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. This is because their parathyroid glands are less mature.