Tumors arising from bones (osteosarcoma) are common and highly aggressive cancers in dogs. Even with limb amputation surgery and chemotherapy, most dogs with osteosarcoma live less than a year, usually because the cancer spreads to the lungs. Investigators are studying small molecules called microRNAs, in tumors, blood, and other bodily fluids. Changes in levels of specific microRNAs can be used to detect cancer, predict survival, and predict response to therapies both in humans and dogs. These molecules are important in the workings of cells and can control malignant behavior in cancers. The research team has measured microRNAs in blood and tumor tissue and found differences in levels in dogs with osteosarcoma compared to healthy dogs, and also found that these molecules can predict survival time of dogs with osteosarcoma. Since these molecules help control how cancer cells behave, they are also potential targets for new therapies. A critical step towards considering microRNAs in novel treatments is to understand what cells within tumors produce them, both in the main bone site but also in the lungs where they spread and lead to death of the dog. The goal of this study is to determine which cell make microRNAs of interest in osteosarcoma by using a specialized microscopy technique. This knowledge will help understand how microRNAs are working in osteosarcoma cells as well as normal body cells and aid in developing treatments, early detection and monitoring of disease, and better prediction of how a dog will respond to therapy.