The ultrastructural identification of Auer body precursors in a case of acute promyelocytic leukemia using high-angle specimen tilt.

Auer body (AB) precursors were identified in a case of poorly differentiated acute promyelocytic leukemia (APL). They consist of azurophilic granules containing membraneous lamellae. In most granules the lamellae were seen only after high-angle specimen tilting. In small but more mature ABs, the periodic tubular structure also was visualized best by specimen tilting. An intermediate granule having both lamellae and tubules is described and discussed in relation to the fusion of azurophilic granules to form ABs. The early diagnosis of APL, in the absence of ABs and intravascular coagulation, is assisted by specimen tilting to resolve the lamellae in the azurophilic granules.