Department of Pathology, Sinai Hospital of Baltimore Pathology, Baltimore, MD, USA
The marrow biopsy specimen is usually taken from the iliac bone. It consists of bony trabeculae surrounded by a mixture of fat and hematopoietic cells . The percent cellularity (nonfat) should be roughly [100 – the patient’s age]. The components of normal trilineage hematopoiesis are (1) megakaryocytes, (2) erythroid precursors, and (3) myeloid precursors. There may also be assorted plasma cells, lymphocytes, and histiocytes. The bone marrow is considered a reflection of what is in the peripheral blood, so disorders of marrow affect blood counts. Lymphomas can involve the marrow, but generally the primary malignancies of the marrow are the leukemias.
The nomenclature is slightly confusing. Myelo- means bone marrow (or sometimes spinal cord, but that’s not relevant here). Technically, all three of the marrow-based hematopoietic lines (megakaryocytes, erythroids, and granulocytic cells) are of the myeloid lineage , in contrast to the lymphoid lineage (B and T cells), because they mature in the marrow. The myeloproliferative neoplasms and myeloid leukemias refer to this broad classification. However, the word myeloid, as used to describe individual components of the marrow, generally refers to those cells in the granulocyte/monocyte pathway only.
This chapter will discuss the morphologic approach to the specimen and will focus on the findings under the microscope. However, in reality, hematopathology , more than any other discipline, is becoming driven primarily by genetic classification of diseases. The recent WHO classification has defined many leukemias by their cytogenetic abnormalities, which can be identified by molecular testing and which supersede the histologic impression. However, the histology is still valuable in providing a preliminary impression of the diagnosis, as well as determining the significance of the molecular finding. In myelodysplastic syndrome , for example, the presence of genetic abnormalities in the absence of recognizable dysplasia is significantly less worrisome than those same abnormalities with extensive dysplasia. For the time being at least, histology is still relevant to hematopathology.
Megakaryocytes are the most easily identified cells, with their lakes of pink cytoplasm and multilobated nuclei (Figure 20.1). Erythroid precursors have a distinct rim of clear cytoplasm and centrally located, perfectly round nuclei; as they mature, the nuclei become small and dense such that erythroid islands in the marrow look like handfuls of buckshot (see Figure 20.1). Myeloid cells make up almost everything else. Myeloid precursors have more open chromatin than the red cell precursors, more cytoplasm, and more convoluted nuclei as they mature. Mature neutrophils and eosinophils should be present in normal marrow. Blasts, the most primitive hematopoietic cells, can be difficult to identify on H&E stain. Lymphoid cells, especially immature, should generally not be found in the marrow, with the exception of hematogones (nonneoplastic B cell precursors), which can be markedly increased in children.
Normal megakaryocytes , erythroids , and myelocytic precursors . In this H&E-stained core biopsy specimen, there are erythroid precursors (1), myeloid precursors (2), megakaryocytes (3), and maturing neutrophils (4).
Usually an aspirate smear will be submitted with the biopsy specimen. The aspirate is stained with the Wright-Giemsa stain, which highlights nuclear detail. Blasts, and the successive stages of maturation, are best seen on an aspirate. The blasts are large cells with a thin rim of cytoplasm and a characteristic nucleus (Figure 20.2). The blast nucleus is large and round with a very finely textured chromatin pattern and a nonstaining nucleolus that shows up as a “hole” in the chromatin. The more differentiated precursors, such as promyelocytes and myelocytes, may have a similarly immature nucleus but acquire cytoplasmic features such as granules and a “hof” (the cleared-out Golgi zone in the cytoplasm, as in a plasma cell; see Figure 20.2).
Blast on aspirate, Wright–Giemsa stain . The key to identifying a blast is the high N/C ratio and immature chromatin pattern, which consists of very finely grained, uniform chromatin with several nucleoli that show up as negative images on this stain (arrowhead). The immature cell nearby is a promyelocyte, which has the same nuclear qualities as a blast but has abundant cytoplasm with granules (arrow).
