(Transcribed from Dr. Kalliecharan’s lecture, 3 Mar 2000 by Brian Buschman)
Return to Semester One Goodies
Return to The Unofficial Ross Page
There are two classes of non-encapsulated lymphoid tissue:
1) Diffuse lymphoid tissue which are collections of immune cells found around the body especially in loose connective tissue deep to the epithelia on body cavities such as in the digestive tract.
2) Dense lymphoid tissue which include lymph nodules and tonsils.
a) Lymph nodules are tight balls of B-lymphocytes. A primary lymph nodule is a solid color and has not yet been challenged by an antigen. Secondary lymph nodules have been antigenically challenged and are recognizable in section because they have a germinal center which stains differently then the outer portion.
b) Tonsils have an epithelium around them which has deep crypts that the antigens get into and cause infection. The tonsils are the palatine, lingual and the pharyngeal.
The thymus has a connective tissue capsule around it which penetrated into it to divide it into lobules. Each lobule has connective tissue that extends into it as trabecula to divide the lobule into a cortex and medulla. The thymus has no reticular fibers which are not to be confused with the cells it does have called epithelial-reticular cells. These cells function as antigen presenting cells (per Dr. Kalliecharan) and have branches that connect to other branches to bundle the lymphocytes together.
One distinctive feature of the thymus in section is the thymic (Hassal’s) corpuscle which is located in the medulla. It’s a bunch of epithelial-reticular cells bundled up together in a spherical structure. With an H&E stain they appear as pink spirals. Their function is yet to be determined.
The cortex has a collection of T-cells which proliferate and are tested to make sure they function properly. Many which do not pass the test will die by apoptosis . Correctly functioning T-cells pass into the medulla where they enter the blood stream.
Arterioles of the thymus enter through the trabecula and at the end branch into cortical and medullary capillary beds. The capillaries of the medulla will join back together as the thymic veins. In the cortex they just end as all blood must go into the medulla to get into the venous system. These veins are what carries the net T-cells into circulation. The thymus has no afferent lymph vessels but does have efferent vessels.
The capillaries have a thick basal lamina and non-fenestrated capillaries which form a blood thymic barrier which functions to protect the developing T-cells from invasion. The blood-thymic barrier is also made up of pericytes, basal lamina and epithelial-reticular cells and is only in the cortex.
Lymph nodes also have a capsule, trabecula and reticular fibers. Like in the thymus the length of the trabecula define where the cortex ends and the medulla begin. Lymph nodes have three functional layers:
1) The outer cortex is made of reticular fibers and is populated by B-cells. The outer cortex also contains the subcapsular sinus directly deep to the capsule which receives afferent lymphatics and is the site of initial antigen infection. The outer cortex also contains lymph nodules (B-lymphocytes).
2) The inner (para) cortex has few nodules but many T-lymphocytes.
3) The medulla is divided by the medullary sinuses which drain into efferent lymph vessels. The medulla contains many B-lymphocytes.
Lymph nodes also have intermediate sinuses that parallel the trabecula as they connect between the subcapsular and medullary sinuses.
The blood supply is via arteries that enter at the hillus. The arteries branch into capillaries that surround the lymph nodules. The veins and afferent lymph vessels leave through the hillus.
Lymph nodes function to filter lymph by macrophages and dendritic cells (antigen presenting cells). Lymph nodes store and allow for the proliferation of B-lymphocytes and T-lymphocytes by pathways previously studied under immune response.
The spleen also has a capsule, trabecula and reticular fibers. The spleen is divided into red and white pulp. The white pulp is a lymph nodule (primarily B-lymphocytes) and PALS (periarterial lymphatic sheath) located around the central arteriole. The PALS is primarily composed of T-cells.
The red pulp consists of splenic cords and (small vessels) which take blood to the splenic sinusoids. The sinusoids can be either attached to the splenic cords or separated by a small space. Of they are attached it is considered to be closed circulation because the blood never has to leave the system but if unattached it is an open system since the blood must diffuse over to the sinusoids.
Blood enters the spleen through splenic arterioles which branch into trabecular arterioles. The trabecular arterioles travel through the trabecula and then branch into central arterioles which are surrounded by the PALS. The central arteriole gives marginal zone capillaries which supply the marginal zone sinuses that have macrophages and dendritic cells to pick up antigens. The central artery also gives penicillar arterioles which supply the splenic sinuses by either open or closed circulation. The splenic sinuses link together to form the pulp veins which goes into the trabecular vein and into the splenic vein.
The spleen functions to:
1) Form lymphocytes (proliferation)
2) Destroy warn out RBCs.
3) Store blood (more so in animals but some in humans).
4) Retrieve iron from Hb.
5) Uses B-lymphocytes, T-lymphocytes and macrophages to defend against infection.
If the spleen breaks down the liver can take over it’s main functions.
Return to Semester One Goodies
Return to The Unofficial Ross Page