The effects of thyroidectomy on the extraocular muscles of the rat: A histochemical study

The effects of thyroidectomy on the metabolic properties of the rat extraocular muscles have been studied using histochemical techniques. Six months after removal of the thyroid gland a significant increase in the proportion of Type 1 fibres was found in the central zone of the superior rectus (SR) and in the levator palpebrae superioris muscles, but not in the medial rectus (MR) or superior oblique (SO) muscles. Other changes included an increased proportion of Type 2a fibres and reduced proportions of Type 2a and 2b fibres in the central zone of the SR, and an increased proportion of type 2a, and a reduced proportion of Type 2a' fibres in the peripheral zone of the SR, MR and SO. In addition, there was a mild hypertrophy of Type 1 fibres in the MR, and a mild atrophy of Type 2 fibres in each of the muscles studied. The physiological significance of these findings is discussed