Lymphoid Tissue and Immmune System

Lymphoid Tissue and Immmune System - 1 Lymphoid Tissue and...

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1 Lymphoid Tissue and Immune System 2010 1. Primary lymphoid tissues are bone marrow and thymus 2. Secondary lymphoid tissues are the encapsulated spleen ( inserted into circulatory system) and all the lymph nodes (inserted in to the lymphatic system) and all the diffuse lymphoid tissues that are not encapsulated like tonsils, GALT, BALT, SALT, Peyer’s Patches, etc. 3. Lymphoid tissue is another type of specialized CT which ultimately derived from the primitive mesenchyme. THYMUS is an H- shaped organ that lies below the thyroid gland just behind the sternal notch. It has a CT capsule which penetrates to form incomplete lobules. Each lobule has a dark staining cortex and a central light staining medulla. Unlike other lymphoid tissues that are derived exclusively from mesenchyme (mesoderm), the thymus has a dual embryonic origin. The lymphocytes are derived from mesenchyme, but the epithelial reticular cells are derived from the endoderm. Cortex contains large numbers of T-cell precursors plus some reticular epithelial cells, and macrophages. Medulla contains differentiated T cells, as well as epithelial reticular cells and Hassall corpuscles. These epithelial reticular cells have long cytoplasmic extensions that stretch out in all direction to come in contact with the cytoplasmic extension of other epithelial reticular cells where they are held together by your old friend, desmosomes. In this manner they form a reticulum (net; rete is L. for net). The interstices of this net are packed with T-lymphs. Maturation of the T cell precursors takes place in cortex and they become CD4 + Helper cells or CD8 + Cytotoxic cells. Mature T cells migrate to medulla and enter capillaries. Note that the thymus has a very complicated physiology. It is here where the T cells learn to recognize “self”. The epithelial reticular cells produce all sorts of hormones but most are probably paracrine type, that is, they act locally on the T cells influencing their differentiation. After puberty the thymus involutes and a lot of the tissue is replaced by adipose. The exact function of Hassall Corpuscles is unknown but they are the most characteristic feature of thymus. Despite presence of reticular endothelial cells, there are no reticular fibers in thymus. (Reticular fibers are made by fibroblasts and smooth muscle cells which were derived from mesoderm).Note that many, if not most, of the CD4 and CD8 cells are carried via the circulation to specific areas of lymph nodes and the spleen but they like to roam Lymph Nodes are encapsulated glands in scattered areas of the body inserted into lymphatic vessels. Shaped like a lima bean. On convex side several afferent
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2 lymphatic vessels pierce the capsule to enter the node. These vessels have valves and they empty into subcapsular sinus which lies just inside the lymph node under the capsule. The lymph flows from the subcapsular sinus into cortical sinuses and then into medullary sinuses and finally these medullary sinuses coalesce to form the
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Lymphoid Tissue and Immmune System - 1 Lymphoid Tissue and...

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