Myosin Heavy Chain Expression Can Vary over the Length of Jaw and Leg Muscles

Abstract

Muscle fiber type classification can be determined by its myosin heavy chain (MyHC) composition based on a few consecutive sections. It is generally assumed that the MyHC expression of a muscle fiber is the same over its length since neural stimulation and systemic influences are supposed to be the same over its length. We analyzed this in detail in three muscle types: the temporalis (closer) and digastricus (opener; both first brachial arch), and the medial gastrocnemius (somite). Sections of the muscles were incubated with monoclonal antibodies against various MyHC isoforms, and the distribution of these isoforms within individual fibers was followed over a distance of approximately 1 mm. The staining intensity of a fiber was measured and compared with the other fibers in the section. In the temporalis, digastricus, and gastrocnemius, 46, 11, and 15%, respectively, of their MyHC-I fibers showed a variation in the staining intensity over the length of their fibers, as well as 47, 87, and 22%, respectively, of their MyHC-IIA fibers. Most variable fibers were found amongst those with an overall relative intermediate staining intensity, which are presumably hybrid fibers. We conclude that different parts of a muscle fiber can have different fiber type compositions and, thus, contractile properties. Some muscle parts might reach their maximum contraction peak sooner or later than a muscle part a few microns further away. Next to stimulation by the nerve and systemic influences, local influences might also have an impact on the MyHC expression of the fiber.

Fig. 1

Example of 10 micrographs of a sample of a rabbit jaw muscle stained for α-cardiac MyHC over a length of 756 μm. Fiber B shows an increase in darkness while the other fibers maintain their shade. The images shown are consistent with intermediate cross sections not shown here. Bar = 5 µm.

Fig. 2

Example of the staining intensities of 10 muscle fibers followed over a length of 1,340 µm (67 slides), incubated with a monoclonal antibody against MyHC-IIA.

Fig. 3

Example of the staining intensity of 4 fibers (nonchanging black, white, and gray, and white changing) over a total length of 1,000 µm of a gastrocnemius muscle.

Fig. 4

An example to illustrate the variability in MyHC expression over the length of a muscle fiber in a small bundle of muscle fibers in toto incubated with an antibody against α-cardiac MyHC. We used a secondary antibody with a fluorochrome (goat anti-mouse Alexa Fluor® 647) to visualize the staining. Bar = 100 µm.