Ultrastructural Changes of the Ciliary Epithelium of Rabbit after the Intravenous Mannitol Injection

ABSTRACT

Hyperosmotic agents such as mannitol are widely used in ophthalmology to lower intraocular pressure as a short-term or emergency method. The mechanism of action of these agents is not fully understood, but probably relates primarily to a reduction in vitreous volume. There are other theories of hypotensive meechanism such as hypothalamic-neural theory and altered epithelium theory. The author performed this animal experiment for the eletronmicroscopic study of ciliary epithelium after the intravenous mannitol injection. Five healthy adult male albino rabbits weighing 2.5 kg were used in this experiment. Four rabbits were administered 25 ml(2 gm/kg) of 20% mannitol and the other one was given 25 ml of normal saline as a control through ear vein within 5 minutes each. The mannitol group was enucleated 10, 20, 40 and 80 minutes after injection and the control one was enucleated 20 minutes after injection. The enucleated eyes were opened and fixed in mixed solution of 2% paraformaldehyde, 3% glutaraldehyde and 0.2M Milonig's buffer. Small pieces consisting of ciliary body were excised, postfixed in 1% osmium tetroxide, dehydrated in ethylalcohol and embedded in Epon 812. Thin section were stained with toluidine blue for general histologic study and ultrathin sections stained with 4% uranyl acetate and 0.4% lead citrate were examined with a Hitachi H-600 transmission electronmicroscopy. The results were as follow 1. The ciliary epithelium showed normal appearence 20 minutes after injection of normal saline and was composed of double layered epithelial cells. The tight juctions(zonulae occludens) were present between nonpigmented epithelial cells. The active Golgi apparatus, numerous mitochondria, rough endoplasmic reticulum and smooth endoplasmic reticulum were visible in the nonpigmented epithelial cells. The intercellular spaces were not dilated. 2. In mannitol group, no cellular necrosis was observed and cells were invariably present and apparently unaltered. 3. The intercellular spaces of ciliary epithelium began to dilate 10 minutes after intravenous mannitol injection, maximally dilated after 40 minutes and recovered after 80 minutes. 4. In view of the morphological changes of cytoplasmic organelles such as Goigi apparatus, the secretory function of nonpigmented epithelial cells after intravenous mannitol seemed to be inhibited maximally at 20 minutes and then recovered after 80 minutes. 5. In conclusion, the hypotensive mechanism of the mannitol on the ciliary epithelium was considered of secretory inhibition of nonpigmented epithelial cells besides diffusion by the osmotic gradient.