <i>versus</i> I-1 are represented as *, <i>versus</i> I-2 are represented as # and, <i>versus</i> IIA are represented as ¤. Measurement of Mean Grey Value does not allow differentiating IMTG content in the 4 fiber types. A more detailed analysis of IMTG stores, as lipid droplets’ density and size (D), allows for the differentiation of 4 metabolically distinct fiber types. Two populations of type I muscle fibers, I-1 and I-2, can be differentiated by muscle fiber crossectional area (E) and lipid droplet density. Type I-2 muscle fibers are similar in fiber crossectional area and lipid content and distribution to type IIA. Quantification of lipid and glycogen content in 10 human <i>VL</i> muscles before (black bars) and after exhausting exercise (grey bars) was performed. Glycogen content is expressed as mean grey value (F), lipid droplets’ density as number of lipid droplets in 3.6 µm² (G) and, lipid droplets’ size as number of pixels (H) (Pixel size is 3.6*10⁻³ µm²). Two-way repeated Measures Analysis of Variance was performed and statistically significant differences between fiber types (C and D: *vs I-1, #vs I-2) and, between basal and exhausted muscle (F–H: *p<0.05, **p<0.01 and ***p<0.005) are represented.