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The primary lymphoid organs, bone marrow and thymus, are specialized sites where lymphocytes undergo maturation, proliferation, and selection for survival. These processes ensure that lymphocytes can effectively distinguish between self and non-self, a critical factor for their development and function in the immune system.
The Bone Marrow
Red bone marrow, characterized by its collection of loosely arranged cells, serves as the site for hematopoiesis—the process of blood cell formation (Figure 1). In contrast, yellow bone marrow primarily functions as an energy reservoir, predominantly composed of fat cells.
During embryonic development, the initial site for blood cell production is situated within the yolk sac. As embryogenesis progresses, the responsibility for blood cell formation is gradually assumed by various organs, including the spleen, lymph nodes, and the liver. The bone marrow emerges as the predominant site for most hematopoietic functions. While the bone marrow oversees most cell differentiation processes, some final stages of maturation for select cell types may occur in organs such as the thymus. For instance, B cells predominantly develop within the red bone marrow, while specific immature lymphocytes depart from the bone marrow and undergo maturation primarily within the thymus.
Figure 1 | Bone Marrow | 2 Red bone marrow fills the head of the femur, and a spot of yellow bone marrow is visible in the center.
The Thymus
The thymus is a bilobed organ located in the mediastinum, above the heart and behind the sternum, encased in a connective tissue capsule. This capsule extends inward as trabeculae, dividing the thymus into smaller lobules. The outer region, or cortex, is rich in thymocytes, epithelial cells, macrophages, and dendritic cells, while the inner medulla contains a more sparsely arranged collection of these cells (Figure 2). Thymic involution, the gradual reduction in the thymus size, is a natural process. This process commences within the first year of life.
Figure 2 | Location, Structure, and Histology of the Thymus | 3 The thymus, located anterior to the heart, is the primary site of T cell maturation. Histological sections show a dark-staining cortex with densely packed thymocytes and cortical epithelial cells, separated into lobules by trabeculae (extensions of the fibrous capsule). The lighter-staining medulla contains dendritic cells, macrophages, and medullary epithelial cells. T cells are transported via lymphatic ducts to lymph nodes and other peripheral lymphoid organs for immune surveillance.
14.4: Primary (Central) Lymphoid Organs and Lymphocyte Development is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
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