There are other supporting cells, similar to neuroglial cells, also present. The photoreceptor cells activate the bipolar cells which in turn activate ganglion cells. However, when examined under the electron microscope, no true "layers" are defined. The histological organization of the neural retina can be understood in terms of their functional groups, mainly:Ĭones: mainly responsible for fine details and color vision Inner neural layerĬlassically, light microscopy defined ten layers of the neural retina. The cell organelles include lysosomes, Golgi apparatus and melanin granules. The basal layer of the hexagonal cells rests on a basement membrane closely related to the Bruch membrane of the choroid. The retinal pigment epithelium consists of a single layer of cells that extends from the margins of the optic disc to the ora serrata, then continues with the pigmented layer of the ciliary body. Histology Outer pigmented layer / retinal pigment epithelium The intracranial course of the optic nerve terminates at the optic chiasma in the floor of the third ventricle. As they pass the lamina cribrosa, they acquire myelin sheaths made of oligodendrocytes (not Schwann cells). The axons are arranged in bundles, and leave exit the globe by passing backward through the lamina cribrosa. There is a complete absence of rods and cones at the disc, making this part of the retina insensitive to light, termed as the "blind spot". The disc lies nasally to the macula lutea. The optic nerve (cranial nerve II) is made up of the neural axons in the ganglionic layer of the retina, converging at the optic disc. The retina is an alymphatic structure, as are the optic nerve and cornea. The central vein of the retina leaves the eyeball through the lamina cribrosa, draining directly into the cavernous sinus or through the superior ophthalmic vein. The inner laminae is drained by the central vein of the retina. The outer laminae is drained by the choroidal circulation which drains into the superior and inferior ophthalmic veins via vorticose veins. The blood-retinal barrier maintains the eye as a privileged site. Tight junctions between the retina capillary endothelial cells and tight junctions between retina pigment epithelial cells allows this function to be achieved. The retina capillary network is most concentrated at the macula but absent at the fovea centralis. It divides into two equal superior and inferior branches, and these branches subsequently divide dichotomously into superior and inferior nasal and temporal branches. The inner lamina is supplied by the central retinal artery, which is the first branch of the ophthalmic artery and is an anatomic end-artery. The outer lamina, including the rods and cones and outer nuclear layer, are supplied by choroidal capillaries. The integrity of the retina depends on both of these circulations, neither of which alone is sufficient. The blood supply of the retina is from two sources, supplying different portions of the organ.
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