Lymphatic Organs

In contrast to the diffuse lymphatic tissue, lymphatic organs have well-defined anatomical sites and at least partial connective tissue capsules that separate the lymphatic tissue from neighboring tissues. These organs include the lymph nodes, tonsils, thymus, and spleen.

Lymph Nodes

Lymph nodes (fig. 21.7) serve two functions: to cleanse the lymph and alert the immune system to pathogens. There are hundreds of lymph nodes in the body. They are especially concentrated in the cervical, axillary, and inguinal

2Johann Conrad Peyer (1653-1712), Swiss anatomist

Collecting vessel

Lymph node

Collecting vessel

Lymph nodes

Collecting vessel

Lymph node

Collecting vessel

Lymph nodes

Upper Deep Cervical Lymph Node

Figure 21.7 Lymph Nodes. Several collecting vessels are especially evident leading to and from the upper lymph node.

'Hieronymus Fabricius (Girolamo Fabrizzi) (1537-1619), Italian anatomist

Figure 21.7 Lymph Nodes. Several collecting vessels are especially evident leading to and from the upper lymph node.

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Physiology: The Unity of Immune System Companies, 2003 Form and Function, Third Edition regions close to the body surface, and in thoracic, abdominal, and pelvic groups deep in the body cavities. Most of them are embedded in fat.

A lymph node is an elongated or bean-shaped structure, usually less than 3 cm long, often with an indentation called the hilum on one side (fig. 21.8). It is enclosed in a fibrous capsule with extensions (trabeculae) that incompletely divide the interior of the node into compartments. The interior consists of a stroma of reticular connective tissue (reticular fibers and reticular cells) and a

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parenchyma of lymphocytes and antigen-presenting cells. Between the capsule and parenchyma is a narrow space called the subcapsular sinus, which contains reticular fibers, macrophages, and dendritic cells.

The parenchyma is divided into an outer cortex and, near the hilum, an inner medulla. The cortex consists mainly of ovoid lymphatic nodules. When the lymph node is fighting a pathogen, these nodules acquire light-staining germinal centers where B cells multiply and differentiate into plasma cells. The medulla consists largely of a branching

Afferent— lymphatic vessel

Valve

Cortex

Lymphatic nodule Germinal center Subcapsular

Capsule

Afferent— lymphatic vessel

Valve

Cortex

Lymphatic nodule Germinal center Subcapsular

Capsule

Medullary Cord Capsule Image

Artery Vein

Hilum

- Efferent lymphatic vessel

-Medullary cords Medullary sinus

Macrophage -Trabecula Lymphocytes Reticular fibers Venule

Medulla

Medullary sinus Medullary cord

Medulla

Medullary sinus Medullary cord

Artery Vein

Hilum

- Efferent lymphatic vessel

-Medullary cords Medullary sinus

Macrophage -Trabecula Lymphocytes Reticular fibers Venule

Medullary Cord Efferent Vessels
Figure 21.8 Anatomy of a Lymph Node. (a) Bisected lymph node showing pathway of lymph flow. (b) Detail of the boxed region in a. (c) Stroma and immune cells in a medullary sinus.

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806 Part Four Regulation and Maintenance network of medullary cords composed of lymphocytes, plasma cells, macrophages, reticular cells, and reticular fibers. The cortex and medulla also contain lymph-filled sinuses continuous with the subcapsular sinus.

Several afferent lymphatic vessels lead into the node along its convex surface. Lymph flows from these vessels into the subcapsular sinus, percolates slowly through the sinuses of the cortex and medulla, and leaves the node through one to three efferent lymphatic vessels that emerge from the hilum. No other lymphatic organs have afferent lymphatic vessels; lymph nodes are the only organs that filter lymph as it flows along its course. The lymph node is a "bottleneck" that slows down lymph flow and allows time for cleansing it of foreign matter. The macrophages and reticular cells of the sinuses remove about 99% of the impurities before the lymph leaves the node. On its way to the bloodstream, lymph flows through one lymph node after another and thus becomes quite thoroughly cleansed of most impurities.

When a lymph node is under challenge from a foreign antigen, it may become swollen and painful to the touch— a condition called lymphadenitis3 (lim-FAD-en-EYE-tis). Physicians routinely palpate the accessible lymph nodes of the cervical, axillary, and inguinal regions for swelling. Lymph nodes are common sites of metastatic cancer because cancer cells from almost any organ can break loose, enter the lymphatic capillaries, and lodge in the nodes. Cancerous lymph nodes are swollen but relatively firm and usually painless. Lymphadenopathy4 (lim-FAD-eh-NOP-a-thee) is a collective term for all lymph node diseases.

Tonsils

The tonsils are patches of lymphatic tissue located at the entrance to the pharynx, where they guard against ingested and inhaled pathogens. Each is covered by an epithelium and has deep pits called tonsillar crypts lined by lymphatic nodules (fig. 21.9). The crypts often contain food debris, dead leukocytes, bacteria, and antigenic chemicals. Below the crypts, the tonsils are partially separated from underlying connective tissue by an incomplete fibrous capsule.

There are three main sets of tonsils: (1) a single medial pharyngeal tonsil (adenoids) on the wall of the pharynx just behind the nasal cavity, (2) a pair of palatine tonsils at the posterior margin of the oral cavity, and (3) numerous lingual tonsils, each with a single crypt, concentrated in a patch on each side of the root of the tongue (see fig. 22.3b, p. 844). The palatine tonsils are the largest and most often infected. Their surgical removal, called tonsillectomy, used to be one of the most common surgical procedures performed on children, but it is done less often today.

