Chapter 21 - Section 21.1 - Study Guide - Assess Your Learning Outcomes - Page 844: 12

**Location and Gross Anatomy of the Thymus:** The thymus is located in the upper chest, behind the sternum (breastbone) and in front of the heart. It extends into the lower part of the neck. The thymus is relatively large in infants and children, but it gradually decreases in size as a person matures. **Gross Anatomy:** - The thymus consists of two main lobes that are connected by a central tissue bridge. - Each lobe is divided into smaller lobules, giving the thymus a segmented appearance. - The thymus is surrounded by a fibrous capsule that divides it into lobules. - Blood vessels and nerves enter the thymus at its outer cortex. **Histology of the Thymus:** - The thymus is composed of two main regions: the cortex and the medulla. - The cortex contains densely packed lymphocytes, primarily immature T lymphocytes (T cells). - The medulla contains fewer lymphocytes and more reticular cells, epithelial cells, and macrophages. **Functional Difference between Cortex and Medulla:** - **Cortex:** The cortex is the outer region of the thymus and is where most of the T cell development and maturation take place. Immature T cells undergo positive and negative selection here, ensuring that only T cells with appropriate antigen recognition and self-tolerance are allowed to mature and exit the thymus. - **Medulla:** The medulla is the inner region of the thymus. It is involved in the final maturation of T cells and the removal of T cells that react strongly against self-antigens. The medulla also contains Hassall's corpuscles, structures formed from concentric layers of epithelial cells. Their exact function is not fully understood, but they may be involved in regulatory processes. **Functions of Reticular Epithelial Cells (RTECs):** - Reticular epithelial cells (RTECs), also known as thymic epithelial cells, are a critical component of the thymus. - They form the structural framework of the thymus, helping to organize and compartmentalize different areas. - RTECs are involved in the positive and negative selection of T cells during their maturation. Positive selection ensures that T cells can recognize antigens presented by major histocompatibility complex (MHC) molecules. Negative selection eliminates T cells that react strongly against self-antigens, helping to establish self-tolerance. - RTECs express a diverse array of self-antigens, helping to educate developing T cells about self-tissues and promoting self-tolerance. **Necessity of the Thymus for Immunity:** - The thymus is critical for the development and maturation of functional T cells, which are essential for adaptive immune responses. - T cells are responsible for recognizing antigens presented by infected or abnormal cells and activating immune responses to eliminate them. - Without a properly functioning thymus, the development of mature and functional T cells is compromised, leading to immune deficiencies and increased susceptibility to infections and other immune-related disorders. In summary, the thymus is a primary lymphatic organ responsible for the maturation and education of T cells, which play a central role in adaptive immunity. The cortex and medulla have distinct roles in T cell selection and maturation, and reticular epithelial cells are essential for these processes. The thymus is necessary for the development of a functional immune system that can effectively protect the body against pathogens and maintain self-tolerance.

**Location and Gross Anatomy of the Thymus:** The thymus is located in the upper chest, behind the sternum (breastbone) and in front of the heart. It extends into the lower part of the neck. The thymus is relatively large in infants and children, but it gradually decreases in size as a person matures. **Gross Anatomy:** - The thymus consists of two main lobes that are connected by a central tissue bridge. - Each lobe is divided into smaller lobules, giving the thymus a segmented appearance. - The thymus is surrounded by a fibrous capsule that divides it into lobules. - Blood vessels and nerves enter the thymus at its outer cortex. **Histology of the Thymus:** - The thymus is composed of two main regions: the cortex and the medulla. - The cortex contains densely packed lymphocytes, primarily immature T lymphocytes (T cells). - The medulla contains fewer lymphocytes and more reticular cells, epithelial cells, and macrophages. **Functional Difference between Cortex and Medulla:** - **Cortex:** The cortex is the outer region of the thymus and is where most of the T cell development and maturation take place. Immature T cells undergo positive and negative selection here, ensuring that only T cells with appropriate antigen recognition and self-tolerance are allowed to mature and exit the thymus. - **Medulla:** The medulla is the inner region of the thymus. It is involved in the final maturation of T cells and the removal of T cells that react strongly against self-antigens. The medulla also contains Hassall's corpuscles, structures formed from concentric layers of epithelial cells. Their exact function is not fully understood, but they may be involved in regulatory processes. **Functions of Reticular Epithelial Cells (RTECs):** - Reticular epithelial cells (RTECs), also known as thymic epithelial cells, are a critical component of the thymus. - They form the structural framework of the thymus, helping to organize and compartmentalize different areas. - RTECs are involved in the positive and negative selection of T cells during their maturation. Positive selection ensures that T cells can recognize antigens presented by major histocompatibility complex (MHC) molecules. Negative selection eliminates T cells that react strongly against self-antigens, helping to establish self-tolerance. - RTECs express a diverse array of self-antigens, helping to educate developing T cells about self-tissues and promoting self-tolerance. **Necessity of the Thymus for Immunity:** - The thymus is critical for the development and maturation of functional T cells, which are essential for adaptive immune responses. - T cells are responsible for recognizing antigens presented by infected or abnormal cells and activating immune responses to eliminate them. - Without a properly functioning thymus, the development of mature and functional T cells is compromised, leading to immune deficiencies and increased susceptibility to infections and other immune-related disorders. In summary, the thymus is a primary lymphatic organ responsible for the maturation and education of T cells, which play a central role in adaptive immunity. The cortex and medulla have distinct roles in T cell selection and maturation, and reticular epithelial cells are essential for these processes. The thymus is necessary for the development of a functional immune system that can effectively protect the body against pathogens and maintain self-tolerance.