Ultrastructural organization of the optic nerve of the tench (Cyprinidae, Teleostei)

Abstract

Different parts of the tench optic nerve, the intraocular and intraorbital segments, the chiasm, and the post-chiasmatic segment, were studied using light and electron microscopy. From the head of the optic nerve, a zone of continuous growth constituted by the younger non-myelinated ganglion axons can be differentiated from a mature zone where almost all the axons are myelinated. The transition from one zone to the other is progressive. The area containing only non-myelinated axons is very restricted, and the presence of myelinated and non-myelinated axons in the same fascicle is frequent. In the head of the optic nerve, the growing zone surrounds the central artery. In the intraorbital segment, where the optic nerve is organized as a folded ribbon, the growing edge is surrounded by other mature folds. In the chiasm and in the post-chiasmatic segment of the optic nerve, the organization as a folded ribbon disappears and the youngest axons are situated on the periphery. In the growing zones, the immature astrocytes predominate; in the transition zones, oligodendrocytes, in different stages of maturity, begin to appear. In the mature zone, almost all the glial cells are differentiated, although immature cells can be found. The microglial cells are not abundant and are of the ramified type. Moreover, in contrast to the descriptions of other teleosts, the tench optic nerve is profusely supplied with blood vessels throughout its length.