Detection of Muscle Fiber Orientation During Human Tongue Development: Analysis Using Diffusion Tensor Imaging.

The tongue consists of intrinsic and extrinsic muscles that enable complex movements. Previous studies, based on gross and histological observations, were limited in their ability to fully distinguish between them, which may have resulted in inaccurate differentiation timelines for intrinsic and extrinsic muscles. This study analyzed tongue muscle fiber orientation during embryonic development using diffusion tensor imaging (DTI). Human embryonic and fetal specimens (n = 29) were used in this study. A 7-Tesla magnetic resonance imaging (MRI) system was used to acquire T1-weighted images and DTI data. Fiber tractography was used to track muscle orientation. Tongue muscles that differentiate into myotubules were first recognized at Carnegie stage (CS) 19, with many muscles becoming identifiable by CS23. Most extrinsic and intrinsic muscles could be clearly distinguished based on differences in their fractional anisotropy values and fiber orientations. Precise discrimination and quantification were only successful for a limited number of muscles. Serial histological sections identified the vertical, transverse, hyoglossus, and genioglossus muscles at CS19. This study indicates that intrinsic and extrinsic muscles differentiate along a similar timeline during the first differentiation of myotubules at CS19. Moreover, this study highlights the capabilities and limitations of DTI for evaluating the embryonic development of tongue muscles. Additionally, DTI can detect immature muscle tissues, suggesting potential applications for other regions at similar stages of embryonic development.