On the aspirate, a myeloblast (as seen in acute myeloid leukemia) cannot always be distinguished from a lymphoblast (as seen in acute lymphoblastic leukemia) on histology alone. However, the presence of granules or Auer rods identifies a blast as myeloid. Erythroblasts have royal blue cytoplasm and very round nuclei. Monocyte precursors tend to have grayer cytoplasm and a folded or creased nucleus. Flow cytometry is performed on the aspirate as well and helps to confirm the lineage.
Approach to the Biopsy Specimen
A full evaluation of the specimen requires an H&E-stained core biopsy, a Wright–Giemsa-stained aspirate, and a peripheral smear. Beginning with the biopsy :
On low power (4×):
Assess the cellularity of the marrow (Figure 20.3). A hypo- or hypercellular marrow will guide your differential diagnosis.
Marrow cellularity . (a) Normocellular marrow for a middle-aged adult; this cellularity is approximately 30%. (b) Hypercellular marrow for an adult; this cellularity is about 95% and is taken from a case of acute myeloid leukemia.
Estimate the cellularity as a percentage range (i.e., 30–40%), as clinicians follow the cellularity to monitor response to therapy.
On medium power (10×) :
Survey the marrow for trilineage hematopoiesis. You should see megakaryocytes, erythroid islands, and myeloid cells. Look to see if each line matures to completion: you should see mature neutrophils and red cells. Estimate the ratio of myeloid to erythroid cells (M/E ratio), which is normally about 2–4 to 1.
Look for things that do not belong in the marrow in large populations or aggregates, such as blue areas (lymphocytes), pink areas (histiocytes, plasma cells), or islands of nonheme cells (metastases). Look for fibrosis, which gives the marrow a streaming texture (Figure 20.4).
Marrow fibrosis . On H&E stain, the marrow has a streaming texture (arrow), indicative of strands of collagen separating the hematopoietic cells into nests and channels.
On high power (20× and 40×) :
Look at the individual cells, especially megakaryocytes . Small megakaryocytes with single nuclei are a feature of myelodysplasia and are also seen in chronic myeloid leukemia (Figure 20.5). Giant-clustered megakaryocytes are a feature of myeloproliferative disorders. A few atypical megakaryocytes are not unusual, but a large population is significant.
Chronic myeloid leukemia . This hypercellular marrow is full of small, hypolobated megakaryocytes (arrow) and maturing and mature neutrophils (arrowhead).
Look for neutrophils. A packed marrow with numerous neutrophils may indicate chronic myeloid leukemia (see Figure 20.5), whereas numerous myeloid precursors with few neutrophils indicates a left shift in maturation. Sheets of immature myeloid cells could represent anything from acute myeloid leukemia to infection; the aspirate needs to be evaluated for blasts (see next section).
Next, look at the aspirate. Hold it up to the light; an adequate aspirate will have little chunks in it (spicules) that are foci of stromal elements. Scan the slide for an optimal area of the smear. Cells should be spread out in a monolayer, with intact cytoplasm and distinct nuclei. “Naked” nuclei, which have been stripped of cytoplasm, are not evaluable. You have already evaluated the megakaryocytes, so with the aspirate, focus on erythroid and myeloid cells . On high power (20× to 100×, with oil if necessary):
Assess the heterogeneity of the marrow. A healthy aspirate should be a random mosaic of all different cell types at all levels of maturity. Sheets of uniform cells is a bad, bad sign (Figure 20.6).
Monomorphic aspirate in acute leukemia. This aspirate is composed of sheets of blasts, identified by high N/C ratios and immature chromatin.
Look for blasts. You should be able to find scattered blasts but not clusters of them. Clumps of immature-looking cells are more commonly promyelocytes and myelocytes, as identified by their granules and cytoplasmic hofs. If you do find lots of blasts, note the shape of the nuclei, the color of the cytoplasm, and the presence of Auer rods. The presence of greater than 5% blasts is abnormal; they should be quantified by a systematic cell count of 200–500 nucleated cells. Flow cytometry will also provide a blast count.
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