3lymph + adeno = gland + itis = inflammation

Tonsils Crypts
Figure 21.9 A Palatine Tonsil. A vertical tissue section showing a branched tonsillar crypt and lymphatic nodules.

Thymus

The thymus is a member of both the lymphatic and endocrine systems. It houses developing lymphocytes and secretes hormones that regulate their later activity. It is located between the sternum and aortic arch in the superior mediastinum. The thymus is very large in the fetus and grows slightly during childhood, when it is most active. After age 14, however, it begins to undergo involution (shrinkage) so that it is quite small in adults (fig. 21.10). In the elderly, the thymus is replaced almost entirely by fibrous and fatty tissue and is barely distinguishable from the surrounding tissues.

The fibrous capsule of the thymus gives off trabec-ulae that divide the parenchyma into several angular lobules. Each lobule has a cortex and medulla populated by T lymphocytes (fig. 21.11). Reticular epithelial cells seal off the cortex from the medulla and surround blood vessels and lymphocyte clusters in the cortex. They thereby form a blood-thymus barrier that isolates developing lymphocytes from foreign antigens. After developing in the cortex, the T cells migrate to the medulla, where they spend another 3 weeks. There is no blood-thymus barrier in the medulla; mature T cells enter blood or lymphatic vessels here and leave the thymus. In

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Lymphatic Organs

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Thymus -

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Thymus Newborn

Figure 21.10 The Thymus. (a) The thymus is extremely large in a newborn infant. (b) The adult thymus is atrophied and often barely noticeable.

Saladin Animal Cell
Figure 21.11 Histology of the Thymus.

the medulla, the reticular epithelial cells form whorls called thymic (Hassall5) corpuscles, useful for identifying the thymus histologically but of no known function. Besides forming the blood-thymus barrier, reticular epithelial cells secrete hormones called thymosins, thy-mulin, and thymopoietin, which promote the development and action of T cells. If the thymus is removed from newborn mammals, they waste away and never develop immunity. Other lymphatic organs also seem to depend on thymic hormones and develop poorly in thymec-tomized animals.

5Arthur H. Hassall (1817-94), British chemist and physician

Spleen

The spleen is the body's largest lymphatic organ. It is located in the left hypochondriac region, just inferior to the diaphragm and dorsolateral to the stomach (fig. 21.12; see also fig. A.15, p. 46). It has a medial hilum penetrated by the splenic artery and vein and lymphatic vessels. Its parenchyma exhibits two types of tissue named for their appearance in fresh specimens (not in stained sections): red pulp, which consists of sinuses gorged with concentrated erythrocytes, and white pulp, which consists of lymphocytes and macrophages aggregated like sleeves along small branches of the splenic artery.

These two tissue types reflect the multiple functions of the spleen. It produces blood cells in the fetus and may resume this role in adults in the event of extreme anemia. It monitors the blood for foreign antigens, much like the lymph nodes do the lymph. Lymphocytes and macrophages of the white pulp are quick to detect foreign antigens in the blood and activate immune reactions. The splenic capillaries are very permeable; they allow RBCs to leave the bloodstream, accumulate in the sinuses of the red pulp, and reenter the bloodstream later. The spleen is an "erythrocyte graveyard"—old, fragile RBCs rupture as they squeeze through the capillary walls into the sinuses. Splenic macrophages phagocytize their remains, just as they dispose of blood-borne bacteria and other cellular debris. The spleen also compensates for excessive blood volume by transferring plasma from the bloodstream into the lymphatic system.

The spleen is highly vascular and vulnerable to trauma and infection. A ruptured spleen can hemorrhage

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Living Without Spleen

Diaphragm (cut)

Spleen

Splenic artery Splenic vein Pancreas

Kidney Inferior vena cava Aorta

Common iliac arteries

Diaphragm (cut)

Spleen

Splenic artery Splenic vein Pancreas

Kidney Inferior vena cava Aorta

Common iliac arteries

Abdominal Quadrants And Organs
Figure 21.12 The Spleen. (a) Position of the spleen in the upper left quadrant of the abdominal cavity. (b) Histology.

fatally, but is difficult to repair surgically. Therefore a common procedure in such cases is its removal, splenec-tomy. A person can live without a spleen, but is somewhat more vulnerable to infections.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

1. List the primary functions of the lymphatic system. What do you think would be the most noticeable effect of clamping the right lymphatic duct closed?

2. How does fluid get into the lymphatic system? What prevents it from draining back out?

3. List five major cell types of lymphatic tissues and state the function of each.

4. Predict the relative seriousness of removing the following organs from a 2-year-old child: (a) a lymph node, (b) the spleen, (c) the thymus, (d) the palatine tonsils.

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Responses

  • Nieve
    Is the spleen in the lower left abdominal quadrant?
    8 years ago
  • Hope
    What do you think would be the most noticeable effect of clamping the right lymphatic duct closed?
    7 years ago
  • Lucas Lang
    What consist of the medulla, medullary sinus and reticular fibers?
    7 years ago
  • Kedija Gabriel
    What is the largest lymphoid organ located at the left hypochondriac region?
    2 months ago

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