Porth's Essentials of Pathophysiology, 4e

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Disorders of White Blood Cells and Lymphoid Tissues

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with a cytoplasm containing many primary granules. In the subsequent metamyelocyte stage, the nuclei dis- tort and become arclike, producing the band devel- opmental stage. Maturation from metamyelocyte to mature neutrophil involves progressive condensation of nuclear chromatin, increasing nuclear lobulation, and the appearance of secondary (specific) granules. Eosinophils and basophils undergo similar develop- mental stages but develop different secondary gran- ules. Like granulocytes, monocytes develop from the granulocyte-monocyte progenitor cell and progress through monoblast and promonocyte stages. Lymphocytes derive from lymphoid stem cells and progress through the lymphoblast and prolymphocyte stages. The prolymphocytes leave the bone marrow and travel to the lymphoid tissues, where further differentia- tion into T and B lymphocytes occurs. LymphoidTissues The body’s lymphatic system consists of the lymphatic vessels, lymphoid tissue and lymph nodes, thymus, and spleen (see Chapter 15). Although both precursor B and T lymphocytes begin their development in the bone marrow, they migrate to peripheral lymphoid structures to complete the differentiation process. B lymphocytes mature in the bone marrow, differentiate into plasma cells, and then move to the lymph nodes, where they continue to proliferate and produce antibodies. T lym- phocytes leave the bone marrow as precursor T lympho- cytes travel to the thymus, where they differentiate into CD4 + helper T cells and CD8 + cytotoxic T cells, after which many of them move to lymph nodes, where they undergo further proliferation. Lymph nodes consist of organized collections of lymphoid tissue located along the lymphatic vessels. 2–4 Typically grayish white and ovoid or bean shaped, they range in diameter from 1 mm to 1 to 2 cm. A fibrous capsule and radiating trabeculae provide a supporting structure, and a delicate reticular network contributes to internal support (Fig. 11-4). The parenchyma of the lymph node is divided into an outer cortex and an inner medulla. The cortex contains well-defined B-cell and T-cell domains. The superficial outer cortex contains aggregates of cells called follicles . Follicles are the B-cell zones of the lymph nodes. There are two types of fol- licles: immunologically inactive follicles, called primary follicles, and active follicles that contain germinal cen- ters, called secondary follicles. Germinal centers contain large lymphocytes (centroblasts) and small lymphocytes with cleaved nuclei (centrocytes). The mantle zone is the small layer of B cells surrounding the germinal centers. The cortex around the follicles is called the paracortex . This region contains most of the T cells in the lymph nodes. Like normal lymphocytes, malignant B and T cells tend to home to particular nodal sites, leading to char- acteristic patterns of involvement. For example, follicu- lar lymphomas develop in the B-cell areas of the lymph node, whereas T-cell lymphomas typically grow in the paracortical T-cell zones.

Myeloid stem cell

Lymphoid stem cell

Committed cells

Lymphoblast

Myeloblast

Monoblast

Bone marrow

Promyelocyte

Promonocyte Prolymphocyte

Neutrophilic myelocyte

Neutrophilic metamyelocyte

Neutrophilic band cells

Granular leukocytes Monocytes Lymphocytes

(some become)

(some become)

Blood and lymphoid tissue

Wandering macrophages

Plasma cells

The granulocytic precursor cells, which are called myeloblasts, have round to oval nuclei, with delicate chromatin and a blue to gray cytoplasm. During their next stage of development, the myeloblasts are trans- formed into promyelocytes with similar nuclei, but FIGURE 11-3. Leukocytes originate from multipotential stem cells in the bone marrow. Granular leukocytes (neutrophils, eosinophils, basophils) have their origin in the myeloid stem cells and develop through a sequence involving myeloblasts. Monocytes, like granulocytes, are progeny of the myeloid stem cell line, but develop along a pathway involving monoblasts. Only lymphocytes originate from the lymphoid stem cell line. They develop through a sequence involving lymphoblasts and are released from the bone marrow as prolymphocytes, which undergo further differentiation in the lymphoid organs.