Research in: Agricultural & Veterinary Sciences Vol.6, No.2, 2022, pp.94-103 94 HISTOLOGICAL EVALUATION OF AVIAN OPTIC NERVE Hatice Yaren Kuloglu * Aksaray University, Aksaray, Turkey Abstract. Nervus opticus (Cr2) is a cranial nerve that carries the sense of sight and has a dense number of fibers and important neighbors. Cr2 enables sight by receiving the sense of sight perceived by the photoreceptor cells in the retina and transmitting it to other parts of the visual pathways. In our study, 24 nervus opticus (Cr2) tissue samples were taken from 12 (6 males, 6 females) chukar partridges ( Alectoris chukar). Triple, Hematoxylin Eosin (H&E), My-Grünwalt Giemsa, Well stain and Gordon Sweet staining methods were applied to the tissue samples. The information about the general structure of the nerve opticus was obtained in the first three staining methods. These are: The dura mater and arachnoid mater were observed at the outermost, while the pia mater, which tightly encircles the nerve, was observed at the bottom. Astrocytes extensions were seen between the septa and nerve fibers. The extrafascicular connective tissue was in the same structure as the pia mater and it partially surrounded the fascicle in many places. An intense vascularization was also seen in the connective tissue. In Well Stain staining, myelin sheaths surrounding the nerve fibers were observed. In Gordon-Sweet staining, it was observed that reticular fiber bundles started from the connective tissue capsule surrounding the organ and spread by branching. These reticular fiber bundles extended into the center of the tissue in thin branches. It was observed that the reticular fiber bundles in the areas where the connective tissue septums expanded were thicker and formed a network. Reticular fibers were also found around blood vessels. In the Well Stain staining, myelin sheaths were observed around the nerve fibers in the sections after the lamina cribrosa. Keywords: Avian, Nervus opticus, histology, partridge, Alectoris chukar. Corresponding Author: Hatice, Yaren Kuloglu, Department of Histology and Embryology, Faculty of Veterinary, Aksaray University, Aksaray, Turkey, Phone: +903822882888, e-mail: haticeyyaren@gmail.com Received: 15 June 2022; Accepted: 19 August 2022; Published: 31 August 2022. 1. Introduction Nervus opticus (Cr2) is the second of 12 cranial nerves known as Nervi craniales (encephalici) and transmits visual information from the retina to the brain. Since it originates from embryonic retinal ganglion cells, which is a diverticulum located in the diencephalon, it cannot regenerate after being cut (Oğul, 1996; Anderson, 1969). Nervus opticus (Cr2) carries special somatic afferent nerve fibers related to the sense of sight. Afferent nerve fibers originate from multipolar ganglion cells in the retina. There are also a small amount of efferent fibers, but their origin is not clear. Cr2 contains an average of 1,200,000 fibers. Nervus opticus (Cr2), which is approximately 4 cm long, consists of four parts: pars intraocularis (1 mm), pars intraorbitalis (25-35 mm), pars intracanalicularis (7 mm) and pars intracranialis (10-15 mm) (Arıcı & Elhan, 1997; Williams et al., 1995; Snell, 2000; Radius, 1994). Nervus opticus is surrounded by the cerebral cortex. In other words, the "Dura mater", "Arachnoid" and "Piamater" layers that make up the cerebral cortex also surround the optic nerve. The retinal nerve fiber layer gradually becomes thinner and extends from the optic disc to the periphery of the retina. If nerve fibers forming the visual pathways in the optic nerve are damaged in any region