Lymphatic System Mike Clark, M.D.

Lymphatic System Composed of lymphoid cells, tissues, organs, and vessels The function of the lymphatic system is to (1) fight infection (2) carry lipids absorbed from the GI tract to the bloodstream and (3) return fluids and plasma proteins that escaped from the bloodstream back to the bloodstream

Microvilli (brush border) Absorptive cells Lacteal Goblet cell Blood Lipids absorbed from GI tract into Central Lacteal of Lymphatic Absorptive cells Lacteal Goblet cell Blood capillaries Vilus Mucosa associated lymphoid tissue Enteroendocrine cells Intestinal crypt Venule Muscularis mucosae Lymphatic vessel Duodenal gland Submucosa (b) Figure 23.22b

Cells and Fibers of the Lymphatic System 1. Lymphocytes 2. Macrophages 3. Dendritic Cells (Antigen Presenting Cells) 4. Reticular Cells 5. Reticular Fibers Lymph is a connective tissue

Lymphoid Tissues Lymph follicles Diffuse Lymphoid Tissue MALT (Mucosal Associated Lymphoid Tissue) GALT (Gut associated Lymphoid Tissue) –example Peyer’s patches BALT (Bronchial associated Lymphoid Tissue)

Lymphoid Organs In order to be lymphoid organ – need a partial or complete connective tissue capsule surrounding the organ Primary lymphoid organs – a lymphoid organ where the lymphocytes receive immunocompetence ( Bone marrow & Thymus) Secondary lymphoid organs – receive lymphocytes for residence that have received immunocompetence in the primary lymphoid organs (lymph nodes, spleen, vermiform appendix, tonsils)

Lymphatic Vessels Lymph capillaries Lymphatic collecting vessels Lymphatic trunks Lymphatic Ducts

Lymphatic Flow Vessels Lymphatic flow is a one way flow – from lymphatic capillaries through lymphatic collecting vessels to lymphatic trunks to lymphatic ducts – then into the blood stream Therefore lymphatic vessels drain the tissue fluids (interstitial fluids) and return substances to the bloodstream Unlike blood flow which is continuous and circular- lymph begins in blind capillaries

Lymphatic Capillaries Similar to blood capillaries, with modifications: Very permeable Loosely joined endothelial mini-valves Withstand interstitial pressure and remain open The mini-valves function as one-way gates that: Allow interstitial fluid to enter lymph capillaries Do not allow lymph to escape from the capillaries

Figure 20.1

Lymphatic Collecting Vessels Have the same three tunics as veins Have thinner walls, with more internal valves Anastomose more frequently Collecting vessels in the skin travel with superficial veins Deep vessels travel with arteries Nutrients are supplied from branching vasa vasorum

Figure 20.2a

Lymphatic Trunks Lymphatic trunks are formed by the union of the largest collecting ducts Major trunks include: Paired lumbar, bronchomediastinal, subclavian, jugular trunks and a single intestinal trunk

Lymphatic Ducts Lymph is delivered into one of two large ducts from the lymphatic trunks allowing lymph to enter the bloodstream Right lymphatic duct – drains the right upper arm and the right side of the head and thorax Thoracic duct – arises from the cisterna chyli and drains the rest of the body

Figure 20.2b

Lymph Transport The lymphatic system lacks a pumping organ Vessels are low-pressure conduits Uses the same methods as veins to propel lymph: Pulsations of nearby arteries Contractions of smooth muscle in the walls of the lymphatics

Organization of Lymphatic System Cells Tissues – both diffuse and compact (nodules) Organs – must have a capsule Lymph nodes, spleen, thymus, tonsils, appendix

Cells of the Lymphatic System Other cells of the Lymphatic System Two main varieties of Lymphocytes: T cells B cells Other cells of the Lymphatic System Macrophages – phagocytize foreign substances and help activate T cells Dendritic cells – spiny-looking cells with functions similar to macrophages Reticular cells – fibroblast–like cells that produce a stroma, or network, that supports other cell types in lymphoid organs

Lymphoid Tissue Diffuse lymphatic tissue – scattered reticular tissue elements in every body organ Larger collections appear in the lamina propria of mucous membranes and lymphoid organs Lymphatic follicles (nodules) – solid, spherical bodies consisting of tightly packed reticular elements and cells Germinal center composed of dendritic and B cells Found in isolation and as part of larger lymphoid organs

