Analysis of the shape changes of muscle fiber cross sections in guinea pigs raised at 22°C and 5°C

Seven control and nine cold-acclimated guinea pigs were selected over a wide range of body weights (256–960 g and 239–1.074 g, respectively) from two larger groups of animals chronically exposed to 22°C or 5°C. Thin cross sections of the frozen soleus muscles were treated by the ATPase method to visualize fibers and capillaries. Photomicrographs of the sections were magnified and a total of 1,067 fiber profiles were cut out and weighed to determine individual fiber cross sectional areas (FCSA). Since most fibers are assumed to be cylinders with a circular cross section, the area of the fiber was compared to the area of a circle with the same diameter as the longest axis of the fiber. The longest axis within the fiber was measured and divided by two (LD/2). The LD/2 of each fiber was treated as the radius of a circle and the calculated area of the circle compared to the real area of the fiber. LD/2 and FCSA were positively correlated in both groups of animals by power functions FCSA=a(LD/2)b, wherea is the intercept andb is the regression coefficient. The values forb (1.58 for the 22°C and 1.59 for the 5°C animals) were both significantly less than 2 (b in the formula for the area of a circle), and the regression lines moved from a line representing a circle towards a line representing an ellipse, indicating that as the fibers grow they tend to become more and more elliptical. The adjusted mean of the 5°C regression line was significantly lower than that for the control line, indicating that the fibers of the cold-acclimated animals were more elliptical at the same FCSA than were the fibers of the control animals.