instnictions): The goal of the Muscle/Bone Phenotyping Core is to support the research alms of all subprojects within the program project. The overall aim of the program project is to understand the mechanisms underiying crosstalk between muscle and bone that may contribute to the age related decline In muscle and bone mass and function. This global question is addressed by each subproject from a different perspective, including: the effects of muscle on osteoblast and osteocyte function with aging (subproject 1); osteocyte modulation of muscle function during aging (subproject 2); osteocyte control of osteoblast dynamics with aging (subproject 3); and effects of aging on osteocyte responses to mechanical stimulation (subproject 4). The Muscle/Bone Phenotyping Core will provide phenotyping support for all subprojects, which will include In vivo and ex vivo X-ray, densitometric and microCT analysis of skeletal tissues, histological preparation and staining of muscle and mineralized tissues, quantitative histomorphometry and dynamic bone and muscle histomorphometry, immunohistochemistry and in situ hybridization. These techniques are essential for all the subprojects in which muscle and bone phenotypes are being characterized as a function of age in transgenic mouse models with altered osteocyte or muscle function, as well as mice that have been subjected to mechanical loading. In addition to providing these sen/lces the core will provide standardized protocols for sample preparation and for all methods within the core to maintain consistency across the subprojects. The core will also act as a resource to provide training in the above techniques for students, postdocs and research personnel conducting research within subprojects 1, 2, 3 and 4. By centralizing these phenotyping methods within a single core, we will standardize these techniques across all the subprojects, accelerate the pace of the research, enhance the education of students and postdocs and enable the research to be completed in a more cost-effective manner. RELEVANCE (See instructions): This research core will provide centralized support for four subprojects within a program project whose goal is to understand the mechanisms underiying muscle and bone interactions that contribute to age-related decline in muscle and bone mass. The core will standardize methods for analysis of muscle and bone in the mouse models used in all projects. This research may lead to new treatments for osteopenia/sarcopenla.