**5. Discussion**

In the erythroblast lineage, nuclear-cytoplasmic dissociation (**Figure 6A** and **B**) [2], evident as hemoglobin production in the cytoplasm without enucleation, is identifiable in H&E-stained

Multinuclear abnormal pro-erythroblasts are also the hallmark of dysplasia (**Figure 6C** and **D**) [2]. Furthermore, basophilic heterogeneity in the cytoplasm is observed. Neutrophils exhibited a "blot" or localized heterogeneous staining in the cytoplasm, which is frequently seen in H&E-stained specimens (**Figure 7A**–**C**) [2]. In addition, a ring-shaped nucleus, which is observed in murine neutrophils, was observed in MDS and MDS/MPN (**Figure 7D**) [2].

Most pathologists are more familiar with H&E-stained samples than with M-G–stained samples in routine diagnostic work. As demonstrated in this study, H&E-stained specimens of BM, as well as M-G–stained samples, were available despite the difficulty in observation of granules in the cytoplasm in the myelocytic lineage. In this chapter, high-power magnification images are provided for understanding features of individual hematopoietic lineages for

As demonstrated, H&E specimens allow the identification of myeloblasts. Therefore, the identification of "blasts" in H&E-stained sections should be clearly defined. In contrast to the M-G-stained sections, H&E-stained sections do show basophilic cytoplasm; rather, the transparency of the nucleus with one or more nucleoli is more identifiable as the feature of blasts in H&E staining. This phenomenon represents another feature of dysplasia with deformity in the shape with or without atypical morphological findings of the nucleus. In dysplasia, hyperlobulated or ring nuclei were observed, which is a diagnostic feature of dysmyelopoiesis. In addition, basophilic heterogeneity, exudation, or a "blot" in the erythroblast cytoplasm is identified in dysplasia. This basophilic exudation was clearer in H&E-stained specimens than in M-G-stained sections. In addition, H&E-stained clot samples are sufficiently useful for the observation of morphological changes and for the assessment of mega-

H&E-stained clot specimens are advantageous for diagnosing AML subtypes because they allow better identification of hematopoietic dysplastic cells than M-G–stained sections do. Pathologists can integrate findings from the three staining methods: H&E, M-G, and IHC: for the decisive diagnosis of AML subtypes. Routine histological observations using M-G or H&E staining will be more useful in combination with flow cytometry analysis for immunohistological assessment using anti-CD33 and anti-CD13, a set of antibodies used to accurately diagnose AML. Myelodysplasia is also identifiable by H&E staining. Using high magnification, such as 1000×, the blot and localized heterogeneous staining or budding of the nucleus is identifiable by H&E staining. In fact, this high-power magnification provides more cytological and histological findings for facilitating the diagnosis of MDS

specimens because of distinct two-color contrast.

further clarifying morphological features of the subjects.

**4. H&E and M-G staining**

16 Myeloid Leukemia

karyocytic features.

and MDS/MPN.

### **5.1. Terminology: dysplastic, atypical, and blastic findings and interrelationship with hematological neoplasm**

Dysplastic, atypical and abnormal, and blastic findings, together with hypercellularity, compose the core concepts of hematological neoplastic entities in BM pathology. It is possible to understand the interrelationship of the entities by organizing concepts on the basis of their morphological findings (**Figure 8**). Dysplasia refers to specified morphological features that are distinct from other abnormal or atypical morphologies. As an important example, dysplastic megakaryocytes have circular, dispersed nuclei, which differ from atypical and irregular megakaryocytes in MPN and MDS/MPN. Thus, the terms "abnormal" and "atypical" should be used to describe the morphological changes other than morphological changes in MDS. The cytological spectrum of atypical lymphocytes is extremely broad. For example, small- to large-sized cells with often regular or indented nuclei, moderately condensed chromatin, moderate-to-faint basophilic cytoplasm without azurophilic granules and sometimes with microvacuoles are atypical findings [14].

On the other hand, the term "blast" refers to "immature" cells, which have a transparent nucleus with a prominent clear nucleus. For example, the "centroblast" is a morphological concept that refers to lymphocytes that have a specified morphological feature, such as prominent nucleoli representing an immunologically activated state. As another example, "B-lymphoblasts" refer to the activated B-lymphocytes that undergo hypersomatic mutations and the class switch of immunoglobulin genes. In other words, blastic lymphocytes correspond to the lymphocytes with gene recombination. Myeloblasts and erythroblasts, however, do not necessarily correspond to specific genetic alterations but correspond to the activated state of the first stage of differentiation. Distinguishing these morphological features from hyperplastic marrow will elucidate the mutual relationships among AML, AML arising from MDS, MDS, MDS/MPN, and MPN. MDS, AML, and MPN are specific entities on the basis of dysplastic, blastic (>20% of nucleated cell in BM), and hypercellular findings, respectively.

**Figure 8.** Interrelationships of hematopoietic neoplasms. Dysplastic, blastic findings, together with hypercellularity, compose the core concepts of hematological neoplastic entities.

AML arising from MDS and MDS/MPN are the hybrid entities of dysplasia and blastic findings and dysplastic and hypercellular findings, respectively. On the basis of these three morphological findings, the interrelationships among hematopoietic neoplasms can well be understood (**Figure 8**).

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