The goals of Core D are to provide electron microscopic ultrastructure support to Projects 1, 2, 3 and 4 We expect that a structural analysis of protein disposition within the calcium release units (CRUs) and of the factors that affect assembly of complete CRUs will significantly enhance to our understanding of their function and the dysfunction caused by MH/CCD mutations in RyR1. A1. To determine the effect of MH/CCD RyR1 mutations on the ultrastructure of affected skeletal muscle. We will define any ultrastructural abnormalities caused my MH/CCD mutations, paying particular attention to triadic RyR-DHPR relationships and calsequestrin (Supporting Projects 1, 2, 3, 4 and in collaboration with Cores B and C). A2. To detect evidence for RyR conformational changes and for structural changes in DHPR/RyR conformational coupling by assessing alterations in DHPR tetrad parameters resulting from MH/CCD mutations in RyR1. We will determine the relative effectiveness of mouse and human MH/CCD RyRs to coordinate tetrad formation and to determine if these mutations alter intra or inter tetrad spacing. (Supporting 1, and 2 in collaboration with Cores B and C). A3. To define the structural relationship between mitochondria and the SR during development/aging and determine if there are differences in this relationship between wt and MH/CCD mice. Our preliminary data indicate that mitochondria gradually acquire very specific locations relative to the triads during postnatal development. In addition we will determine the structural integrity of both the SR and the mitochondria in homozygous and heterozygous MH mice. (Supporting Projects 1, 3, and 4). A4. To detect CRUs in mature dendritic cells isolated from control and homozygous MH/CCD muscle and to define their structure. We will define the ultrastructure of sites where RyR1 and CaV1.3 are co-localized in dendritic cells (DC) and detect any variations introduced by the MH/CCD mutations (Supporting project 2)