Quantitative analysis of three-dimensional-resolved fiber architecture in heterogeneous skeletal muscle tissue using NMR and optical imaging methods

The determination of principal fiber directions in structurally heterogeneo us biological tissue substantially contributes to an understanding of its m echanical function in vivo. In this study we have depicted structural heter ogeneity through the model of the mammalian tongue, a tissue comprised of a network of highly interwoven fibers responsible for producing numerous var iations of shape and position. In order to characterize the three-dimension al-resolved microscopic myoarchitecture of the intrinsic musculature of the tongue, we viewed its fiber orientation at microscopic and macroscopic len gth scales using NMR (diffusion tenser MRI) and optical (two-photon microsc opy) imaging methods. Diffusion tenser imaging (DTI) of the excised core re gion of the porcine tongue resulted in an array of 3D diffusion tensors, in which the leading eigenvector corresponded to the principal fiber orientat ion at each location in the tissue. Excised axially oriented lingual core t issues (fresh or paraffin-embedded) were also imaged with a mode-locked Ti- Sapphire laser, (76 MHz repetition rate, 150 femtosecond pulse width), allo wing for the visualization of individual myofibers at in situ orientation. Fiber orientation was assessed by computing the 3D autocorrelation of discr ete image volumes, and deriving the minimal eigenvector of the center voxel Hessian matrix. DTI of the fibers, comprising the intrinsic core of the to ngue, demonstrated directional heterogeneity, with two distinct populations of fibers oriented orthogonal to each other and in-plane to the axial pers pective. Microscopic analysis defined this structural heterogeneity as disc rete regions of in-plane parallel fibers, with an angular separation of sim ilar to 80 degrees, thereby recapitulating the macroscopic angular relation ship. This analysis, conceived at two different length scales, demonstrates that the lingual core is a spatially complex tissue, composed of repeating orthogonally oriented and in-plane fiber patches, which are capable of joi ntly producing hydrostatic elongation and displacement.