Architecture of the pectoralis muscle of the Japanese quail (Coturnix japonica): histochemical and ultrastructural characterization, and distribution of muscle fiber types

The muscle fibers in the pectoralis muscle of the Japanese quail (Coturnix japonica) are classified as fast-twitch glycolytic (FG), fast-twitch oxidative-glycolytic (FOG), and slow-tonic on the basis of their histochemical and ultrastructural characteristics. There is an increasing proportion of FOG/FG fibers along a superficial to deep gradient throughout the entire belly of the muscle. Slow-tonic fibers are present in low numbers, and are restricted to a tiny area located in the deepest fasciculi of the cranial third of the muscle. This distribution of muscle fiber types is typical of those vertebrate muscles adapted to a locomotory function. The slow-tonic fibers are alkali-stable and acid-stable when preincubated for myofibrillar adenosine triphosphatase (mATPase) activity. Slow fibers in the chicken pectoralis and mouse soleus muscle, both types previously described as alkali-labile, acid-stable for mATPase activity, cannot be distinguished from each other or Japanese quail slow-tonic fibers on the basis of several ultrastructural characteristics: Z-line width, metabolic differences, or fusion of myofibrils. While mammalian slow fibers have one large motor end plate, all avian slow fibers have small multiple motor end plates. Mammalian slow fibers are slow-twitch, and avian slow fibers are probably slow-tonic. More complex secondary synaptic clefts distinguish mammalian from all avian fiber types.