Eosinophilic cytoplasmic inclusions in fetal leukocytes: are Auer bodies a recapitulation of fetal morphology?

Among the most striking morphological features of acute nonlymphoblastic leukemias (ANLL) is the occurrence of eosinophilic cytoplasmic inclusions known as Auer rods on Auer bodies. We examined immature myeloid cells from the peripheral blood of 9 human fetuses of 16–19 wk gestation for the presence of such structures. Five of these 9 samples contained cytoplasmic inclusions, which were identical to the Auer rods typically seen in blast cells from patients with ANLL. The incidence of positive cells was low (1–5 cells/10,000 cells surveyed). The inclusions were azurophilic with Wright-Giemsa staining and were cytochemically positive with peroxidase, acid phosphatase, and Sudan black staining. We observed no inclusions in identically prepared control myeloid cells from the bone marrow of 5 patients with acute lymphoblastic leukemia in remission and 3 patients with chronic myelogenous leukemia in stable phase. Nor were they present in peripheral blood myeloid cells of 10 normal adults. Myeloid precursors in long-term bone marrow culture from 2 normal adult donors did not develop the inclusions during 24 hr of incubation with prostaglandin F2 (the abortifacient). These observations suggest that Auer rod formation is an occasional but normal phenomenon in fetal hematopoiesis.