Histological and histochemical characterisation of the equine soft palate muscles.
Authors: Hawkes C S, Hahn C N, Dixon P M
Journal: Equine veterinary journal
Summary
Soft palate muscle dysfunction is widely suspected in the aetiology of intermittent dorsal displacement of the soft palate (DDSP), yet baseline histological data for these structures have been lacking until now. Hawkes and colleagues examined four soft palate muscles (palatinus, palatopharyngeus, levator veli palatini and tensor veli palatini) from six Thoroughbred-type horses using standard histological and histochemical techniques to characterise fibre-type composition and morphometry. All four muscles displayed a notably low proportion of oxidative type 1 fibres, with the three vagus-innervated muscles (palatopharyngeus, palatinus and levator veli palatini) containing significantly fewer type 1 fibres than the tensor veli palatini—a finding consistent with these muscles' role in rapid, forceful palatal elevation during exercise. Considerable heterogeneity in fibre diameter was evident across all muscles, particularly pronounced in the levator veli palatini and caudal palatopharyngeus, though the palatinus demonstrated greater uniformity and thus emerges as the most reliable candidate for histological assessment of palatal pathology. These baseline morphometric parameters provide essential reference values for future comparative studies of horses with palatal dysfunction, potentially enabling earlier identification of degenerative or inflammatory changes underlying DDSP.
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Practical Takeaways
- •Establish normal baseline histological characteristics of soft palate muscles to enable detection of pathological changes in horses with DDSP or other palatal dysfunction
- •When investigating muscle abnormalities in soft palate dysfunction cases, prioritise histological examination of the palatinus muscle due to its more consistent fibre morphology making pathology easier to identify
- •The predominance of type 2 fibres in these muscles suggests they are adapted for sustained postural control rather than rapid contraction, which has implications for understanding how muscle dysfunction contributes to DDSP during exercise
Key Findings
- •Normal equine soft palate muscles have a low proportion of type 1 fibres, with vagus-innervated muscles (palatopharyngeus, palatinus, levator veli palatini) having significantly fewer type 1 fibres than the tensor veli palatini (P<0.005)
- •Type 1 fibres were significantly smaller in diameter than type 2 fibres in all muscles except the palatinus (P<0.05)
- •Marked variability in fibre size was observed in levator veli palatini and rostral palatopharyngeus type 1 fibres, and caudal palatopharyngeus fibres (coefficients of variation >250%), with high endomysial connective tissue in the latter
- •The palatinus muscle showed the least variation in fibre size and is recommended as the optimal muscle for histological examination of palatal dysfunction