Suspensory ligament or interosseous muscle? Histology of the interosseous muscle of South American Camelids

The interosseous muscles (IOM) have evolved to largely collagenized structures in many large grazers, including South American Camelids (SACs). SACs include lama (Lama glama), alpaca (Vicugna pacos), guanaco (Lama guanicoe) and vicuna (Vicugna vicugna). This study reports on a comparative histomorphological evaluation of the IOM in these four species. First, a descriptive qualitative analysis of the tissue composition within the physiological IOM clearly demonstrates that the IOM of all SAC species is predominantly tendinous. However, we can always find a small amount of striated muscle fibres supporting the claim that the term "interosseous muscle" should be preferred over the denomination "suspensory ligament". Second, we quantify the ratio of muscle tissue (MTr) occupying the cross-sectional area (CSA) of 80 limbs at three levels and examine it in relation to limb, sample localization and species. Values for MTr are highest at the middle level, ranging from 0.4% to 15.2% with a mean of 3.4% +/- 3.2%. We also detect notable amounts of muscle tissue at the distal sample level in the area of the branches of the IOM. Of all SACs, alpacas present the highest MTr. Lamas have a significantly larger MTr than guanacos. These results suggest the influence of domestication and/or physical activity on the muscle tissue content of the IOM of SACs. Amounts and distribution of muscle, adipose and lose connective tissue vary greatly for each sample localization and within individuals of one species. This may complicate the interpretation of MRI images of SACs' IOM for diagnostic purposes.