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Flashcards in Histology of Lymphatics Deck (24)
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Please describe the structure and function of the spleen.

The bulk of the lymphoid tissue is arranged as a sheath around the central arterioles called the periarteriolar lymphoid sheath (PALS). Germinal centers are arranged within these lymphoid sheaths.The blood flows through loosely-arranged channels/sinuses that constitute collectively what is called the red pulp, while the more organized lymphoid tissue is called the white pulp. Function: Filter red blood cells and facilitate adaptive immune response.


Please describe the structure and function of the thymus.

The thymus is a specialized organ of the immune system. The only known function of the thymus is the production and education "of T-lymphocytes (T cells) which are critical cells of the adaptive immune system. The thymus is composed of two identical lobes and is located anatomically in the anterior superior mediastinum in front of the heart and behind the sternum. Histologically the thymus can be divided into a central medulla and a peripheral cortex which is surrounded by an outer capsule. The cortex and medulla play different roles in the development of T-cells. Cells in the thymus can be divided into thymic stromal cells and cells of hematopoietic origin (derived from bone marrow residenthematopoietic stem cells). Developing T-cells are referred to as thymocytes and are of hematopoietic origin. Stromal cells include thymic cortical epithelial cells thymic medullary epithelial cells and dendritic cells."


Please tell me about the structure and function of lymph nodes

A lymph node is a small ball-shaped organ of the immune system, distributed widely throughout the body including the armpit and stomach/gut and linked by lymphatic vessels. Lymph nodes are garrisons of B, T, and other immune cells.Lymph nodes are found all through the body, and act as filters or traps for foreign particles. They are important in the proper functioning of the immune system. Primary follicle: where B cells live Secondary follicle: dividing B cells and plasma cell diferentiation. Follicle center cells, centroblasts, centrocytes, an occasional T helper cell, follicular dendritic cells, tingible body macrophage


Please describe the structure and function of MALT.

The mucosa-associated lymphoid tissue (MALT) (also called mucosa-associated lymphatic tissue) is the diffuse system of small concentrations of lymphoid tissue found in various sites of the body such as the gastrointestinal tract, thyroid, breast, lung, salivary glands, eye, and skin. MALT is populated by lymphocytes such as T cells and B cells, as well as plasma cells and macrophages, each of which is well situated to encounter antigens passing through the mucosal epithelium. In the case of intestinal MALT, M cells are also present, which sample antigen from the lumen and deliver it to the lymphoid tissue.


Describe the basic structure and general movement of lymph and lymphocytes through a lymph node.

Lymph circulates to the lymph node via afferent lymphatic vessels and drains into the node just beneath the capsule in a space called the subcapsular sinus. The subcapsular sinus drains into trabecular sinuses and finally into medullary sinuses. The sinus space is criss-crossed by the pseudopods of macrophages, which act to trap foreign particles and filter the lymph. The medullary sinuses converge at the hilum and lymph then leaves the lymph node via the efferent lymphatic vessel towards either a more central lymph node or ultimately for drainage into a central venous subclavian blood vessel, most via the post-capillary venules, and cross its wall by the process of diapedesis.


What differentiates activated nodules from non-activated?

The presence of a germinal center differentiates activated (challenged) nodules from non-activated ones.Germinal centers are sites within lymph nodes or lymph nodules in peripheral lymph tissues where intense mature B lymphocytes, otherwise known as centroblasts rapidly proliferate, differentiate, mutate through somatic hypermutation and class switch during antibody responses.


Outline the vasculature of lymph nodes. Know the importance of the high endothelial venule.

Each lymph node has a blood supply that enters via a small artery at the hilus and leaves via a small vein. The artery branches repeatedly to supply essentially the entire node with oxygen and nutrient. In lymph nodes, the vessels are lined by a special endothelium, the so-called high endothelial venule, which are the sites for recognition and diapedesis of lymphocytes from the blood into the lymphatic space of the node. Endothelial cells of these venules contain docking “receptors” (selectins are one of them) that function in initiating pas- sage through the endothelium. These are very unusual endothelia in that the cells are rounded and protrude into the lumen of the vessel, unlike typical squamous endothelial cells of most venules.


Describe the blood flow through the thymus.

