Histological and histochemical characterisation of the equine soft palate muscles

Dysfunction of the muscles is implicated in the pathogenesis of intermittent dorsal displacement of the soft palate (DDSP) in exercising horses. The histological features of normal equine soft palate muscles have not been previously described.

To describe the histological and morphometric features of normal equine soft palate muscles.

The palatinus, palatopharyngeus, levator veli palatini and tensor veli palatini muscles of 6 Thoroughbred type horses were examined histologically and histochemically to assess their general morphology, fibre-type distribution and mean fibre diameter.

The muscles of all 6 specimens showed very similar characteristics, including a low proportion of type 1 muscle fibres in the 4 different muscles examined, with the 3 muscles innervated by the pharyngeal branch of the vagus (i.e. palatopharyngeus, palatinus and levator veli palatini) having significantly fewer (P<0.005) type 1 muscle fibres compared to the tensor veli palatini. The mean fibre diameters were significantly (P<0.05) smaller in type 1 than type 2 fibres in all muscles except the palatinus. Considerable variability in mean fibre size diameter was evident in all muscles examined, with type 1 fibres in the levator veli palatini and rostral fasciculus of the palatopharyngeus and both fibre types in the caudal fasciculus of the palatopharyngeus having the most marked variation, with coefficients of variation >250 and the latter also having high levels of endomysial connective tissue. The palatinus had least variation in fibre size.

Morphological characterisation of the normal soft palate muscles may provide reference values for future comparative studies with samples obtained from horses with palatal dysfunction. The palatinus appears to be the best muscle to histologically examine for evidence of muscle abnormality.