Suzanne de Bruijn

15 General introduction is produced by the epithelial cells of the ciliary body and is a clear watery liquid that supplies nutrients to the cornea and the lens. This liquid has a high turnover and is replaced approximately twelve times a day. The vitreous humor is a thick gelatinous substance and makes up ~80% of the volume of the eye and is responsible for maintaining the shape of the eyeball. The vitreous humor contains phagocytic cells that remove unclarities that might interfere with light transmission. 6,7 The different liquids, their contents, and the cell types located in the surrounding tissue layers all work together to allow optimal perception of light. Pupil Cornea Ciliary body Retina Vitreous chamber Posterior chamber A B Photoreceptor layer Lens Iris Anterior chamber Choroid Sclera Optic disc Macula (Fovea) RPE Bipolar layer Ganglion layer Horizontal cell Amacrine cell Rod Cone Bipolar cell Ganglion cell Figure 1. Anatomy of the human eye. (A) A schematic representation of the different cells and tissue types present within the eye. (B) The cellular organization of the human retina. The neuroretina contains five types of neurons that are divided over three nuclear layers. The photoreceptor layer contains the light sensitive rod and cone photoreceptor cells, the bipolar layer contains the bipolar cells and ganglion layer contains ganglion cells that will form the optic nerve. Communication between the different layers is supported by the horizontal and amacrine cells. RPE; retinal pigment epithelium. The figures are adapted and published with permission from Servier Medical Art. The retina and photoreceptor cells The sensory part of the eye, the retina, is part of the central nervous system and can be separated in the neural retina and the retinal pigment epithelium (RPE). The neural retina has a highly organized layered structure and contains five types of neurons that are organized in three nuclear layers and two synaptic layers ( Figure 1B ). The neurons are interconnected via synapses located in the inner and outer plexiform synaptic layers, the neuron cell bodies are divided over the three nuclear layers: the inner and the outer nuclear layer and the ganglion cell layer. The photoreceptor cells (1 st layer) are the light-sensitive neuronal cells which communicate with the axons of bipolar cells (2 nd layer), the terminals of the bipolar cells are connected to the synapses of the ganglion