Lymp

Lymphatic vessels (lymphatics): are thin-walled vessels (tubes) structured like blood vessels, that carry lymph. As part of the lymphatic system, lymph vessels are complementary to the cardiovascular system.

Lymph: protein containing fluid transported by lymphatic vessels 

Lymph nodes: small lymphoid organ that filters lymph; contains macrophages and lymphocytes 

Lymph capillaries: tiny thin-walled vessels, closed at one end and located in the spaces between cells throughout the body, except in the central nervous system and non-vascular tissues. Lymphatic capillaries are slightly larger in diameter and have greater oncotic pressure than blood capillaries.

Collecting lymphatic vessels: lymph vessels act as reservoirs for plasma and other substances including cells that have leaked from the vascular system and transport lymph fluid back from the tissues to the circulatory system. Without functioning lymph vessels, lymph cannot be effectively drained and lymphedema typically results.

Right lymphatic duct: also called the right thoracic duct, is about 1.25 cm long. It drains lymphatic fluid from the right thoracic cavity (this is the section of the trunk on the upper right side), the right arm, and from the right side of the neck and the head.

Thoracic duct: large lymphatic duct that receives lymph drained from the entire lower body, the upper left extremity, and the left side of the head and thorax 

T cells (T lymphocytes): lymphocytes that mediate cellular immunity; include helper, cytotoxic, regulatory, and memory cells. 

B cells (B lymphocytes): oversee humoral immunity; their descendants differentiate into antibody producing plasma cells 

Plasma cells: members of a B cell clone; effector B cells specialized to produce and release antibodies 

Macrophages: immune cell type common in connective tissue, lymphoid tissue, and many body organs; phagocytizes tissue cells, bacteria, and other foreign debris; presents antigens to T cells in the immune response 

Lymphoid tissue: organized structures that support immune responses. The bone marrow and thymus are primary lymphoid tissues and the sites of lymphocyte development. The lymph nodes, spleen, tonsils and Peyer's patches are examples of secondary lymphoid tissue.

Diffuse lymphoid tissue: mucosa-associated lymphoid tissue (MALT), also called mucosa-associated lymphatic tissue, is a diffuse system of small concentrations of lymphoid tissue found in various submucosal membrane sites of the body, such as the gastrointestinal tract, nasopharynx, thyroid, breast, lung, salivary glands, eye, and skin.

Lymphoid follicles (nodules): small masses of tissues that contain aggregations of inflammatory cells, mainly B cells with some T cells and dendritic cells.

Lymphoid organs: primary lymphoid organs are the red bone marrow, in which blood and immune cells are produced, and the thymus, where T-lymphocytes mature. The lymph nodes and spleen are the major secondary lymphoid organs; they filter out pathogens and maintain the population of mature lymphocytes

Primary lymphoid organs: the red bone marrow and thymus; lymphoid organs in which lymphocytes develop and mature 

Secondary lymphoid organ: lymph nodes, spleen, and mucosa-associated lymphoid tissue (MALT); lymphoid organs in which lymphocytes encounter antigens and are activated 

Capsule: A fibrous, membranous, or fatty sheath that encloses an organ or body part; the capsule of a lymph node is a thin layer of dense fibrous connective tissue penetrated by various afferent and efferent lymphatic vessels.

Trabeculae: any of the fibrous bands extending from the capsule into the interior of an organ 

Cortex: outer surface layer of an organ 

Medulla: central portion of certain organs 

Lymph sinuses: coated with T and B lymphocytes as well as phagocytic cells, all of which filter the passing lymph.

Afferent lymphatic vessels: flow into a lymph node and carry unfiltered lymph fluid.

Hilum: the indented region of an organ from which blood and/or lymphatic vessels and nerves enter and exit 

Efferent lymphatic vessels: flow out of a lymph node and carry filtered lymph fluid.

Spleen: largest lymphoid organ; provides a stie for immune surveillance, lymphocyte activation, and erythrocyte destruction 

Splenic artery: the blood vessel that supplies oxygenated blood to the spleen. It branches from the celiac artery, and follows a course superior to the pancreas. It is known for its tortuous path to the spleen.

