Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Chapter 22 The Lymphatic System Part B

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 20.4a Lymph Nodes  Principal lymphoid organs of the body  Embedded in connective tissue  Clustered along lymphatic vessels  Large lymph node clusters occur:  Near the body surface in the:  Inguinal region,  Axillary region  Cervical region  These are the places where collecting vessels form trunks

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Lymph Nodes  Have two basic protective functions:  Lymph filtration (filters)  Macrophages remove & destroy microorganisms and other debris before reaching blood  Immune system activation  Lymph nodes-resident lymphocytes monitor lymph stream for antigens  They (lymphocytes) mount an attack against these antigens

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 20.4a Structure of a Lymph Node  Bean shaped surrounded by a fibrous capsule  Capsule forms trabeculae dividing the node into com- partments  A lymph node Has two distinct regions:  Cortex:  Outer part  Medulla:  Inner part

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Structure of a Lymph Node  Cortex:  Superficial cortex contains follicles with germinal centers  Germinal centers contain dividing B cells  Dendritic cells nearly encapsulate the follicles  Deep cortex houses T cells in transit  T cells (patrolling) circulate continuously between the:  Blood  Lymph nodes  Lymph

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Structure of a Lymph Node, cont’d.  Medulla:  Is defined by inward cortex extensions called medullary cords  Medullary cords:  Contain B cells, T cells, and plasma cells  Lymph sinuses:  Large lymph capillaries found throughout the node  Are spanned by crisscrossing reticular fibers  Macrophages reside on these reticular fibers and phagocytize foreign matter in lymph as it flows by

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Structure of a Lymph Node Figure 20.4a, b

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Circulation in the Lymph Nodes  Lymph enters via afferent lymphatic vessels  Then enters a large subcapsular sinus  Then travels into smaller sinuses  Exits the node at the hilus  Leaves the node via efferent vessels  Efferent vessels are fewer than afferents vessels  This helps:  Stagnate lymph in the node  Give lymphocytes &macrophages more time to carry out their protective functions

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Other Lymphoid Organs  Other lymphoid organs include:  The spleen  The thymus  The tonsils  Peyer’s patches  Bits of lymphatic tissue scattered in connective tissue  All lymphoid organs, except thymus, are composed of reticular connective tissue  All lymphoid organs help protect the body  Only lymph nodes filter lymph  Other lymphoid organs only have efferent vessels (lack afferent)

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Lymphoid Organs Figure 20.5

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Spleen  Largest lymphoid organ  Location:  On the left side of the abdominal cavity  Beneath the diaphragm  Served by:  The splenic artery and vein  They enter and exit at the hilus (concave side)

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Spleen (cont.)  Functions:  Site of lymphocyte proliferation  Site of immune surveillance & response  Cleanses blood:  Extracts aged & defective RBCS (RBCs graveyard) & platelets  Its macrophages remove debris & foreign matter from blood flowing thru its sinuses

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Additional Spleen Functions  Stores some breakdown products of RBCs for later reuse (iron for Hb synthesis)  Stores blood platelets  Site of fetal erythrocyte production (normally ceases after birth)

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Structure of the Spleen  Surrounded by a fibrous capsule  Has trabeculae that extend inward  Contains:  Lymphocytes  Macrophages  Huge numbers of erythrocytes

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Structure of the Spleen Figure 20.6a, b

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Structure of the Spleen, cont’d.  Two distinct areas:  White pulp:  Lymphocytes suspended on reticular fibers  Forms clusters “cuffs” around central arteries  Involved in immune functions  Red pulp:  The remaining part of splenic tissue  Concerned with disposing off:  Worn-out RBCs  Bloodborne pathogens

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Structure of the Spleen Figure 20.6a, b

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Thymus  A bilobed organ  Secretes hormones:  Thymosin  Thymopoietin  Both hormones cause:  T lymphocytes to become immunocompetent

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Thymus, cont’d.  Size of the thymus varies with age:  Prominent in newborn  Found in the inferior neck  Extends into the mediastinum & partially overlies the heart  Increases in size and is most active during 1 st yr  Starts to atrophy gradually after puberty  By old age is replaced by fibrous &fatty tissue

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Internal Anatomy of the Thymus  Thymic lobules contain:  An outer cortex  Inner medulla  Cortex contains:  Rapidly dividing densely packed lymphocytes  Scattered macrophages  Has no follicles (lacks B cells)

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Internal Anatomy of the Thymus (cont.)  Medulla contains:  Fewer lymphocytes  Thymic (Hassall’s) corpuscles (keratinized epithelium):  Involved in development of regulatory T lymphocytes  Regulatory T lympocytes prevent autoimmune response

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Thymus  Differs from other lymphoid organs in important ways:  Lacks follicles and B cells  Functions strictly in T lymphocyte precursors maturation  Does not directly fight antigens  Its stroma:  contains no reticular fibers  consists of star-shaped epithelial cells (thymocytes)

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Thymus (cont.)  Thymocytes:  Secrete hormones that stimulate immunocompetency of lymphocytes  Blood-thymus barrier:  Keeps bloodborne antigens from entering the cortex  Prevents premature activation of immature lymphocytes

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 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 auditory tubes-pharynx openings

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Tonsils (cont’d)  Lymphoid tissue contains follicles with germinal centers  Tonsil masses are not fully encapsulated  Epithelial tissue (overlying tonsil masses) invaginates, forming blind-ended crypts  Crypts trap and destroy bacteria and particulate matter

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 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 the breaching of the intestinal wall  Generate “memory” lymphocytes for long-term immunity

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings MALT  Mucosa-associated lymphatic tissue (MALT ):  In digestive tract:  Peyer’s patches  Tonsils  Appendix :  In respiratory tract:  Lymphoid nodules in the walls of the bronchi  In genitourinary tract:  In the mucosa of the genitourinary organs MA  MALT:  Protects these systems from foreign matter