Lymphatic and Immune System

Diffuse Lymphatic Tissue

Appendix
Here the lamina propria and submucosa are heavily infiltrated with lymphatic tissue. Look near the lining epithelium for individual lymphoid cells. Lymphoid cells are also very abundant in the many lymphatic nodules in the appendix.

Ileum
Scattered cells and lymphoid tissue, as well as slightly more organized lymphatic nodules, are commonly found in the mucosa of the GI tract. Lymph nodules in the ileum are referred to as Peyer's patches. Observe the presence of the lymphoid cells (mostly lymphocytes and plasma cells).

Organs

Lymph node
Lymph nodes are more organized organs than are lymphatic nodules. They have a capsule and an underlying subcapsular sinus. The hilus is the site at which the artery and nerves enter the lymph node, and where the efferent lymphatic vessels and veins exit the lymph node. Supporting dense connective tissue trabeculae penetrate into the lymph node, along with the accompanying trabecular sinuses. Afferent lymphatic vessels penetrate the capsule and drain into the subcapsular sinus.
Note the outer cortex containing lymphatic nodules and its transition to the inner medulla. In the medulla, the lymphoid tissue is organized as irregular cords of cells, medullary cords, surrounded by medullary sinuses. The node is composed of many free cells, mostly lymphocytes and plasma cells. Also note the distribution and density of reticular cells, which have large pale-staining nuclei and eosinophilic branching cellular processes. Identify the high endothelial venules that are located in the cortex.

Lymph node
Identify the capsule, subcapsular sinus, lymphatic nodules, supporting connective tissue trabeculae with trabecular sinuses, and medullary cords with medullary sinuses.

Lymph node
Identify the capsule, subcapsular sinus, lymphatic nodules, trabeculae with trabecular sinuses, and medullary cords with medullary sinuses. In the medullary sinuses, identify reticular cells, whose processes form a meshwork within the sinus. Their large nuclei typically have a prominent nucleolus.
Identify the many afferent lymphatic vessels as they pass through the capsule, and observe the valves in their lumens. In the cortex, identify high endothelial venules, which is where diapedesis occurs; the site of entry of lymphocytes into the lymph node from the vascular circulation.

Spleen, human.
Note the dense connective tissue capsule covered with mesothelium (peritoneum). Dense connective tissue trabeculae extend into the organ from the capsule to provide support. The trabeculae also serve to carry blood vessels to/from the vascular spaces in the spleen. Within a within trabecula, observe a trabecular artery and trabecular vein. Examine the two major components of the spleen the white pulp and red pulp. The white pulp can be organized as splenic (lymph) nodules with the characteristic central artery (arteriole) in each nodule. The germinal centers of the nodules are mainly B cells. In addition, the lymphatic tissue not organized as nodules can be seen as the periarterial lymphatic sheath (a thin cuff of tissue made up of T lymphocytes that surrounds the central arteries). Both red and white pulp is supported by reticular fibers that form a supporting framework of the spleen.

Spleen, baboon.
Examine the baboon spleen, which has active germinal centers, central arteries, and enlarged splenic nodules. This specimen demonstrates the venous sinuses and splenic cords (cords of Billroth). Identify the capsule, mesothelial lining of the capsule, trabeculae, and trabecular arteries and trabecular veins.

Thymus, infant.
Note that the thymus consists of an outer capsule, a darkly-stained cortex, and a lighter central medulla. The cortex is darker because of the high concentration of small lymphocytes. Smaller subdivisions of the gland are called lobules. Trabeculae extend from the capsule to separate the lobules from each other. Within the medulla are eosinophilic, whorled structures called thymic (Hassall’s) corpuscles composed of a concentric mass of epithelioreticular cells joined together by many desmosomes. These are diagnostic of the thymus, although they are not particularly well-demonstrated in this specimen (they are more obvious in the next specimen). Among the numerous small lymphocytes are many thymic reticular cells (epithelial origin). Note particularly that the thymus lacks lymph nodules and sinuses. Recall that after puberty the thymus undergoes involution and is infiltrated by fat as lymphocytes degenerate (the organ is, however, still functional).

Thymus, human infant.
In this thymus specimen the thymic (Hassall’s) corpuscles in the medulla are more obvious, although the epithelioreticular cells are difficult to identify. Also observe the darkly-stained cortex, and the lighter-stained central medulla, lobules, and trabeculae.

Palatine Tonsil
Note in the tonsil that the lymphoid tissue is arranged as nodules (with germinal centers) and diffuse accumulations of lymphocytes. Try to identify the rudimentary capsule. The free surface is covered with non-keratinized stratified squamous epithelium (continuous with the oral cavity) which dips into the organ forming epithelial invaginations called tonsillar crypts. Note that many lymphocytes (very small darkly stained, round nuclei) migrate through the epithelium and may sometimes obscure its identity.