Splenic vein: a blood vessel that drains blood from the spleen, the stomach fundus and part of the pancreas. It is part of the hepatic portal system.

White pulp: a histological designation for regions of the spleen (named because it appears whiter than the surrounding red pulp on gross section), that encompasses approximately 25% of splenic tissue. White pulp consists entirely of lymphoid tissue.

Red pulp: The red pulp of the spleen is composed of connective tissue known also as the cords of Billroth and many splenic sinusoids that are engorged with blood, giving it a red color. Its primary function is to filter the blood of antigens, microorganisms, and defective or worn-out red blood cells.

Thymus: lymphoid organ active in immune response; site of maturation of T lymphocytes 

Palatine tonsils: a pair of soft tissue masses located at the rear of the throat (pharynx). Each tonsil is composed of tissue similar to lymph nodes, covered by pink mucosa (like on the adjacent mouth lining). Running through the mucosa of each tonsil are pits, called crypts.

Lingual tonsil: a collection of lymphatic tissue located in the lamina propria of the root of the tongue. This lymphatic tissue consists of the lymphatic nodules rich in cells of the immune system (immunocytes).

Pharyngeal tonsil: In anatomy, the adenoid, also known as the pharyngeal tonsil or nasopharyngeal tonsil, is the superior-most of the tonsils. It is a mass of lymphatic tissue located behind the nasal cavity, in the roof of the nasopharynx, where the nose blends into the throat.

Peyer’s patches: groupings of lymphoid follicles in the mucus membrane that lines your small intestine. Lymphoid follicles are small organs in your lymphatic system that are similar to lymph nodes.

Appendix: a narrow, finger-shaped pouch that projects out from the colon. Appendicitis occurs when the appendix becomes inflamed and filled with pus. Appendicitis is an inflammation of the appendix, a finger-shaped pouch that projects from your colon on the lower right side of your abdomen.

Mucosa-associated lymphoid tissue (MALT): a diffuse system of small concentrations of lymphoid tissue found in various submucosal membrane sites of the body, such as the gastrointestinal tract, nasopharynx, thyroid, breast, lung, salivary glands, eye, and skin.

OBJECTIVES:

1. State the components and functions of the lymphatic system

Overall, the lymphatic system contains lymphatic vessels, lymph, and lymph nodes. 

• Function of the lymphatic system: the lymphatic system returns leaked fluid to the blood.  This ensures that the cardiovascular system has sufficient blood volume. 

• Lymphatic vessels: form a one way system in which lymph can flow through the heart 

• Lymphatic capillaries: weave between the tissue cells and blood capillaries in the loose connective tissues of the body. They are widespread and aren’t in bones or teeth. They are limited to locations in the meninges, and help drain the extracellular fluids in the brain. 

• Two structural modifications: 

• Endothelial cells forming the walls of lymphatic capillaries are not tightly joined – the edges of adjacent cells overall loosely, forming minivalves. 

• Collagen filaments anchor the endothelial cells to surrounding structures so that any increase in interstitial fluid volume opens the minivalves. 

• One way swinging door system: when fluid pressure is greater inside the lymphatic capillary, it forces the endothelial minivalve flap shut, preventing lymph from leaking back out as the pressure moves it along the vessel

• Proteins in the interstitial space are unable to enter blood capillaries but are able to enter lymphatic capillaries with ease

• When tissues are inflamed, lymphatic capillaries develop openings that permit uptake of even larger particles like cell debris, pathogens, and cancer cells. These particles can then travel with the lymph to the lymph nodes where they are removed by cells of the immune system. This is critical for activating the immune response!

• Larger lymphatic vessels: 

• Lymph flows successively: lymphatic capillaries -> collecting lymphatic vessels -> lymphatic trunks -> lymphatic ducts

• Collecting lymphatic vessels: same three tunics as veins, but have thinner walls and more internal valves, and they anastomose more. 

