**2. Overview of various blast cells**

#### **2.1. Erythroblasts**

In H&E-stained samples, it is difficult to distinguish between erythroblasts, myeloblasts, and megakaryoblasts. Erythroblasts have a large nucleus that occupies the whole cell. In H&Estained sections, the nucleus is observed to be pale, basophilic, and homogenous, with a fine appearance. Myeloblast lineages have more eosinophilic and rough cytoplasm than the erythroblasts, with distinct nucleoli (**Figure 1A** and **B**). Megakaryoblasts have transparent, lobulated nucleoli, with a higher nuclear-cytoplasmic ratio than that in megakaryocytes. The cytoplasmic border is frequently observed, and cell membranous projections are occasionally observed in some of the megakaryoblasts (**Figure 1C**).

#### **2.2. Immature myelocytic lineage**

In general, the morphological characteristics of myelocytic developmental stages have not been sufficiently described using H&E-stained clot samples. This is probably because of the difficulty in recognizing the cytoplasmic granules in the H&E-stained sections. However, H&E-stained sections provide incomplete morphological information regarding the developmental stages. In comparison with erythroblasts, myelocytic lineages are larger and have eosinophilic cytoplasm. Pro-myelocytes are smaller than myeloblasts and have a halo surrounding the nucleus, which is sometimes observed in M-G–stained samples (**Figure 2A**). Myelocytes and meta-myelocytes are easily identifiable owing to their segmented nuclei and smaller size than that of myelocytes (**Figure 2B** and **C**). Eosinophilic and basophilic lineages are identifiable because of the presence of distinct granules (**Figure 2D**).

To assist in morphologic diagnosis, immunohistochemistry (IHC) is available for assessment and immunophenotyping. For instance, by IHC, the marker glycophorin can assess the myeloid-erythroid cell ratio, and spectrin can efficiently detect erythroid cellular lineage [9]. P53 can also be used for differential diagnosis between refractory anemia and aplastic anemia [10]. An international Working Party for the Standardization of Bone Marrow IHC was formed by the International Council for Standardization in Hematology to prepare a set of guidelines for the standardization of handling BM specimens and reports [11]. While the histological examination of BM introduces new aspects of blood disease, the diagnosis of hematopoietic disorders using H&E-stained BM clot sections remains a challenge for nonhematopathologists. In fact, hematopoietic disorders have been diagnosed using May-Grünwald Giemsa (M-G) staining by hematologists and pathologists, and the findings obtained from H&E-stained BM

clot sections are used as a reference for those obtained from M-G–stained samples.

cytic proliferative neoplasm (MDS/MPN).

**2. Overview of various blast cells**

observed in some of the megakaryoblasts (**Figure 1C**).

**2.2. Immature myelocytic lineage**

**2.1. Erythroblasts**

10 Myeloid Leukemia

This review discusses the information obtained from H&E-stained BM clot sections and its advantages and limitations over M-G–stained BM clot sections. The techniques are discussed with respect to examination of erythrocytes and myelocytes and the diagnosis of acute myeloid leukemia (AML), myelodysplastic syndrome (MDS), and myelodysplastic syndrome/myelo-

In H&E-stained samples, it is difficult to distinguish between erythroblasts, myeloblasts, and megakaryoblasts. Erythroblasts have a large nucleus that occupies the whole cell. In H&Estained sections, the nucleus is observed to be pale, basophilic, and homogenous, with a fine appearance. Myeloblast lineages have more eosinophilic and rough cytoplasm than the erythroblasts, with distinct nucleoli (**Figure 1A** and **B**). Megakaryoblasts have transparent, lobulated nucleoli, with a higher nuclear-cytoplasmic ratio than that in megakaryocytes. The cytoplasmic border is frequently observed, and cell membranous projections are occasionally

In general, the morphological characteristics of myelocytic developmental stages have not been sufficiently described using H&E-stained clot samples. This is probably because of the difficulty in recognizing the cytoplasmic granules in the H&E-stained sections. However, H&E-stained sections provide incomplete morphological information regarding the developmental stages. In comparison with erythroblasts, myelocytic lineages are larger and have eosinophilic cytoplasm. Pro-myelocytes are smaller than myeloblasts and have a halo surrounding the nucleus, which is sometimes observed in M-G–stained samples (**Figure 2A**). Myelocytes and meta-myelocytes are easily identifiable owing to their segmented nuclei and smaller size than that of myelocytes (**Figure 2B** and **C**). Eosinophilic and basophilic lineages

are identifiable because of the presence of distinct granules (**Figure 2D**).

**Figure 1.** Morphology of erythroblasts and myeloblasts. (A) an erythroblast showing a transparent nucleus with condensed nucleoli (600×). (B) a myeloblast showing a transparent nucleus with one prominent eosinophilic nucleolus. The cytoplasm is eosinophilic, rough, and heterogeneously condensed (600×). (C) Two megakaryoblasts show transparent and lobulated nuclei with prominent eosinophilic nucleoli. The cytoplasm is eosinophilic, rough, and heterogeneously condensed. The edges are irregular, with a number of villi (600×).

**Figure 2.** Morphology of myelocyte lineages detected by H&E staining. Immature neutrophil lineages (A–C) and immature eosinophil lineages (D). (A) A pro-myelocyte showing a nucleus surrounded by halo (represented by an arrow). Nucleoli are not prominent relative to myeloblasts. The cytoplasm is eosinophilic, rough, and heterogeneously condensed (600×). (B) A myelocyte showing a regular oval nucleus with a contour and without indentation. The cytoplasm is eosinophilic, rough, and heterogeneously condensed (600×). (C) A meta-myelocyte showing a U-shaped or bent nucleus. The cytoplasm is eosinophilic, rough, and heterogeneously condensed. No nucleoli are visible (600×). (D) Variable immature eosinophil lineages. The arrow indicates a pro-myelocyte, and the arrowhead indicates a myelocyte. A mature eosinophil can be seen in the upper-left corner (600×).
