• Structure of Lymph Nodes OH-Structure of Lymph Nodes 20838

• Lymph nodes are oval-shaped of bean-shaped structures.
  • Some are as small as a pinhead and others as large as a lima bean.
• Each lymph node is enclosed by a fibrous capsule.
• Note that lymph moves into a node via several different lymphatic vessels and emerges by one or two efferent vessel.
  • Think of a lymph node as a biological filter placed in the channel of several afferent lymph vessels.
  • Once lymph enters the node, it "percolates" slowly through the spaces known as sinuses before draining into a single efferent draining vessel.
    • One-way valves in both the afferent and efferent vessels keep lymph flowing in one direction.
• Fibrous septa or trabeculae extend from the covering capsule toward the center of the node.
  • Cortical nodules found within the sinuses along the outer region of the node are separated from each other by these trabeculae.
  • Each cortical nodule is composed of packed lymphocytes that surround a less dense area called a germinal center.
• When an infection is present, germinal centers form and the node begins to release lymphocytes.
• Lymphocytes begin their final stages of maturation within the germinal center of the nodule and then are pushed to the more densely packed outer layers as they mature to become antibody-producing plasma cells.
• The center or medulla of a lymph node is composed of sinuses and cords.

• Both the cortical and medullary sinuses are lined with specialized reticuloendothelial cells (fixed macrophages) capable of phagocytosis.

• Location of Lymph Nodes OH-The Lymphatic System

• Submental and submaxillary groups 20819

• 
• Location Figure 1
  • In the floor of the mouth
  • Lymph from the nose, lips, and teeth drains through these nodes.
  • Superficial cervical lymph nodes 20819 (See Location Figure 1 Above)
    • In the neck along the sternocleidomastoid muscle.
    • Drain lymph from the head and neck.
  • Superficial cubital or supratroclear lymph nodes 20820
• 
• Fig. Location 2
  • Located just above the bend of the elbow.
  • Lymph from the forearm passes through these nodes.
  • Axillary lymph nodes 20819 (See Location Figure 1 Above)
    • 20-30 large nodes clustered deep within the underarm and upper chest regions.
    • Lymph from the arm and upper part of the thoracic wall, including the breast, drains through these nodes.
  • Inguinal lymph nodes 20820 (See Location Figure 2 Above)
    • In the groin
    • Lymph from the leg and external genitals drains through these nodes.

• Functions of Lymph Nodes

• Defense functions--filtration and phagocytosis
  • The structure of the sinus channels within the lymph nodes slows the lymph flow through them.
    • This gives the reticuloendothelial cells that line the channels time to remove microorganisms and other injurious particles (soot) from the lymph and phagocytose them.
    • Sometimes such large numbers of microorganisms enter the node that the phagocytes cannot destroy enough of them to prevent their injuring the node.
      • An infection of the node, adenitis, then results.
    • Because cancer cells often break away from a malignant tumor and enter the lymphatics, they travel to the node, where they may set up new growths.
• Hematopoiesis
  • The lymphatic tissue of lymph nodes serve as the sites of the final stages of maturation for some types of lymphocytes and monocytes that have migrated from the bone marrow.

• Tonsils

• Masses of lymphoid tissue located in a protective ring under mucous membranes in the mouth and back of the throat.
• Help protect against bacteria that may invade tissues in the area around the openings between the oral and nasal cavities.
• Palatine tonsils--located on each side of the throat. OH-Palatine Tonsil

• Pharyngeal tonsils--also known as adenoids are near the posterior openings of the nasal cavity.

• Lingual tonsils--near the base of the tongue

• The tonsils serve as the first line of defense from the exterior and as such are subject to chronic infection--tonsillitis.

• Thymus

• In an unpaired organ consisting of two pyramid-shaped lobes with delicate and finely lobulated surfaces.
• Is located in the mediastinum, extending up into the neck as far as the lower edge of the thyroid and inferiorly as far as the fourth intercostal cartilage.

• a. Structure of the thymus
  • The pyramid-shaped lobes of the thymus are subdivided into small lobules by connective tissue septa that extend inward from a fibrous covering capsule.
  • Each lobule is composed of a sense cellular cortex and an inner less dense medulla.
    • Both cortex and medulla are composed of lymphocytes in an epithelial framework.
  • c.Medullary tissue can be identified by the presence of rather large laminated spherical structures called thymic corpuscles or Hassall's corpuscles.
• Functions of the Thymus
  • Thymus performs two important functions:
    • Serves as a final site of lymphocyte development before birth.
      • The fetal bone marrow forms immature lymphocytes which then move to the thymus.
      • Many lymphocytes leave the thymus and circulate to the spleen, lymph nodes, and other lymphatic tissue.
    • Soon after birth the thymus begins secreting a group of hormones collectively called thymosin that enable lymphocytes to develop into mature T-cells.
• Spleen
• Location of the Spleen
  • Located in the left hypochondrium directly below the diaphragm, above the left kidney and descending colon, and behind the fundus of the stomach.

• Structure of the Spleen
• Roughly ovoid in shape.
  • Is surrounded by a fibrous capsule with inward extensions that roughly divide the organ into compartments.
    • Arteries leading into each compartment are surrounded by dense masses (nodules) of developing lymphocytes.
      • Because of its whitish appearance, this tissue is called white pulp.
    • Near the outer regions of each compartment is tissue called red pulp made up of fine reticular fibers submerged in blood that comes from nearby arteries.
    • After passing through the reticular meshwork, blood collects in venous sinuses and then returns to the heart through veins.
• Functions of the Spleen
• Defense
  • As blood passes through the sinusoids, reticuloendothelial cell (macrophages) lining these venous spaces remove microorganisms from the blood and destroy them by phagocytosis.
• Hematopoiesis
  • Monocytes and lymphocytes complete their development to become activated in the spleen.
  • Before birth, red blood cells are also formed in the spleen.
• Red blood cell and platelet destruction.
  • Macrophages lining the spleen's sinusoids remove worn-out red blood cells and imperfect platelets from the blood and destroy them by phagocytosis.
  • Also break apart the hemoglobin molecules from the destroyed red blood cells and salvage their iron and globin content by returning them to the blood stream for storage in bone marrow and liver.
• Blood reservoir
  • Pulp of the spleen and its venous sinuses store considerable blood which can be released during hemorrhage.

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