The hematopoietic system is not fully developed at birth, and the normal hematologic values of newborns and infants differ as compared to older children and adults. The differences are a manifestation of the unique characteristics of the embryonal and fetal development of the hematopoietic system that continues to evolve after birth. Furthermore, preanalytical and analytical factors unique for neonates and young children also contribute to these differences. This chapter will explore these factors and discuss how they define the normal hematologic values for different age groups.

Developmental hematopoiesis: a general view

The hematopoietic development, unlike any other organ system, occurs in successive anatomic sites where the hematopoietic stem cells (HSCs) are generated, maintained, and differentiate into various cell types [1]. The hematopoiesis begins in the yolk sac with the generation of angioblastic foci or “blood islands” that contain primitive erythroblasts. It then progresses further in several waves involving multiple anatomic sites: the aorta–gonadal–mesonephros (AGM) region, fetal liver, and bone marrow (BM) [2, 3]. Depending on the site of major hematopoietic activity, the hematopoiesis has been divided into three stages: the mesenchymal, hepatic, and myeloid stages with the yolk sac, liver, and bone marrow as major hematopoietic sites where hematopoietic cells with characteristic features are generated [4] (Fig. 1.1). There is a considerable temporal overlap between different stages. At birth and thereafter, the hematopoiesis is restricted to the bone marrow and continues to evolve in order to adapt to the new oxygen-rich environment and the needs of the growing organism.

Pediatric hematopathology is a special and highly challenging field. Despite its importance, however, and despite the sea of publications that exist in the field of hematopathology, few texts focus on the diagnosis of benign and malignant hematologic disorders found in children. As a result, even though the uniqueness of developmental factors and pathology in children is well recognized and appreciated, the specifics are not widely known or understood, which poses great difficulties for the diagnosis of hematologic diseases in children.

Diagnostic Pediatric Hematopathology presents an accurate and up-to-date examination of such diseases in children – both non-neoplastic and neoplastic. One goal is to provide knowledge about how the hematopoietic and lymphoid systems develop and how this development affects what can be considered normal and abnormal findings in children at various ages. A second key goal is to focus on the morphologic, immunophenotypic, cytogenetic, and molecular genetic characteristics of most pediatric-specific hematologic diseases so as to provide a resource that can be helpful in reaching a proper diagnosis when evaluating pediatric peripheral blood, bone marrow, and lymph nodes. The text addresses these goals through a team of experienced pediatric hematopathologists and clinical scientists drawn from major academic children's hospitals in the United States, Canada, and Europe, and this text is a result of our collaborative efforts.

The hematopoietic system is unique in comparison to other organ systems because its anatomic location shifts during the embryogenesis and fetal development from the yolk sac to the fetal liver and finally to the bone marrow (BM). At birth and thereafter, the hematopoiesis is restricted to the BM, which continues to evolve in order to accommodate the changing oxygenation needs of the growing child. As a result, the composition of the BM depends on the child's age – particularly early in life – as well as on the demands of the growing child, and so differs from the BM of adults. Knowledge of these differences needs to be considered when evaluating a child's BM in order to distinguish between normal development and pathologic processes.

Ontogeny of the hematopoietic system

Until definitive (adult) hematopoietic organs are fully developed, hematopoiesis occurs in successive anatomic sites where hematopoietic stem cells (HSCs) are generated, maintained, and expended to differentiate into blood cells [1, 2]. The HSCs develop from the hemangioblast, a mesoderm-derived multipotent precursor that gives rise to hematopoietic as well as endothelial and vascular smooth muscle cells [3, 4]. This process is initiated in the yolk sac between days 16 and 19 of gestation, with the formation of angioblastic foci or “blood islands” that contain primitive erythroblasts surrounded by endothelial cells. The yolk sac hematopoiesis is transient and generates HSCs that differentiate in the vasculature into primitive and definitive erythroblasts and rare macrophages.

Introduction

Bone marrow (BM) studies are often performed in children with solid tumors for initial diagnosis, for staging of disease, for monitoring of response to therapy, and for detecting recurrent diseases. In general, the presence of BM metastasis indicates high-stage disease and is associated with inferior outcome. Different pediatric solid tumors, however, metastasize with variable frequency depending on the tumor type. While neuroblastoma and alveolar rhabdomyosarcoma frequently present with BM metastases, other tumors such as pediatric brain tumors, non-rhabdomyosarcoma soft tissue sarcomas, and retinoblastoma metastasize less often. Some tumors, such as Wilms tumor, almost never metastasize to the BM and seldom require bone marrow evaluation. There is no single clinical finding that can predict the presence of BM metastasis in a child with a solid tumor. Bone pain, spinal cord compression, or pathologic fractures, although frequently reported, are non-specific and unreliable in confirming BM metastasis.

