The process in which a relatively unspecialized cell acquires the specialized features of a tendon cell. Tendon cell are elongated fibrocytes in which the cytoplasm is stretched between the collagen fibres of the tendon. Tendon cells have a central cell nucleus with a prominent nucleolus, a well-developed rough endoplasmic reticulum, and are responsible for synthesis and turnover of tendon fibres and ground substance. [ http://www.ncbi.nlm.nih.gov/pubmed/21412429 http://purl.obolibrary.org/obo/CL_0000388 GOC:yaf ]