Lymphatic tissue is found in five different locations in the body; 1) the lamina propria of mucous membranes (walls of alimentary canal, upper respiratory passages and urinary tract) 2) lymph nodes; 3) spleen; 4) bone marrow; and 5) the thymus. In most cases lymphoid tissue consists of stroma and parenchyma. The stroma is formed of a network of reticular fibers to support the parenchymal cells and the walls of sinuses. The cells of the parenchyma include lymphocytes at various stages of development, reticular cells, macrophages, and plasma cells. [Note that the structure of the bone marrow will be covered in hematopoiesis.]
[15-3]:Lymphatic tissue is often found in the lamina propria underlying mucous membranes (those membranes lining cavities which have openings to the outside of the body). This lymphatic tissue can assume three forms: diffuse lymphatic tissue, solitary lymphatic nodules and aggregated lymphatic nodules, the latter being present in the small intestine (Peyer’s patches), the vermiform appendix, and tonsils.
Tonsil, Palatine (H&E) [#38]: This lymphoid organ has no complete capsule or trabeculae. It consists of a collection of nodules, some of which contain germinal centers (arrows), which are lighter staining areas. The tonsil is covered on its free surface by a stratified squamous non-keratinized epithelium which penetrates deeply through the lymphoid tissue of the tonsil to form crypts lined by the same type of epithelium. In some areas, the epithelium is infiltrated with lymphocytes (presumably on their way to the oral cavity).
[15-4]:Colon (H&E) [#71]: Examine the inner lining of the colon. Note the presence of a thin band of smooth muscle upon which the inner lining of the colon rests. Follow this thin ring of smooth muscle around the colon and observe that there are discontinuities in the band that correspond to the presence of large numbers of lymphocytes. This is a lymphoid nodule (follicle) which may or may not contain a germinal center.
[15-5]:Colon (H&E) [#71]: Examine the inner lining of the colon. Note the presence of a thin band of smooth muscle upon which the inner lining of the colon rests. Follow this thin ring of smooth muscle around the colon and observe that there are discontinuities in the band that correspond to the presence of large numbers of lymphocytes. This is a lymphoid nodule (follicle) which may or may not contain a germinal center (arrow).
[15-6]:Appendix (H&E) [#73]: The appendix contains numerous follicles which may comprise a large fraction of this organ.
[15-7]:II. LYMPH NODES
The lymph nodes are the only lymphatic organs which are interposed in the course of lymphatic vessels and thus possess both afferent and efferent connections. Grossly, the lymph node has a bean or kidney shaped form which may be slightly indented in one side (arrow).
Lymph Node (H&E) [#37]: Study the arrangement of the stroma which consists of the capsule and trabeculae of connective tissue (mainly collagen), and the reticular connective tissue stroma (which cannot be distinguished in H&E preparations). Note that the lymph node has a peripheral zone (cortex) which is divided by thin connective tissue trabeculae into compartments that contain lymph follicles surrounded by lymph sinuses. Locate the subcapsular lymph sinus. The central part of the lymph node is the medulla. It contains irregularly-arranged trabeculae enclosing the medullary lymph cords and sinuses. Locate the hilum of the lymph node and find the efferent lymphatic vessel. Locate an afferent vessel on the convex surface of the node.
[15-8]:II. LYMPH NODES
The lymph nodes are the only lymphatic organs which are interposed in the course of lymphatic vessels and thus possess both afferent and efferent connections. Grossly, the lymph node has a bean or kidney shaped form which may be slightly indented in one side.
Lymph Node (H&E) [#37]: Study the arrangement of the stroma which consists of the capsule and trabeculae of connective tissue (mainly collagen), and the reticular connective tissue stroma (which cannot be distinguished in H&E preparations). Note that the lymph node has a peripheral zone (cortex) which is divided by thin connective tissue trabeculae into compartments that contain lymph follicles surrounded by lymph sinuses. Locate the subcapsular lymph sinus (arrow). The central part of the lymph node is the medulla. It contains irregularly-arranged trabeculae enclosing the medullary lymph cords and sinuses. Locate the hilum of the lymph node and find the efferent lymphatic vessel. Locate an afferent vessel on the convex surface of the node.
[15-9]:Lymph Node (Silver reticulum) [#36]: The reticular fibers are selectively stained brown-black by the silver reticulum stain and are arranged in loose spidery meshworks. These reticular fibers may be observed to blend with the collagenous fibers of the capsule and trabeculae, and they are usually intimately associated with stellate reticular cells (that are unstained in this section). Note also that the meshwork of reticular fibers criss-cross the various sinuses and are denser in the vicinity of blood vessels.
