Quantitative Morphology and Histochemistry of Intrinsic Lingual Muscle Fibers in Macaca fascicularis

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Abstract
Histological, histochemical, and quantitative morphometric techniques were used to determine muscle fiber-type distributions and sizes in four intrinsic tongue muscles of Macaca fascicularis. Histologically, fiber interdigitation among muscles was striking with pronounced infiltration of endomysium into muscle fascicles. Endomysium and perimysium were most prominent anteriorly. Histochemically, with a sample of 20,106 fibers, type II fibers predominated (73.9%) over type I fibers (26.1 %) and 99% of type II fibers were categorized as IIA. The relative density of type I and type IIA fibers varied consistently within all muscles from anterior to posterior and to a lesser degree from superficial to deep. Although the tongue apex was composed almost exclusively of type IIA fibers, the proportion of type I to type IIA fibers increased posteriorly. Most posteriorly, type I and IIA fibers were in about equal proportions. These nonuniform fiber-type concentrations may suggest that different segments of individual primate intrinsic tongue muscles may be functionally independent, supporting recent models of tongue motor system biomechanics. For example, predominantly type IIA fibers in the anterior segments of a given muscle may underlie characteristic rapid tongue tip movements while slower movements of the posterior tongue are executed via separately activated type I fibers. Mean fiber diameters were quantified in one animal (n = 7,758). The distribution was unimodal (5.61–63.03 um) and overall type IIA fibers were larger than type I fibers. However, within all muscles studied, sizes of both fiber types were greater at posterior sites, further suggesting functional intramuscular segregation.

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