An even smaller fraction of the morphological cell types have been characterized in the rat, mouse, cat, or monkey. How does the multitude of retinal cells array itself across the retinal surface? The answer reveals an elegant feat of developmental engineering (review, Reese et al., 2011). Each of the retina’s >60 cell types is regularly spaced, so that the cells cover the retinal surface evenly. This assures that the cell types survey the visual scene efficiently (Cook, 1996; Wässle et al., 1981; Wässle and Riemann, 1978). But retinal cells of a particular type are evenly spaced only with respect to other

cells of the same type. With respect to cells of other types—even those to which they are synaptically connected—their positions are random (Rockhill et al., 2000). Not only do the cell bodies space themselves, the dendritic arbors of most cell types arrange not MDV3100 to overlap very much, as though dendrites of neighboring cells of the same type repel each other. This efficient coverage is observed physiologically as well as morphologically (Devries and Baylor, 1997; Gauthier et al., 2009). The phenomenon is called “tiling,”

but the term—invoking bathroom tiles—conflates two different concepts: regular spacing of the cell bodies (mosaic spacing), and fitting together of the dendritic arbors at their edges. A measure of the latter is the coverage factor, given by the spatial density of the cells (cells/mm2) times the dendritic field area of each cell (mm2/cell). A coverage factor of 1.0 represents

perfect tiling: no empty spaces Volasertib nmr between the arbors, and no overlap between the arbors. Bathroom tiles have both a regularly spaced mosaic and a coverage factor of one. All genuine cell types thus far discovered have regular mosaics. Many ganglion cells and the axon terminals of bipolar cells have coverage factors near 1.0. Other types of ganglion cells, especially in lower mammals, have coverage factors of three to five, and thus partial overlap in their Non-specific serine/threonine protein kinase arbors. And wide-field amacrine cells have enormous coverage factors, representing the specialized functions of these cells. The starburst amacrine cell of a rabbit has a coverage factor that ranges from 25 centrally to 70 peripherally, an overlap that serves their unique function for direction selectivity. Because of their regular spacing, the arbors of each of the ∼20 types of retinal ganglion cells cover the retina completely and evenly. This means that every point in the retinal surface is reported upon at least once—in the limiting case, exactly once—by each of the diverse types of retinal ganglion cell. This is represented pictorially in Figure 8, where the mosaics of four different types of ganglion cell are superimposed on an image. The first represents the X-type cell, responding in a linear way to the total brightness captured within its aperture. The second represents the Y cell, with a larger aperture and sensitivity to movement.