The dual connective tissue system of rat soleus muscle.

Rat soleus muscle has two types of connective tissue associated with it which do not overlap except at the myotendinous junction. One, an areolar type, is restricted mainly to the spaces between muscle fibers and the other, a regular dense type, is located only beyond the ends of muscle fibers in structures called microtendons. The definitive tendon of a muscle thus represents the accumulation of all the microtendons, one of which is contributed by each muscle fiber. The diameter distributions were the same for collagen fibrils from areolar tissue and microtendons but different from tendon collagen. Marked changes occur in the diameter distribution of the microtendon collagen as it passes into and blends with that of the tendon.