QUANTITATIVE MORPHOLOGY AND HISTOCHEMISTRY OF INTRINSIC LINGUAL MUSCLE-FIBERS IN MACACA-FASCICULARIS

Histological, histochemical, and quantitative morphometric techniques were used to determine muscle fiber-type distributions and sizes in fo ur intrinsic tongue muscles of Macaca fascicularis. Histologically, fi ber interdigitation among muscles was striking with pronounced infiltr ation of endomysium into muscle fascicles. Endomysium and perimysium w ere most prominent anteriorly. Histochemically, with a sample of 20,10 6 fibers, type II fibers predominated (73.9%) over type I fibers (26.1 %) and 99% of type II fibers were categorized as ILA. The relative den sity of type I and type IIA fibers varied consistently within all musc les 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 eq ual proportions. These nonuniform fiber-type concentrations may sugges t that different segments of individual primate intrinsic tongue muscl es 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 characteristi c rapid tongue tip movements while slower movements of the posterior t ongue 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 mu m) and overall type IIA fibers were larger than type I fibers. However, within all muscles studied, sizes of bot h fiber types were greater at posterior sites, further suggesting func tional intramuscular segregation.