diaphysis diaphysis the diaphysis is the main or midsection (shaft) of a long bone. it is made up of cortical bone and usually contains bone marrow and adipose tissue (fat). diaphysisa long bone, with the diaphysis labeled at right.detailspronunciation/daɪˈæfɪsɪs/ dy-af-iss-isspart oflong bonesidentifiersmeshd018483ta98a02.0.00.017ta2391fma24013anatomical terminology it is a middle tubular part composed of compact bone which surrounds a central marrow cavity which contains red or yellow marrow. in diaphysis, primary ossification occurs. ewing sarcoma tends to occur at the diaphysis. additional images long bone