Lateral rectus muscle changes after bilateral neonatal labyrinthectomy in the ferret

The vestibular system is essential to the coordination of eye movements during head movements. Exercise, such as the eye movements mediated by the vestibular system, is a major factor in the development of muscle fiber types and the strength of muscle. In this study, the contents of the inner ear were removed (labyrinthectomy) from (LAB) ferrets at postnatal day 10 (P10) and raised with control (SHAM) animals. At P30, the lateral rectus muscles (LR) were removed to analyze the expression of myosin heavy chain (MHC) isoforms and to measure the least diameter of the developmental, slow and fast type muscle fibers. Another set of animals were sacrificed at P120 to analyze MHC isoform expression and muscle fiber diameters, as well as the contractile characteristics of the LR were measured prior to sacrifice. The average LAB LR was significantly stronger than the SHAM LR and the muscle fibers of the LAB animals were larger in diameter. The LAB animals expressed more type IIx and less slow type MHC. These results support the hypothesis that input from the inner ear influence the development of strength and muscle diameter in the ferret extraocular muscles.