This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The goal is to obtain three-dimensional images of photoreceptor cells and their internal structures from wildtype retinas and from retinas of mice with mutations corresponding to human retinal disease. Understanding these structures will help us to understand the normal structure of healthy photoreceptors, and the structural correlates of disease pathogenesis. The most interesting structures will be the disk membranes, flattened membrane sacks which fill up the photoreceptor outer segments. These are packed with rhodopsin at high density and the organization of rhodopsin in these membranes is a subject of considerable controversy. Disorganization of these membranes is observed in a number of mouse models of human retinal degeneration. We hope to see the structural organization of these membranes and their connections to cytoskeletal elements and the plasma membrane. These have been previously observed only in samples that have been sectioned and negatively stained, or subjected to freeze-etch procedures. Cryo-electron tomography offers the opportunity to observe these in intact cells (at least on the margins of the cell) with no manipulations other than rapid freezing. Also of interest will be the connecting cilium, a bundle of microtubules that connects the inner and outer segments and serves to transport outer segment components to their proper location. Defects in genes involved in the structure and function of this cilium and the trans-cilium transport also lead to retinal degeneration.