Ultrastructural and ultracytochemical differences between transient myeloproliferative disorder and megakaryoblastic leukaemia in Down's syndrome.

Ultrastructural and ultracytochemical studies were performed on blast cells from 12 Down's syndrome neonates with transient myeloproliferative disorder (TMD) and 13 Down's syndrome patients with megakaryoblastic leukaemia (MKL), in order to clarify the cytological characteristics of these cells. Average platelet peroxidase-positivity in blast cells of TMD patients was similar to that found in cases of MKL. Blast cells from subjects with TMD contained a number of different granules, namely, alpha granules, those that were myeloperoxidase (MPO)-positive, electron-lucent or basophil-like, and those containing membrane components or ferritin particles. On the other hand, granules found in the blast cells of MKL patients with Down's syndrome included the electron-lucent variety, those with membrane components and a few that were basophil-like, but not alpha and MPO-positive granules nor those containing ferritin particles. A demarcation membrane system was observed in blasts from the TMD group, but not in the MKL group. These findings suggest that blast cells in TMD patients differentiate to megakaryocytes, neutrophils, basphils and erythroblasts, while those in cases of MKL show limited differentiation to immature megakaryocytes, erythroblasts and, sometime, basophils. Such results correspond well with those of culture studies, in which TMD blasts were found to be precursors of various types of blood cells.