A morphological and morphometric analysis of the optic nerve in the hypothyroid rat

This investigation was designed to morphologically evaluate the effects of hypothyroidism on the development of myelin and axons in the rat optic nerve. Four pups from each group of normal and propylthiouracil-induced hypothyroid rats were sacrificed at 14, 21, 28, and 35 postnatal days. Optic nerves were studied by both light and electron microscopes. The hypothyroid animals had significantly reduced body and brain weights compared to those of their age-matched controls. In the hypothyroid animals, the cross-sectional area of the optic nerve, the fiber density, and fiber occupancy were significantly diminished compared to those of the controls. The mean individual fiber size was unaffected. However, the relationship between the total axonal area to myelin thickness was similar in the control and experimental groups, implying that the feedback mechanism between myelinating cells and axons was not affected by hypothyroidism. Thus, this study indicates that the principal insult of neonatal hypothyroidism results in a delay in myelin acquisition of myelinated fibers, resulting in diminished cross-sectional area of the optic nerve, fiber density, and fiber occupancy.