Characteristics of perioptic and intraoptic arterial vascularization of extracranial segment of optic nerve

78 years, with no signs of changes in the structures of the nervous system. The vasculature of the optic nerves was microdissected and examined under the stereoscopic microscope, after injecting their arteries with 10% mixture of India ink and gelatin, and 5% formaline ixation. Results . The intracanalicular part of the optic nerve received ine blood vessels from the ophthalmic artery, usually in two branches in 29 (82.9%) cases, while in 6 (17.1%) specimens there was only one branch for the nerve. The intraorbital segment of the optic nerve was supplied by two interconnected systems of blood vessels, peripheral and axial, both coming from the central retinal artery. Conclusion . On the basis of the obtained results, it has been conirmed that both segments of the optic nerve have a rich and very delicate vascular network. The supericial pial arterial plexus of the intracanalicular segment of the optic nerve has been identiied as the only source of its vascular supply. The intraorbital part of the optic nerve has two interconnected systems of blood vessels: peripheral and axial.


Introduction
Opticus (Latin) is an ancient Greek word (optikós), and it means seeing, sight or vision. The irst description of the optic nerve dates from the period around 500 BC, and it comes from Alcmaeon of Croton (Croton was a Greek colony in the south of Italy). For a long period of time, it was considered that the optic nerve was hollow and that contained spiritual luid. The more modern description of the optic nerve came from an Italian anatomist Costanzo Varolio. Constantius Varolius (latinized) (1543-1575) was a professor of anatomy and surgery in Bologna, and later in Rome, and the subject of his anatomical interest was the brain. The pons as part of the brainstem were described in his book "De nervis opticis", published in Padua in 1573 [1]. Optic chiasm, as the place of decus-Godište 10 Jun 2019 sation and partial crossing of the optic nerves, was irst described by Rufus of Ephesus, around 50 AD. Hundreds of years later, it was showed that chiasm was responsible for the coordination of the eye bulb movements. Hermann Ludwig Ferdinand von Helmholtz (1821-1894), was a German physicist, a professor of philosophy, pathology, and anatomy. With his discovery and introduction of ophthalmoscope in 1851, he showed the inner part of eyeball with the area around the optic disc. In 1856, he published the irst volume of his "Handbook of physiological optics", being one of the most important works on this subject [2,3,4].
We have accepted the division of the optic nerve (ON) into two parts: 1) cranial part and 2) extracranial part, whereby the extracranial part can be subdivided into two segments: a) intracanalicular part (in the optic canal) and b) intraorbital part (in the orbit) [5].
The earliest descriptions have pointed out that pial arterial plexus vascularize optic nerve over its entire length [6]. This vascular scheme was completed with the indings of other researchers, who described more branches, 1-3, coming from the ophtalmic artery (OA), as it passed through the optic canal [7][8][9]. The intracanalicular part is vascularized from recurrent ophthalmic artery (lateral and ventral), while intraorbital part is supplied from the posterior ciliary arteries and the central retinal artery [9]. The already-mentioned perioptical arterial network gives even smaller branches, which enter the optic nerve and supply all deep structures [10]. Therefore, we clearly differentiate between two mutually connected vascular systems of blood vessels: peripheral and axial [11].
Traumatic optic neuropathy is a condition in which acute injury of the optic nerve, caused by direct or indirect trauma, results in vision loss. Orbital hemorrhage, hematoma of the optic nerve coverings, or direct injury of the optic canal walls by bony fragments can lead to this condition. Vision loss is usually the result of the compression or injury of ine vascular network of the optic nerve, which is of utmost importance for our research [12].
The aim of the study was to investigate the micromorphological characteristics (origin, the numbers of arteries, calibers and the way of branching) of perioptic arterial vascularization and intraoptic vascular network of the extracranial section of the optic nerve, composed of intracanalicular and intraorbital parts.

Methods
The study included 18 pairs of the optic nerves, obtained from persons of both sexes (11 males and 7 females), aged from 51 to 78 years, without any signs of change concerning the optic system structure, using the collection of the Laboratory for Vascular Anatomy of the Brain of the Institute of Anatomy at the Faculty of Medicine in Belgrade. The material was obtained during the regular autopsies at the Institute of Pathology of the Faculty of Medicine in Belgrade. The study was approved by the Ethics Committee of the Faculty of Medicine, University of Belgrade. The optic nerve, regardless of its size, is fragile and positioned deep in the optic canal and in the orbit. Therefore, a special method of approaching and cutting the nerve was used. Intraorbital and intracanalicular sections of the optic nerve were taken in a block with surrounding bones of the skull base and dural covering, to the level of the eyeball, including carvenous segment of the internal carotid artery and ophthalmic artery. A plastic tube was ixed in the isolated segment of the internal carotid artery, and the arterial system of the optic nerve was injected, irst with saline solution, then with 5% formaldehyde, and inally with 10% mixture of melted gelatin and India ink.
Microdissection of injected blood vessels was performed with the help of micro-instruments, and all measurements were obtained under Leica MZ6 stereo microscope using 35 well-injected nerves. All specimens were photographed with Canon Power Shot A710 digital camera, and all details recorded by using Leica DFC 295 digital camera under stereo microscope. Vascular network of the optic nerves and topographic relations were sketched in the already prepared schemes. An original software Leica Interactive Measurements was used for morphometric analysis.

