ULTRASTRUCTURAL-STUDY OF THE OPTIC-NERVE IN BLIND MOLE-RATS (SPALACIDAE, SPALAX)

The optic nerve in two species of subterranean mole-rats (Spalacidae) has been examined at the ultrastructural level. The axial length of th e eye and the diameter of the optic nerve are 1.9 mm and 52.5 mu m in Spalax leucodon, and 0.7 mm and 80.8 mu m in Spalax ehrenbergi, respec tively. An anti-glial fibrillary acidic protein postembedding procedur e was used to distinguish glial cell processes from axons. In both spe cies, the optic nerve is composed exclusively of unmyelinated axons an d a spatial distribution gradient according to the size or the density of fibers is lacking. The optic nerve of S. leucodon contains 1790 fi bers ranging in diameter from 0.07-2.30 mu m (mean = 0.57 mu m), where as in S. ehrenbergi, only 928 fibers, with diameters of 0.04-1.77 mu m (mean = 0.53 mu m) are observed. In S. ehrenbergi, a higher proportio n of glial tissue is present and the fascicular organization of optic fibers is less obvious. Distribution gradients according to size frequ ency or density of fibers in the optic nerve are absent in both specie s. Comparison with other mammals suggests that although ocular regress ion in microphthalmic species is correlated with a significant decreas e in the total number of optic fibers and the relative proportion of m yelinated fibers, no difference in the absolute size range of unmyelin ated axons is observed. The total absence of myelinated fibers in Spal ax may be related to the subcutaneous location of the eyes. The unique presence of unmyelinated fibers in the optic nerve is discussed in re lation to the possible conservation of a single class of W-like gangli on cells in the retina, in relation to photoperiodic perception.