Retinal Ganglion Cell Death

1.1 Topographic and cellular organization of the retina The retina is the thin (0.2 mm) lining of the back of the eye that gathers light focused on it by the cornea and lens. The retina has a complex laminar organization; cells are organized into layers (Fig. 1). These layers are named by reference to the middle of the eyeball; the innermost layers are located nearest the vitreous chamber, whereas the outermost lie adjacent to the retinal pigment epithelium and choroid. The most important layers, progressing from the inner to the outer, are: (1) the inner limiting membrane (formed by astrocytes and the conical end-feet of Müller cells); (2) the nerve fiber layer, composed of the axons of ganglion cells; (3) the ganglion layer, containing the cell bodies of ganglion cells; (4) the inner plexiform layer, composed of synapses formed between bipolar, amacrine, and ganglion cells; (5) the inner nuclear layer, containing the cell bodies and nuclei of horizontal, bipolar, and amacrine cells; (6) the outer plexiform layer, composed of synapses connecting photoreceptor cells from the outer nuclear layer with bipolar and horizontal cells from the inner nuclear layer; (7) the outer nuclear layer, containing the synapses and cell bodies of two classes of photoreceptors, namely the rods and cones; (8) the outer limiting membrane, a junction line between photoreceptor cells and Müller cells; (9) the photoreceptor layer, which contains the light-sensitive outer segments of the photoreceptors; and (10) the retinal pigment epithelium (RPE), which is a monolayer of melanin-containing cells forming part of the blood/retina barrier. Although the RPE is not a component of the neural retina, this layer provides critical metabolic support to photoreceptors and the integrity thereof is fundamental in terms of proper retinal function [Bok, 1993; Krstić, 1997]. Retinal tissue contains both neuronal and non-neuronal elements, which work together to enable vision and to maintain retinal homeostasis Neurons: The retina contains five types of neurons: (1) photoreceptors (cone and rod cells); (2) bipolar cells (of the flat, midget, and rod types); (3) horizontal cells; (4) amacrine cells; and, (5) ganglion cells [Krstić, 1997]. Photoreceptors are photosensitive neurons that absorb photons from the field of view and, using a specific complex biochemical pathway, turn this information into electrical signals via the process termed phototransduction [Sung & Chuang, 2010] to bipolar cells. Horizontal cells connect rods and cones that horizontally convey information within the retina. The horizontal cells receive input from one or more photoreceptors and transmit information to other photoreceptors and to bipolar cells [Poche & Reese, 2009]. Amacrine cells modulate signaling between bipolar and ganglion cells. The amacrine cells receive inputs from one or more bipolar cells and contact ganglion cells that