Primary Lymphoid Structures PDF

Page 840 21.3 Primary Lymphoid Structures The general structure and function of the primary lymphoid structures are described in this section. These include the red bone marrow and the thymus. 21.3a Red Bone Marrow LEARNING OBJECTIVES 6. Describe the location and general function of red bone marrow....

Page 840 21.3 Primary Lymphoid Structures The general structure and function of the primary lymphoid structures are described in this section. These include the red bone marrow and the thymus. 21.3a Red Bone Marrow LEARNING OBJECTIVES 6. Describe the location and general function of red bone marrow. 7. Identify the unique step required by T-lymphocytes in their maturation. Red bone marrow is located within trabeculae in portions of spongy bone within the skeleton. In adults, these include the flat bones of the skull, the vertebrae, the ribs, the sternum, the ossa coxae, and proximal epiphyses of each humerus and femur (see section 7.2d). Red bone marrow is responsible for hematopoiesis (or hemopoiesis), which is the production of formed elements. Hematopoiesis is described in detail in section 18.3a. Recall that the formed elements include erythrocytes, platelets, granulocytes (neutrophils, eosinophils, and basophils), and agranulocytes (monocytes and lymphocytes) ( figure 21.4). Two of the major types of lymphocytes are B-lymphocytes (also called B-cells) and T-lymphocytes (also called T-cells). Most formed elements move directly from the red bone marrow into the blood and circulate following hematopoiesis. Unlike the other formed elements, Tlymphocytes must migrate to the thymus to complete their maturation. The functions of both B-lymphocytes and T-lymphocytes are described in detail in chapter 22. The “T” in the name T-lymphocytes originates from the requirement of these cells to complete their maturation in the thymus. Figure 21.4 Formed Elements of Hematopoiesis. Red bone marrow produces all formed elements through the process of hematopoiesis. Immature T-lymphocytes (pre-T-lymphocytes) migrate from the bone marrow to the thymus to complete their maturation. (Note that the figure does not accurately display the relative size of the different formed elements.) WHAT DID YOU LEARN? 6 Why is red bone marrow considered a primary lymphoid structure? INTEGRATE LEARNING STRATEGY 21.2 Lymphocytes can be identified according to the tissue or organ where they mature: T-lymphocytes mature in the Thymus. B-lymphocytes mature in the Bone marrow. However, the “B” in the name B-lymphocytes did not originate because B-lymphocytes develop in the bone marrow. Rather, these cells were first discovered in chickens and were named for where they develop in chickens—in the bursa of fabricius (a lymphoid structure in birds). Humans lack the bursa of fabricius, and B-lymphocytes complete their maturation in the bone marrow. 21.3b Thymus LEARNING OBJECTIVE 8. Describe the structure and general function of the thymus. The thymus (thī′mŭs) is a bilobed organ that is located in the superior mediastinum and functions in T-lymphocyte maturation ( figure 21.5). In infants and young children, the thymus is quite large and extends into the anterior mediastinum as well. The thymus continues to grow until puberty, when it reaches a maximum weight of 30 to 50 grams (approximately 1/10 of a pound). Cells within the thymus begin to regress after it reaches this size. Thereafter, much of the thymic tissue is replaced by adipose connective tissue ( figure 21.5c). Figure 21.5 Thymus. (a) The thymus is a bilobed lymphoid organ that is most prominent in children. (b) A micrograph of a child’s thymus reveals the arrangement of the outer cortex and the central medulla within a lobule. (c) A micrograph of an adult’s thymus showing loss of thymic tissue and replacement with adipose connective tissue. (b) ©McGraw-Hill Education/Alvin Telser (c) ©Dr. Thomas Caceci, Virginia-Maryland Regional College of Veterinary Medicine APR Module 10: Lymphatic: Dissection: Thoracic: Thymus : Adult: Remnant of Thymus INTEGRATE CONCEPT CONNECTION Recall from section 17.2a how some organs, in addition to their primary function, also house clusters of endocrine tissue that produce hormones. The thymus, for example, contains (a) lymphoid cells and is considered part of the lymphatic system, and (b) epithelial tissue that produces and secretes hormones (e.g., thymosin, thymulin, thymopoietin) and is considered part of the endocrine system (see section 17.11c). Page 841 The thymus in a child consists of two fused thymic lobes, each surrounded by a connective tissue capsule ( figure 21.5b). Fibrous extensions of the capsule, called trabeculae (trăbek′ū-lē), or septa, subdivide the thymic lobes into lobules. Each lobule is arranged into an outer cortex and inner medulla. Both parts are composed primarily of epithelial tissue infiltrated with T-lymphocytes in varying stages of maturation. The cortex contains immature T-lymphocytes, and the medulla contains mature T-lymphocytes. The epithelial cells secrete thymic hormones (e.g., thymulin) that participate in the maturation of T-lymphocytes (see section 17.11c). Because the thymus contains both lymphoid cells and epithelial tissue, it is described as a lymphoepithelial organ. The details of T-lymphocyte maturation are described in section 22.5. WHAT DID YOU LEARN? 7 How are the two types of T-lymphocytes arranged in the cortex and medulla?