ABSTRACT
In humans the retina is the innermost layer of the eye, which consists of retinal pigment epithelium (RPE) and neural retina. The neural retina has several layers and various cell types, which are illustrated in Figure 1. RPE is a single layer of hexagonal cells that maintains the homeostasis of neural retina. It has essential biochemical, physiological, physical, and optical functions in maintaining the visual system, including phagocytosis of rod outer segments, transport of substances between photoreceptors and choriocapillaries, and uptake and conversion of the retinoids, which are needed in visual cycle. Together with endothelial cell linings of retinal capillaries, RPE forms the blood-retinal barrier. The neural retina is a complicated and delicate multilayer. The thickness of neural retina varies from 0.4 mm near the optic nerve to about 0.1 mm anteriorly at the ora serrata. The photoreceptors are the light-sensing part of retina. The electric impulses are amplified and integrated by bipolar, horizontal, amacrine, and ganglion cells. The principal glial cell of the retina is the Mu¨ller cell. The bipolar cells
are the first and ganglion cells the second neuron of the visual pathway from photoreceptors to brain. Macula is the central part of retina located temporally of optic nerve head between the upper and lower temporal vessels. Fovea is the central, approximately 1.5 mm wide sloping part of macula. Visual acuity is decreased quickly in the paramacular areas. Of the photoreceptors, the cones take care of photoptic and color vision and are located mainly in the macula. Rods are the main photoreceptor type in the periphery; they are specialized to scotopic vision.
