Auer rods and faggot cells: A review of the history, significance and mimics of two morphological curiosities of enduring relevance

Abstract

Myeloid differentiation in blasts is distinguished by the presence of one or more needle-shaped crystalline structures called Auer rods. Auer rods manifest either alone or as faggot cells (containing bundles of Auer rods) in various types of acute myeloid leukemia (AML), myelodysplastic syndromes (MDS) and myelodysplastic/ myeloproliferative neoplasms (MDS/MPN). Their presence largely portends a better prognosis in AML (as markers of maturation/differentiation) and upstages cases of MDS and MDS/MPN. Observation of these rods in residual blasts in treated cases of AML indicates an absence of remission. This article traces their historical discovery and examines their pathogenetic intricacies, as well as our current understanding of their relevance in myeloid neoplasms. Studies evaluating their prognostic impact in AML and MDS are catalogued. We also discuss a variety of other hematological and non-hematological neoplasms where structures potentially mistakable for Auer rods have been described. Even as the diagnostic approach to hematological malignancies has evolved from a morphology + cytochemistry + immunophenotyping-dependent one in the last century to a predominantly molecular genetics-based classification currently, and even as high-throughput sequencing and structural variation detection techniques surpass morphology in detecting clinically-relevant sub-categories of similar-appearing tumours, we review these curious microscopic structures that have withstood the test of time with respect to their diagnostic relevance.

Acute myeloid leukemia with misleading cytology

“…They often present with cytogenetic abnormalities [(inv)16, t(8;21), t(7;11), etc.] 1,2 . Our patient displayed normal karyotype but a myelodysplastic syndrome mutational profile.…”

“…They often present with cytogenetic abnormalities [(inv)16, t(8;21), t(7;11), etc.]. 1,2 Our patient displayed normal karyotype but a myelodysplastic syndrome mutational profile. This case highlights the importance of rapid F I G U R E 1 (A-C) Peripheral blood smear with immature hypergranular promyelocytes containing bundles of Auer rods (100Â objective, May-Grünwald-Giemsa stain), (D) Bone marrow aspiration smear with abnormal promyelocytes with bundles of Auer rods (100Â objective, May-Grünwald-Giemsa stain).…”

Pharmaceutical/Clinical Strategies in the Treatment of Acute Promyelocytic Leukemia: All-Trans Retinoic Acid Encapsulation by Spray-Drying Technology as an Innovative Approach–Comprehensive Overview

“…APL diagnosis can be first performed by the morphological characterization of APL. The abnormal promyelocytes can present a typical hyper-granular form, with a bilobed or reniform nuclear membrane, a densely granulated cytoplasm and Auer rods or Faggot cells (cells with bundles of Auer rods) [ 17 , 18 ]. In turn, the hypo-granular (microgranular) form of abnormal promyelocytes—observed with less frequency—presents bilobed nuclei, several sub microscopic granules and only a few cells with multiple Auer rods.…”