• Lymphatic trunks: largest collecting vessels unite to form these, and they drain large areas of the body

• Paired lumbar, bronchomediastinal, subclavian, and jugular trunks

• Single intestinal trunk 

• Ducts: lymph is eventually delivered to one of two large ducts in the thoracic region

• Right lymphatic duct: drains lymph from the right upper limb and the right side o the head and thorax

• Thoracic duct: larger, receives lymph from the rest of the body. In about half of individuals, the thoracic duct begins as an enlarged cisterna chyli located in the region between the last thoracic and second lumbar vertebrae 

2. Name and describe the structures and vessels through which lymph flows from the interstitial compartment to the point where it is returned to the bloodstream, and list the factors that aid its flow.

• The lymphatic vessels form a one way system in which lymph flows only toward the heart 

• Steps: 

  1. Lymphatic capillaries

  2. Collecting lymphatic vessels

  3. Lymph nodes (objective 3)

  4. Lymphatic trunks

  5. Lymphatic ducts

  6. Terminal ducts empty lymph into the venous circulation at the junction of the internal jugular vein and subclavian vein on its own side of the body 

• Lymph transport: 

• Lacks an organ but acts as a pump

• Low pressure conduit

• Action of active skeletal muscle, pressure chances, and valves to prevent backflow

• Connective tissue sheaths and pulsations of nearby arteries promote lymph flow

• Smooth muscle also contracts rhythmically, to help pump lymph allow

• Sporadic

• When physical activity or passive movement increase, lymph flows more rapidly 

3. Describe the structure, locations and functions of diffuse lymphoid tissue, lymphoid follicles, lymph nodes, spleen, thymus, tonsils, and MALT.

• Lymphoid cells: consist of immune system cells found in lymphoid tissues together with supporting cells that form scaffolding

• Lymphocytes: the warriors

1. T cells/T lymphocytes: manage immune response and directly attack and destroy infected cells

2. B cells/B lymphocytes: protect the body by producing plasma cells, daughter cells that secrete antibodies into the blood 

• Macrophages: phagocytizing foreign substances and activate T cells

• Dendritic cells: capture antigens and bring them to the lymph nodes 

• Reticular cells: fibroblast like cells that produce stroma, which supports the other cells types 

• Lymphoid tissue: an important component of the immune system!

  1. Houses lymphocytes and provides a site where they can be activated and proliferate

  2. Furnishes an ideal surveillance vantage point for lymphocytes and macrophages 

• Largely composed of reticular connective tissue (everywhere but the thymus)

• Lymphoid tissue comes in various “packages” 

• Diffuse: loose arrangement of lymphoid cells and some reticular fibers

• Lymphoid follicles/lymphoid nodules: solid, spherical bodies consisting of tightly packed lymphoid cells and reticular fibers

• Lymphoid organs: grouped into two functional categories

  1. Primary: where B and T cells mature, in the red bone marrow and the thymus. Both originate in red bone marrow, but B cells mature in the red bone marrow and T cells mature in the thymus. 

  2. Secondary: mature lymphocytes first encounter their antigens and are activated. Includes lymph nodes, the spleen, MALT, Peyer’s patches, and the appendix.

• Lymph nodes: cleanse lymph and house lymphocytes

• Lymph nodes cluster along the lymphatic vessels of the body

• Hundreds of small organs and are embedded in connective tissue

• Two basic protective functions: 

1. Cleansing the lymph: acts as a filter. Macrophages remove and destroy microorganisms and other debris, to prevent them from being delivered to the blood and spreading.

2. Immune system activation: strategically located at sites where lymphocytes encounter antigens and are activated to mount an attack against them. 

• Structure of a lymph node: 

• Bean shaped

• Capsule: dense and fibrous, surrounds each node

• Trabeculae: connective tissue that extends inwards to divide the node into several compartments 

• Two histologically distinct regions: 

1. Cortex: densely packed follicles, superficial part. Has geminal centers heavy with dividing B cells. The deeper part houses T cell in transit. Dendritic cells are also abundant. 

