Normal function of the vertebrate nervous system depends on a precise pattern of distribution of the different cell types. For example, cone photoreceptors, which constitute a sheet of light-sensitive neurons in the retina of the eye, come in three main types, each type maximally sensitive to light of a certain color, red, green or blue. The different cone-cell types are arranged in a regular, repeating lattice or mosaic pattern. The developmental mechanisms that build the lattice, i.e. that control the spacing and determine the fate of the presumptive cone cells, are not well understood. Most biologists agree that the immature cells send signals to their neighbors that determine the fate of the neighboring cells. To begin to understand this process, the earliest stages in the development of the cone mosaic will be examined. Molecular markers will used to identify the early developing cone types. The hypothesis to be tested is that one cone type acts as the "founder" to establish a blueprint of the mosaic pattern, and the other cone types develop later, filling in the pattern. This study will give new insight into how the distribution of different neuron types are established and will provide new information about the way cells communicate with each other during development.
