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The descriptive term “Leukamie/leukemia” was introduced by Rudolf Virchow who noticed the “white” color of blood samples obtained from patients presenting with high peripheral white blood cell counts and a reversed white and red blood cell balance [1]. The designation “acute leukemia” (AL) is actually applied to a heterogeneous group of immature clonal hematopoietic neoplasms occurring in all age groups. The large majority of AL is derived from either the myeloid or the lymphoid lineage, allowing a definite classification as acute myeloid leukemia (AML) or acute lymphoblastic leukemia (ALL). The rare category “AL of ambiguous lineage” includes AL with no lineage-specific features and AL of mixed lymphoid/myeloid phenotype (MPAL) [2, 3].

Reactive plasmacytosis is common in patients with infectious diseases (especially HIV), cirrhosis, and rheumatic disorders as well as autoimmune diseases, especially in patients with lupus.

Myeloid/lymphoid neoplasms (M/LN) with eosinophilia and gene rearrangement are listed in a distinct WHO category. This category includes three specific disease groups involving rearrangement of platelet-derived growth factor receptor α (PDGFRA), platelet-derived growth factor receptor β (PDGFRB), fibroblast growth factor receptor 1 (FGFR1) or is associated with PCM1-JAK2, a provisional WHO entity [1–3] (Table 10.1). The presentation of the disorders is heterogeneous: Phenotypic features may vary between those of a myeloproliferative neoplasm (MPN), especially chronic eosinophilic leukemia (CEL), myelodysplastic syndrome/MPN (MDS/MPN), or occasionally of an acute myeloid leukemia (AML), a myeloid sarcoma (MS), or an acute lymphoblastic leukemia/lymphoma (ALL/LBL). It has been strengthened that although the classification of this category of neoplasms is predominantly based on genetic aberrations, diagnosis should still be anchored to a combination of histomorphology and clinical and laboratory criteria [3].

Myelodysplastic syndromes (MDS) comprise a heterogeneous spectrum of clonal stem cell disorders.

The skeleton function is diversified. It is the instrument for direct locomotion and movement and protects vulnerable tissues. It is important for the homeostasis of minerals, and it shelters the bone marrow which provides the blood elements. The metabolites of Vitamin D, parathyroid hormones, and calcitonin are among the most important regulators in the complex mechanism for the control of the calcium mechanism and the sound condition of bone. These complex mechanisms explain the vulnerability of trabecular bone which is variably affected in many primary or secondary diseases. A bone marrow biopsy can be helpful in diagnosis since morphological changes can be detected before abnormalities are evident on X-rays. Bone remodeling and bone production are typical features in pediatric marrows. However, even in adults bone remodeling is continuing for mechanical support and to provide adequate calcium levels. This bone remodeling in adults is morphologically not noticeable in the figurative sense of peculiar osteoblasts or osteoclasts [1]. The evaluation of trabecular bone on bone marrow biopsies comprises an estimation of the thickness, size, and shape and a general survey of the matrix and cellular components of bony trabeculae. Normally, a structural support for medullary cavities is evident by anastomosing bone spicules. Continuously connected anastomosing bony trabeculae are a pathologic finding. Careful evaluation of the mesenchymal developed osteoblasts and osteocytes and the osteoclasts, which are of hematopoietic origin, is essential. The mononuclear osteoblasts are located in the periphery of trabeculae. In young patients, these cells often form a row along bony trabeculae. Osteoclasts, mostly multinucleated cells, occupy small depressions on the bone’s surface, called Howship lacunae. A patchy distribution of osteoclasts along bony trabeculae is normal in young patients. Mineralized bone predominates and comprises osteocytes. Unmineralized newly formed bone matrix, the osteoid is prominent in young children, but minimal in adults.

MPNs comprise various groups of clonal hematopoietic stem cell disorders. The WHO classification 2017 subdivides MPNs into BCR-ABL1-positive chronic myeloid leukaemia (CML) and BCR-ABL1-negative or Philadelphia (Ph) chromosome negative MPNs.

Histological investigation of bone marrow biopsy is an integral examination method for diagnosis of various hematological and also non-hematological diseases [1]. Since sampling of bone marrow biopsies (BMB) from newborns as well as geriatric patients can easily be performed, the indications for retrieving BMB have increased, especially in hematology, internal medicine, oncology, but also osteology [2]. Various fixatives and different methods are described for preparation of bone marrow biopsy sections; however, each hematopathologist prefers his own technique used in his laboratory. Therefore, it is not our intention to persuade the investigating pathologists to use a special method, but the need for well-fixed and well-stained thin sections cannot be emphasized enough.

The normal red blood cell count is kept by the balance between erythrocyte production and erythrocyte destruction. The regulation of progenitor cells to the erythroid lineage and their maturation are controlled by growth factors, cytokines and, very important, by the bone marrow microenvironment signaling directing nuclear transcription factors and gene expression. Erythropoietin is an obligate growth factor for the erythropoiesis and is required for the proliferation, differentiation, and survival of erythropoietic progenitor cells. GATA1 and GATA2 are transcription factors with key roles in gene regulation during erythropoiesis.

The investigation of the bone marrow and the bone marrow biopsy is one of the most valuable diagnostic tools for evaluating hematologic disorders. Furthermore, bone marrow analysis has great importance in the evaluation of non-hematologic conditions. Various chemical toxins, including minerals, as well as radiation may severely affect the bone marrow cells and stroma. In patients with fever of unknown origin (FUO), among others in patients with autoimmune deficiency syndrome (AIDS), the marrow may reveal various microorganisms, such as leishmaniosis, tuberculosis, and fungal as well as viral infections.