Small arteries enter the thymus through the outer capsule and penetrate into the thymus and bifurcate within the connective tissue septa between the lobules. The endothelial cells of the vessels have tight junctions, and are surrounded by a region of connective tissue that is then ensheathed in endothelioreticular cells. These combined layers (but functionally primarily the endothelioreticular cells) form what is called the blood- thymus barrier. Hence maturing thymocytes are not exposed to just any molecules that might be circulating in the blood. Efferent lymphatics too travel in the septum.


Be able to recognize the nuclei and cell bodies of reticuloendothelial cells in the thymus, Hassall’s corpuscles, and know how they are relevant to lymphocyte selection.

Reticuloendothelial cells: involved in selection process (positive and negative) for thymocytes as thymocytes progress towards the medulla.Hassall's corpuscles: cells that thickly populate medulla-- produce lymphokines that promote thymocyte maturation into adult T cells.


Be able to identify the general parts of the thymus, including the cortex, medulla, trabeculae, and blood vessels. 

Examination of the thymus at low magnification reveals a dark-staining cortex (C) and lighter-staining medulla (M). Note blood vessels (V) that bypass the cortex via trabeculae (T) and enter the medulla.


Describe the blood flow through the spleen. How does it differ markedly from that of the thymus and lymph node?

It is completely different in organization than either the lymph node or the thymus in that it has an open blood circulation through porous splenic sinuses. The spleen receives blood via the splenic artery and is drained by the splenic vein. The splenic artery branches into central arterioles that run deeper into the pulp of the spleen, but they then become lined with discontinuous endothelial cells, the discontinuities being so large that platelets, red blood cells and leukocytes can leave the vessels and enter sinuses that contain loosely packed arrangements of cells


Be able to recognize the cellular components of white pulp and red pulp.

The white pulp is composed of lymphatic nodules and the red pulp consists of irregular chords of cells and blood sinuses lined with endothelial cells and macrophages. The white pulp contains T cells, B cells and accessory cells. There are many similarities with lymph node structure. The purpose of the white pulp is to mount an immunological response to antigens within the blood. The white pulp is present in the form of a periarteriolar lymphoid sheath. This sheath contains B cell follicles and T cells.


Be able to recognize regions of mucosal-associated lymphoid tissue.

Notable collections include the tonsils (palatine, lingual and pharyngeal (adenoids), esophageal nodules, the appendix, bronchial nodules, and a large number of aggregations of lymphocytes in the intestine, usually increasing in size and abundance along its length until in the colon, there are very abundant multiple groups of nodules both in the mucosa and submucosa known as Peyer’s patches.


Describe the functions and distribution of the lymph system.

Lymphoid organs play a major role in cleansing blood and lymph and in providing adaptive immunity. The lymphatic system is responsible for the production and storage of the agranular white blood cells or lymphocytes. Lymphatic tissue is found in four different forms in the body:Non-encapsulated aggregates of lymphocytesLymph nodesThymusSpleenThese tissues are composed of two principle components (1) free cells (lymphocytes), and (2) a supporting frame work of extracellular fibers (reticular) and the fibroblasts which produce them.


Describes the types of lymphoid cells. Understand the functions and interactions between these cells.

T-cells, B-Cells, Natural Killer cellsSeriously? Interactions are several, please see other lectures. 


Delineate the differences between primary and secondary lymph organs.

The primary organs are the bone marrow and the thymus gland. These are the major sites of development of B lymphocytes and T-lymphocytes, respectively (although T cell precursors actually colonize the thymus from other hematopoetic tissue during embryonic development then reside, divide and mature in the thymus). Secondary lymphoid tissues are seeded with cells from primary tissues (GALT, Peyers Patches, etc.)


What is this?

A lymph node


What is this organ?What is the function of the dark cells?

The spleen. With dark macrophages.Culling old RBCs and platelets.


What region and spaces are in the red pulp? Where are these cells going next?What cells are in the PALS?What cells are in the nodule?



What is going on here? LS? BV?

Efferent lymph vessel and blood vessels in a lymph node.


What kind of cells enter the subcortex space? From where?

Leukocytes, afferent lymph vessels


What is going on in this challenged nodule?

Rapid B Cell division.


What do M cells do?

Take in foreign antigens and feed them to macrophages in the GI. 


What do stromal cells do?

Select T lymphocytes.