Results
The intracanicular and intraorbital segments of the optic nerve had prominent and delicate vascular network.
A) An intracanalicular part of the optic nerve received the gracile blood vessels from the ophthalmic artery on which it lay. With respect to the point of entrance of the ophtalmic artery in a ibrous tissue of the dural sheath of the optic nerve, we described two groups of intracanicular optic branches: extradural and intradural. a) Optic branches, which were separated from the ophthalmic artery before its entrance into the dural layer of the optic nerve covering, were present in 14 (40%) cases, always one artery ( Figure 1); b) Optic branches, which were coming from the ophthalmic artery after its entrance into the dural sheath of the optic nerve covering, were present in all cases (100%), one artery in 20 (57.1%) cases, and two arteries in 15 (42.9%) cases ( Figure 2).
Overall, the intracanalicular part of the optic nerve most commonly received two small arteries, branches of the ophthalmic artery, in 29 (82.9%) cases, while on 6 (17.1%) specimens there was only one branch for the nerve supply, from 104.3 to 162.9 µm, an average of 148.3 µm. Having been separated from the ophthalmic artery, these branches continued to branch upwards and backwards through intracanalicular segment of the optic nerve, oriented towards the orbital opening of the optic canal. They anastomosed with branches coming from the cranial cavity, as well as with branches from the orbit, forming the pial arterial network or plexus of this part of   the optic nerve. At this level, we did not notice the existence of any kind of axial arterial blood vessel of the segment.
B) The intraorbital part of the optic nerve was the only one vascularized by two, interconnected, separated systems of blood vessels: peripheral and axial. a) The peripheral vascular system consisted of the pial arterial network, formed by two or three branches of the central retinal artery (CRA). They branched from the CRA before it entered into the lower surface of the optic nerve (Figures 3, 4). Each of these branches, most frequently in 3 in 27 (77.1%) cases, ramiied over the lower part of the nerve, proceeding to its upper part. b) The axial vascular system was composed of many branches, separated radially from the central retinal artery and located in the central part of the optic nerve. These small intraneural branches are the main source of vascularization of the central part of the nerve, and are connected with the supericial pial arterial plexus via capillary anastomoses (Figures 5, 6).

Discussion
Anatomical variations of the origin of branches forming pial arterial plexus of the optic nerve, as well as the morphometric results of our study,   are important not only for understanding variability of arterial vascularization of the intracanalicular and intraorbital segments, but also for developing practical strategies for the surgical interventions in this region [11].
The intracanalicular part of the optic nerve normally gave off two small arteries, ophthalmic artery branches, in 29 (82.9%) cases, while on 6 (17.1%) specimens there was only one branch for the nerve. On the surface of the optic nerve, the small arteries were interconnected with branches coming from the cranial cavity, as well as with branches from the orbit, forming the pial arterial network or plexus covering this part of the nerve. The fragility of the intracanalicular arterial system of the optic nerve was noticeable only on the specimens injected with India ink and gelatin. Mainly due to this, there are not so many descriptions of the blood vessels of this part of the optic nerve, except from one author. He stated that more small branches (one to three), coming from the ophthalmic artery during its passage through the optic canal, pierced dural sheath and ended up in the optic nerve [7]. The mentioned author did not document his results by means of photographs.
The intraorbital segment of the optic nerve was the only one who had two, mutually connected, segregated systems of the blood vessels: peripheral (made of the pial arterial network) and axial. The results of the authors who gave detailed descriptions of the existence of the central artery of the optic nerve showed that it proceeded to the retina. Nevertheless, they estimated that it did not participate in vascularization of the optic nerve [6]. The other groups of authors had doubts about the existence of this kind of vascular division. Our study has also showed that axial vascular system is made of numerous branches, which are radially directed from the central retinal artery, situated centrally in the optic nerve, and originated from the ophthalmic artery [13]. Those small intraneural branches are the main source of vascularization of the central part of the nerve and are interconnected with the supericial pial arterial network via capillary anastomoses.
The optic nerve can be damaged by the compression of different origin. The disruption in vascularization of intracanalicular and intraorbital parts of the optic nerve is commonly accepted as the cause of visual loss in traumatic optic neuropathy, an irreversible process resulting in neuronal loss. In this case, the damage to the endothelial cells of the nutritive arteries of the optic nerve can lead to thrombocytes aggregation, as well as to thrombus formation. This trauma can also cause relex vasospasm, as the reaction to the injury of the smooth musculature of the arterial wall, which leads to ischemic optic neuropathy. Hemorrhages in the optic nerve, dural sheath and the spaces between the optic coverings, soft tissue edema, tearing of nerves, as well as the contusion necrosis of the nerve, are primary lesions in indirect trauma of the optic nerve. All these data show special sensibility of the perineural and intraneural capillary networks of the optic nerve to traumatic damage, emphasize their recognition, and acknowledge the importance of our research [12][13][14][15][16].

Conclusion
The intracanalicular part of the optic nerve most commonly received two small arteries, branches of the ophthalmic artery, in 29 (82.9%) cases, while on 6 (17.1%) specimens there was only one branch for each nerve. The intraorbital segment of the optic nerve was the only one that had two, interconnected with anastomoses, separated systems of blood vessels: peripheral and axial. The peripheral vascular system of the optic nerve was made of supericial pial arterial plexus, made from three branches of the central retinal artery, in 27 (77.1%) cases. The axial vascular system was formed from numerous branches that were radially separated from central retinal artery, located centrally in the optic nerve. The results showed that both segments of the optic nerve had a rich and very delicate vascular network.