The innate immune cells dendritic cells, macrophages, etc. The adaptive immune cells T-lymphocytes and B-lymphocytes , on the other hand, form specialized lymphoid tissues in addition to circulating the body. A lymphoid tissue can simply be described as any tissue containing large accumulations of lymphocytes. These lymphoid tissues include:.
The term primary lymphoid tissue is used to describe sites of lymphocyte maturation, including the bone marrow and thymus. Secondary lymphoid tissues are sites of lymphocyte activation and proliferation.
These include the spleen, lymph nodes, and MALT. The term lymphoid organ is used to describe lymphoid tissues that are well organized and encapsulated. These include the lymph nodes, thymus, and spleen. The bone marrow is the site of B-cell maturation. It has been covered extensively in the Laboratory on Peripheral Blood and Bone Marrow and will not be discussed further in this laboratory.
The thymus is the primary lymphoid organ engaged in the maturation of T-cells. It is most active during childhood and slowly degenerates after puberty. The thymus is encapsulated and divided into lobules by the interlobular septa, which contain blood vessels. Each lobule features an outer cortex densely populated by lymphocytes and an inner medulla less heavily infiltrated with lymphocytes and thus less heavily stained.
Unlike secondary lymphoid organs, the thymus lacks nodules and afferent lymphatic vessels, and it is not a site where humoral immune responses are mounted. Rather, it is the site of maturation of T-cells. This process occurs in the peripheral to central direction within thymic lobules.
The thymic cortex contains immature and maturing T-cells. The larger and more immature T-cells are found in the outer cortex. They undergo mitosis and mature further as they move inward towards the medulla.
T-cells that fail to mature properly undergo apoptosis and are removed by the macrophages at the interface of cortex and medulla. Immunocompetent T cells exit the parenchyma via post-capillary venules or efferent lymphatics.
Cells forming the structural framework of the thymic parenchyma are epithelial in origin. This is important to keep in mind because the structural cells of all other lymphatic organs are mesodermal. Parencyhmal cells can most easily be recognized in the medulla as large, stellate, pale staining cells. In the thymic medulla, one can find characteristic Hassall's corpuscles, which are composed of keratinized epithelial cells. While the function of these structures is unknown, they may represent a degenerative process since they increase over the course of a lifetime.
The medulla is also the site of negative selection, in which maturing T cells that react to self-antigens are eliminated by apoptosis in order to prevent autoimmunity. The gross tissue structure of the thymus depends upon the age of the individual. They are surrounded by groups of T-cells that a form a structure called the periarteriolar lymphoid sheath.
In the white pulp, the vessel is surrounded by the periarteriolar lymphoid sheath PALS , which is made up of mostly T-cells. Surrounding the follicle are the narrow mantle zone, which contains packed lymphocytes, and a broader marginal zone, which contains more diffuse lymphocytes. The follicle here is a secondary follicle, as evidenced by the presence of a pale-staining germinal center.
The germinal center is surrounded by a narrow ring of deep-stained mantle that contains resting B-cells. The marginal zone separates the follicle from the red pulp. The red pulp is mainly filled with venous sinuses, which contain red blood cells and occasionally white blood cells. The sinuses are lined by elongated, rod-shaped endothelial cells. In the sinuses, old or abnormal red cells that have lost their ability to deform are removed by macrophages, while healthy blood cells can squeeze through the walls of the sinuses and are transported out of the organ by the splenic vein.
Slits between the endothelial cells allow viable red blood cells to squeeze into the sinuses from the splenic parenchyma. These sinuses eventually drain into the splenic vein. Systems Cell Biology Yale Lymphatics Lab Learning Objectives Explain the flow of lymph through the lymph node and blood through the spleen, and how the structure of these organs facilitates their function Distinguish the between B- and T-cell regions of lymphoid tissue Recognize the differences in appearance and function between red and white pulp in the spleen.
Lab Content Introduction The immune system is important for the elimination of harmful foreign pathogens, which it achieves via its ability to distinguish self from non-self. Lymph Nodes Lymph nodes occur along the course of the lymphatic vessels. Lymph Node Capsule This is a high power view of the lymph node capsule and subcapsular sinus. Lymph Node Secondary Follicle This is a medium power view of a secondary follicle in a lymph node. Lymph Node Paracortex The paracortex of the lymph node is a T-cell zone.