[15-10]:III. SPLEEN
The spleen is interposed directly into the blood stream. Unlike the lymph nodes, it possesses neither afferent lymphatic vessels nor a lymphatic sinus system. One of the major functions of the spleen is to remove old and worn out red blood cells.
Spleen, Human (H&E) [#40]: The spleen is characterized by a robust capsule from which heavy irregular trabeculae of connective tissue and smooth muscle extend into the organ (arrow). From the appearance of fresh unfixed tissue, the spleen is divided into red and white pulp. The white pulp consists of periaterial lymphoid sheaths (PALS) which may also contain lymphoid follicles scattered irregularly throughout the organ. Characteristically, what is called the central artery can usually be located at the periphery of the follicle. Most of the spleen is red pulp which consists of splenic sinusoids and splenic cords which consist of various blood cells (mostly R.B.C.’s) in reticular stroma. Between the white pulp and red pulp is the marginal zone which consists of many sinuses and loose lymphoid tissue. Except at the hilum, the spleen is covered completely with peritoneum.
[15-11]:III. SPLEEN
The spleen is interposed directly into the blood stream. Unlike the lymph nodes, it possesses neither afferent lymphatic vessels nor a lymphatic sinus system. One of the major functions of the spleen is to remove old and worn out red blood cells.
Spleen, Human (H&E) [#40]: The spleen is characterized by a robust capsule from which heavy irregular trabeculae of connective tissue and smooth muscle extend into the organ. From the appearance of fresh unfixed tissue, the spleen is divided into red and white pulp. The white pulp consists of periaterial lymphoid sheaths (PALS) which may also contain lymphoid follicles scattered irregularly throughout the organ. Characteristically, what is called the central artery (arrow) can usually be located at the periphery of the follicle. Most of the spleen is red pulp which consists of splenic sinusoids and splenic cords which consist of various blood cells (mostly R.B.C.’s) in reticular stroma. Between the white pulp and red pulp is the marginal zone which consists of many sinuses and loose lymphoid tissue. Except at the hilum, the spleen is covered completely with peritoneum.
[15-12]:Spleen (Iron, Prussian blue) [#41]: The spleen is one of the sites involved in trapping aged red blood cells. This process involves the reutilization of iron from the hemoglobin. This section was specifically stained to demonstrate the presence of this iron, as indicated by a light blue color. Note that most of the iron is present in regions of the red pulp (arrow).
[15-13]:Spleen (Iron, Prussian blue) [#41]: The spleen is one of the sites involved in trapping aged red blood cells. This process involves the reutilization of iron from the hemoglobin. This section was specifically stained to demonstrate the presence of this iron, as indicated by a light blue color. Note that most of the iron is present in regions of the red pulp.
[15-14]:IV. THYMUS
The thymus is a bilobed, encapsulated organ which is situated immediately dorsal to the heart. In the embryo, it is one of the first tissues to become lymphoid and is responsible for the maturation of T-lymphocytes. It is well developed at birth, achieves its greatest weight at puberty, and thereafter undergoes progressive involution.
Thymus, early human (H&E) [#42]: A thin capsule surrounds the two lobes of the thymus. Each lobe is incompletely divided into lobules by thin trabeculae. Each lobule has a cortex packed with lymphocytes (thymocytes) that mask other types of cells, and a medulla which is paler because it contains fewer lymphocytes. Branching irregular reticular endothelial cells are present in the medulla but are difficult to distinguish in this slide. A characteristic feature of the thymic medulla is the presence of many Hassall’s (thymic) corpuscles, which are encapsulated structures serving no known function.
[15-15]:IV. THYMUS
The thymus is a bilobed, encapsulated organ which is situated immediately dorsal to the heart. In the embryo, it is one of the first tissues to become lymphoid and is responsible for the maturation of T-lymphocytes. It is well developed at birth, achieves its greatest weight at puberty, and thereafter undergoes progressive involution.
Thymus, early human (H&E) [#42]: A thin capsule surrounds the two lobes of the thymus. Each lobe is incompletely divided into lobules by thin trabeculae. Each lobule has a cortex packed with lymphocytes (thymocytes) that mask other types of cells, and a medulla which is paler because it contains fewer lymphocytes. Branching irregular reticular endothelial cells are present in the medulla but are difficult to distinguish in this slide. A characteristic feature of the thymic medulla is the presence of many Hassall’s (thymic) corpuscles, (arrow) which are encapsulated structures serving no known function.
[15-16]:Thymus, Involuted, Adult (H&E) [#43, 45]: Compare the structure on these slides with that on the previous slide. There is little remaining thymopoietic tissue (arrow). More of the thymus has become involuted, being replaced with fatty connective tissue. Hassall’s corpuscles should be apparent.
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