Definition is a rhetorical mode that can be used in something smaller or shorter than a full paper. You can use extended definition for several paragraphs only in a paper of much greater length. You also can add to a paper a one-paragraph definition--like a brief encyclopedia definition.
The research group, headed by Dr. Lukas Sommer at the Swiss Federal Institute of Technology, has identified a heretofore unknown role for the TGF cell-to-cell signaling pathway in the pathogenesis of DiGeorge syndrome.
Low parathyroid hormone levels can cause the blood calcium to be low. A number of conditions, such as DiGeorge syndrome, may cause low parathyroid hormone levels. Hypocalcemia is more common in premature and low birthweight babies.