Figure 20.3

Lymph Nodes Principal lymphoid organs of the body Embedded in connective tissue and clustered along lymphatic vessels Aggregations of these nodes occur near the body surface in inguinal, axillary, and cervical regions of the body Two basic functions: Filtration – macrophages destroy microorganisms and debris Immune system activation – monitor for antigens and mount an attack against them

Figure 20.2a

Figure 20.4

Figure 20.4a

Figure 20.4b

Thymus A bilobed organ that secretes hormones (thymosin and thymopoietin) that cause T lymphocytes to become immunocompetent The lobes are separate from one another – but are hooked together by dense irregular collagenous connective tissue Size of the thymus varies with age: In infants, it is found in the inferior neck and extends into the mediastinum where it partially overlies the heart It increases in size and is most active during childhood It stops growing during adolescence and then gradually atrophies (involutes)

Figure 20.5

The dense connective tissue capsule of the Thymus – sends septa into the two lobes of the Thymus – partitioning them into lobules. Each lobule is composed of an outer cortex and an inner medulla

Figure 20.7

Cells of Thymus The thymus does not have a reticular fiber stroma like the other lymphoid organs – instead it has “epithelial reticular cells” Cortical Epithelial Reticular Cells are the Type I, II, and III – these three types of epithelial reticular cells form tight junctions around the blood vessels in the Thymus to form – the Thymic- Blood Barrier

Medullary Epithelial Reticular Cells Type IV, V and VI. The Type IV form the Hassall’s corpuscles of the Thymus The Hassall’s corpuscles used to be thought of as a place where T-cells were destroyed – recent evidence shows that this is the site of production of regulatory T-cells important in preventing autoimmune actions

The thymus differs from other lymphoid organs in important ways 1. It functions strictly in T lymphocyte maturation It does not directly fight antigens (Blood – Thymus Barrier) 2. The stroma of the thymus consists of star-shaped epithelial cells (not reticular fibers) These thymocytes secrete the hormones that stimulate lymphocytes to become immunocompetent

Spleen Largest lymphoid organ, located on the left side of the abdominal cavity beneath the diaphragm Fibroelastic connective capsule with some smooth muscle It is served by the splenic artery and vein, which enter and exit at the hilus No cortex or medulla – white pulp, red pulp and marginal zone Functions: Site of lymphocyte proliferation Immune surveillance and response Cleanses the blood – old red blood cells Traps some platelets

Additional Spleen Functions Stores breakdown products of RBCs for later reuse Spleen macrophages salvage and store iron for later use by bone marrow Site of fetal erythrocyte production (normally ceases after birth) Stores blood platelets

Structure of the Spleen Surrounded by a fibrous capsule, it has trabeculae that extend inward and contains lymphocytes, macrophages, and huge numbers of erythrocytes Two distinct areas: White pulp – containing mostly lymphocytes suspended on reticular fibers and involved in immune functions Red pulp – remaining splenic tissue concerned with disposing of worn-out RBCs and bloodborne pathogens

Figure 20.6

Tonsils Simplest lymphoid organs; form a ring of lymphatic tissue around the pharynx Location: Palatine tonsils – either side of the posterior end of the oral cavity Lingual tonsils – lie at the base of the tongue Pharyngeal tonsil – posterior wall of the nasopharynx Tubal tonsils – surround the openings of the auditory tubes into the pharynx

Ring of Waldeyer Figure 22.3b

Figure 20.8

GALT- Peyer’s patches, tonsils, and the appendix (digestive tract) MALT MALT – mucosa-associated lymphatic tissue: GALT- Peyer’s patches, tonsils, and the appendix (digestive tract) BALT- Lymphoid nodules in the walls of the bronchi (respiratory tract) MALT protects the digestive and respiratory systems from foreign matter

Aggregates of Lymphoid Follicles Peyer’s patches – isolated clusters of lymphoid tissue, similar to tonsils Found in the wall of the distal portion of the small intestine Similar structures are found in the appendix Peyer’s patches and the appendix: Destroy bacteria, preventing them from breaching the intestinal wall Generate “memory” lymphocytes for long-term immunity

Figure 20.9