Identify the palatine tonsil and understand its function. Slide 55 Appendix. Here the lamina propria and submucosa are heavily infiltrated with lymphatic tissue. Look near the lining epithelium for individual lymphoid cells. Slide 53 Ileum. Scattered cells and lymphoid tissue, as well as slightly more organized lymph nodules, are commonly found in the mucosa of the GI tract.
Lymph nodules in the ileum are referred to as Peyer's patches. Again, examine the lymphoid cells mostly lymphocytes and plasma cells. Slide 59 Lymph node. Lymph nodes are more organized organs than are nodules. They have a capsule , subcapsular sinus , hilus , supporting connective tissue trabeculae , and trabecular sinuses. Arteries, veins, and efferent lymphatics are all located at the hilus.
This barrier consists of:. Non-fenestrated, continuous endothelium of blood capillaries. Pericytes and epithelial-reticular cells that form a sleeve around the capillaries in addition to surrounding connective tissue. Macrophages that are present in the connective tissue. Thick basal lamina of epithelial reticular cells.
Desmosome connections forming tight junctions between adjacent endothelial cells of capillaries, as well as similar connections between surrounding epithelial-reticular cells.
Medullary zone. Contains mostly epithelial-reticular cells and fewer T-lymphoblasts and lymphocytes than the cortex. Also contains specialized structures known as Hassall's corpuscles - function unknown. Blood supply. Branches from the internal thoracic and inferior thyroid arteries that penetrate the capsule surrounding the thymus. Capillaries branch into the cortico-medullary junction area and extend into the cortex. Blood-thymus barrier is present as described above.
These eventually arch into the medulla where they drain into venules. Venules connect to veins that exit thymus along connective tissue septa. Cells entering thymus from bone marrow arrive via the circulatory system and enter in the medullary zone from which they can move to the cortical areas. There are no afferent lymphatic vessels in thymus. So it does not act as filter for lymphatic fluids.
Only a few efferent lymphatic vessels are present, and these are associated with the blood vessels. Physiology of thymus relative to immune system. Recall that T-lymphocytes are responsible for cell-mediated immune responses and also for interacting with B-lymphocytes to cause those cells to produce antibodies against certain antigens.
The thymus can be considered a proliferation and maturation center for T-lymphocytes. Precursor cells migrate from bone marrow to thymus entering the organ via blood vessels in medullary zone.
These cells undergo mitosis and maturation in cortical zone and then leave thymus through blood vessels of medullary zone to go about their various activities.
General characteristics. Largest piece of lymphatic tissue in body. Site of formation of activated lymphocytes that enter the circulatory system. Also important in recycling of components of worn-out blood cells. Can be said to act as filter of blood both in an immunologic sense that is it mediates components of immune response , but also in the sense of removing worn out erythrocytes from circulation. Surrounded by a dense connective tissue capsule that extends processes trabeculae into lymphatic tissue of this organ.
There are no lymph vessels in the pulp. Color designations have to do with appearance in freshly cut open organ. Red pulp. Cells can enter or leave spleen through large spaces between endothelial cells lining sinusoids. White pulp. Central artery is identifying characteristic.
Mainly T-lymphocytes in periarterial lymphatic sheath PALS sheath surrounding central artery, B-lymphocytes in lymphatic tissue of white pulp surrounding sheath i. The B-lymphocytes and associated cells are organized into typical lymphatic nodules. Blood circulation in spleen. Arteries enter pulp via trabeculae. Branches of arteries extend into white pulp forming the central arteries of the white pulp.
These arteries are surrounded by a sheath of lymphocytes mainly T-lymphocytes that form the periarterial lymphatic sheath PALS extending along the length of the artery. Branches of central artery extend into white pulp. Some of these leave white pulp and then loop back toward it emptying into sinusoids that form part of marginal zone of loose lymphoid tissue that surrounds white pulp.
Other branches of central arteries extend into red pulp to form the pulp arteries that empty into various sinusoids of this tissue.
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