2. Medulla: consists of medullary cords, thin inward extensions from the lymphoid tissue. B and T lymphocytes are found. 

• Circulation of lymph nodes: 

• Lymph enters the convex side of a lymph node through afferent lymphatic vessels

• Moves through the subcapsular sinus, into a number of smaller sinuses through the cortex to enter the medulla

• Lymph goes through the medullary sinuses and exits the node at its hilum, the indented region on the concave side, via efferent lymphatic vessels 

• Spleen:

• Soft, blood rich

• Size of a fist, and is the largest lymphoid organ

• Location: left side of the abdominal cavity, just beneath the diaphragm, and curls around the anterior aspect of the stomach

• Served by the splenic artery and vein 

• Function: provides a site for lymphocyte proliferation and immunise surveillance and response. Most importantly, it cleanses blood – it extracts aged and defective blood cells and platelets from the blood, and its macrophages remove debris and foreign matter. 

• The spleen also: 

1. Recycles the breakdown of red blood cells for later reuse

2. Stores blood platelets and monocytes for release into the blood when needed

3. May be a site of erythrocyte production it the fetus 

• Histologically, the spleen consists of red and white pulp: 

• White pulp: where immune functions take place. Composed mostly of lymphocytes suspended on reticular fibers. The white pulp forms a cuff around central arteries. 

• Red pulp: where worn-out red blood cells and blood borne pathogens are destroyed, so it contains large numbers of erythrocytes and macrophages that engulf them. It consists of splenic cords, which are regions of reticular connective tissue, that separate the blood filled splenic sinusoids. 

• MALT: mucosa-associated tissues. Includes the tonsils, Peyer’s patches and the appendix. 

• Tonsils: form a ring of lymphoid  tissue around the entrance to the pharynx, where they look like swellings of mucosa. The tonsils gather and remove pathogens entering the pharynx in food or in inhaled air. Their blind ended tonsillar crypts trap bacteria and particulate matter, and when the bacteria works through the mucosal epithelium to lymphoid tissue, they are destroyed.They are named according to location. 

• Palatine tonsils: located on either side at the posterior end of the oral cavity. Largest and the ones most often infected.

• Lingual tonsil: collective term for a lumpy collection of lymphoid follicles at the base of the tongue

• Pharyngeal tonsil: adenoids if enlarged, posterior wall of the nasopharynx

• Tubal tonsils: surround the openings of the auditory tubes into the pharynx 

• Peyer’s Patches: aggregated lymphoid nodules. Large clusters of lymphoid follicles, that are structurally like tonsils. They are located in the wall of the distal portion of the small intestine. 

• Appendix: tubular offshoot of the large intestine and contains a high concentration of lymphoid follicles. The appendix is in an ideal position to prevent bacteria from breaching the intestinal wall, and to generate “memory” lymphocytes for long-term immunity.  

• Thymus: 

• Bilobed

• Location: inferior neck and extends to the superior thorax, where it partially overlies the heart deep to the sternum

• T lymphocytes mature here: this is where they become able to defend us against specific pathogens in the immune response 

• Atrophies at it ages, but continues to produce immunocompetent cells at a declining rate

• Comparison to a head of cauliflower: 

• Florets: represent thymic lobules, each containing and outer cortex and inner medulla. In the cortical regions, the rapidly dividing lymphocytes are densely packed, with a few macrophages scattered about. 

• Lighter staining medullary areas contain some thymic corpuscles, that are involved in the development of regulatory T cells, a class of T lymphocytes important for preventing autoimmune responses

• The thymus is a primary organ and differs from secondary organs in three ways: 

1. Has no follicles because it lacks B cells

2. Does not directly fight antigens: instead, it functions strictly as a maturation site for T lymphocyte precursors. The blood thymus barrier keeps bloodborne antigens out of the thymus. 

3. Stroma consists of epithelial cells rather than reticular fibers. These epithelial cells provide the physical and chemical environment in which T lymphocytes mature.