The standard for staging BM evaluation for solid tumors requires bilateral aspirates and biopsies with at least 1 cm of BM, not including the bone cortex or cartilage. BM biopsy may not be feasible in young infants, and a combination of MRI scans, BM aspirates, and ancillary studies may be acceptable for proper diagnosis. This approach, while sufficient for the diagnosis of metastasis, is limited in monitoring response to therapy and further follow-up. Thus, staging BM biopsy should be attempted when possible to overcome this limitation.

Introduction

As a result of combination chemotherapy administered from several months to 2–3 years, depending upon the type of malignancy and risk stratification, the cure rate in children with acute lymphoblastic leukemia (ALL) who are younger than 15 years of age has risen from less than 10% in the pre-chemotherapy era to 88% presently [1]. Moreover, the cure rate for acute myeloid leukemia (AML), although not as successful, has also markedly improved. This success came mostly as a result of randomized clinical trials conducted by the Children's Oncology Group (COG), the St. Jude Children's Research Hospital, the Medical Research Council in the United Kingdom, and the German Berlin–Frankfurt–Münster clinical trials group. However, while the chemotherapy is necessary for a cure, most of the chemotherapeutic agents have serious myelosuppressive effects resulting in short- and long-term abnormalities in the peripheral blood and bone marrow. Knowledge of these changes is essential for the proper evaluation of such specimens in children undergoing chemotherapy.

Factors affecting treatment response

The treatment response and leukemia clearance differ depending on multiple factors associated with the host response to chemotherapy. Recent studies have shown that these differences in response to drugs are due to individual genomic variations such as single nucleotide polymorphisms (SNPs), insertion, inversions, copy-number variations, epigenetic variants, or loss of heterozygosity that are able to influence the absorption, distribution, metabolism, and excretion of chemotherapeutic agents [2] (Table 19.1 [3]).

Introduction

Lymphadenopathies occur frequently during childhood, and their incidence varies with age. In a majority of children, lymph node enlargement is transient, self-limited, and resolves without sequel, and lymph node biopsies are rarely performed. When a lymph node biopsy is performed, the primary goal of histologic examination is to determine the nature of the process: whether it is a malignant or benign, lymphoid or non-lymphoid proliferation.

This chapter focuses on reactive lymphoid proliferations in children. The chapter will first summarize key points in the development and functional anatomy of lymph nodes, then will discuss the epidemiology of pediatric lymphadenopathies and finally will describe the most frequent types of reactive lymphoid proliferations in children. Since characterization of all lymphadenopathies is beyond the scope of a single chapter, here we will focus on those entities that present major diagnostic challenges in distinguishing benign from malignant disorders, as well as those pediatric lymphadenopathies that are seen in conjunction with specific syndromes.

Embryonal development of lymph nodes

Lymph nodes, similarly to the other secondary lymphoid organs, have a specialized architecture and microenvironment promoting controlled interactions of immune cells in order to elicit a rapid and appropriate immune response to infectious agents. Knowledge of the development and functional anatomy of lymph nodes is important since many of the morphologic changes observed in reactive conditions reflect lymph node function and its role in the normal immune response.

Congenital anemias due to impaired red cell production

Congenital anemias are a heterogeneous group of rare disorders due to impaired red cell production, resulting from either pure red cell aplasias and lack of erythroid progenitors in the marrow, or from ineffective erythropoiesis, dyserythropoiesis, and an increased cell death (Table 7.1). Either pathogenic mechanism leads to a variable degree of anemia with a low reticulocyte count. The peripheral blood findings are non-specific, and the diagnosis requires a bone marrow evaluation and ancillary studies. Anemias due to hemoglobinopathy, nutritional deficiency, increased red cell destruction, or bone marrow metastases are discussed in other chapters.

Diamond–Blackfan anemia [Online Mendelian Inheritance in Man (OMIM) 105650]

Epidemiology and clinical presentation

Diamond–Blackfan anemia (DBA), also known as congenital hypoplastic anemia, is a clinically and genetically heterogeneous group of disorders manifested in early infancy with anemia and reticulocytopenia due to absolute erythroid hypoplasia in otherwise normocellular bone marrow (BM). DBA is the first and, so far, the only known disease of abnormal ribosome biogenesis. It is characterized by mutations at structural ribosomal proteins, which result in intrinsic disorders of erythropoiesis, congenital anomalies, and an increased predisposition to malignancies [1, 2].