Contrast Enhanced Imaging as an Alternative to Dissection in Tree Shrew (Tupaiidae: Scandentia) Chewing Musculature

Abstract

A burgeoning technique in the field of comparative biology involves staining specimens with iodine, followed by CT scanning. Along with providing excellent 3D visualization of soft tissue by giving it a higher attenuation, iodine staining enables digital isolation of muscles and the potential for measuring individual muscle fascicles. It is well suited for visualizing muscles of mastication, especially the medial and lateral pterygoid muscles; located internally, they are often difficult to dissect in small mammals. Here, we test the reliability of this method as an alternative to traditional dissection for obtaining accurate volume and muscle fascicle length measurements. This was accomplished by digitally isolating the masticatory muscles in three tree shrew cadaver specimens that had been subjected to iodine staining. These specimens represent the largest (Tupaia tana), smallest (T. minor), and average (T. belangeri) sizes of tree shrew. All specimens were frozen from death. After fixing with 10% buffered formalin, specimens were stained in a 3% solution of I2KI (T. tana; T. minor) or 5% solution of I2KI (T. belangeri) for 14 days. The heads were microCT scanned (at resolutions of 44.5 μm, 26.5 μm, and 32.2 μm respectively), enabling their muscles to be digitally isolated. Fascicles were visible only in T. minor due to a higher resolution, and were not measurable in any of the specimens. Larger bundles of fascicles contained within connective tissue compartments were identified and used to delineate separate muscles layers (eg. superficial vs deep masseter). These results indicate that while muscle fascicles are not measureable using these scans, they may be measurable if resolution could be increased. Isolating the muscles digitally provided more accurate measures of volume as compared to traditional dissection, while also preserving their orientation. Future work will focus on using these data to digitally map tree shrew muscles of mastication onto fossil primate mandibles as an analogue for studying early primate chewing systems.Support or Funding InformationThis work was supported through an American Association of Anatomists Short‐term Visiting Scholarship to H. Kristjanson