Lymphatic System

The lymphatic system includes the lymphoid tissues (e.g., lymph nodes and nodules and the spleen) and the lymphatic vessels distributed throughout the body (Fig. 16-2). It drains tissue fluid (called lymph within the lymphatic system) and is a framework for the circulation, production, and maturation of immune cells.

Lymphoid tissue consists of accumulations of lymphocytes trapped in the spaces between fibers of reticular connective tissue. The lym­phoid tissue may be scattered diffusely in some regions (as is commonly found in mucous membranes), may appear as nodules (as in the intestinal submucosa), or may be encapsulated to form specific organs, including lymph nodes, the spleen, thymus, and tonsils. The lymphatic vessels and tissues are arranged so that tissue fluid is exposed to aggregates of immune cells, which scrutinize the fluid for foreign cells and substances, thereby assisting in the control of infection.

Lymphatic Vessels

The lymphatic vessels constitute a one-way pathway that parallels the venous system and eventually empties into the cranial vena cava or some of its tributaries. The smallest lymphatics begin blindly between tissue cells as lymphatic capillaries, which collect the tissue fluid not absorbed by the venous system. When the tissue fluid enters the lymphatic vessels, it is known as lymph, which consists of fluid origi­nally derived from the blood, a variety of blood cells, and sometimes microbes. Lymphatic vessels carry lymph back to the great veins of the heart.

Lymph Nodes

Lymph nodes are discrete knots of lymphoid tissue scattered along the course of lymph vessels. Lymph nodes filter the lymph and act as one of the first defenses against infection by harboring lymphocytes, plasma cells, and macrophages.

Each lymph node is surrounded by a connec­tive tissue capsule that sends numerous connec­tive tissue trabeculae into the substance of the node (Fig. 16-3). The node is roughly divided into cortex, paracortex, and medulla, with large numbers of lymphocytes and macrophages in all three. Lymphocytes in the cortex are arranged in nodules (Fig. 16-3). Dark-staining groups are primary nodules, and light-staining groups are secondary nodules. secondary nodules are areas of rapid B cell proliferation, and for that reason their cores are called germinal centers. The paracortex, deep to the cortex, is populated pri­marily with T lymphocytes and dendritic cells.

Lymphocytes in the medullary portion of the lymph node are arranged in medullary cords rather than nodules (Fig. 16-3). These tend to be primarily accumulations of plasma cells.

immediately deep to the capsule of the node is a space, the subcapsular sinus, which com­municates with other sinuses of the cortex and medulla. Lymph delivered by afferent lymph vessels enters the subcapsular sinus and is slowly filtered through the cortex and medulla, to emerge finally at the hilus of the node, where blood vessels and nerves enter and the efferent lymphatic vessels emerge, carrying the lymph that has percolated through the node. This arrangement is ideally suited for the presentation to immune cells of antigens col­lected in the tissue fluid.

Figure 16-2. The lymphatic system of the cow. (Reprinted with permission of Wiley-Blackwell from McCracken, T.O., Kainer, R.A., and Spurgeon, T.S. Spurgeon’s ColorAtlas OfLargeAnimalAnatomy. Baltimore: Lippincott Williams & Wilkins, 1999.)

Figure 16-3.

Oddly enough, the porcine lymph node’s histologic architecture is a reverse of that seen in other species, with the nodules found in central regions and medullary cords on the periphery of the node. Flow of lymph likewise is reversed, with afferents entering at the hilus and percolating through the lymphatic tissue to emerge at the capsule.

Lymph nodes are scattered throughout the body, and in general their number and location are fairly consistent within a given species (Table 16-2). For convenience, groups of related nodes are often called a Iymphocenter. The condition of a lymph node often reflects health of the area from which it receives lymph. If a specific area is infected, the lymph nodes in that area tend to enlarge as the germinal centers begin produc­ing additional lymphocytes in response to the antigens delivered to the node. For example, a horse with strangles, a bacterial infection of the nasal cavity and pharynx, frequently shows enlargement of the mandibular and retropha­ryngeal lymphocenters. The lymph nodes at these locations receive their afferent vessels from the nasal cavity, mouth, and pharynx.

Neoplastic (cancerous) cells may spread throughout the body by way of the lymphatic channels; this is metastasis. When a tumor (cancer) is removed surgically, it may also be necessary to remove the regional lymph nodes draining the cancerous area to prevent further spread of the condition if it is suspected that neoplastic cells have infiltrated the nodes. The meat inspector uses his or her knowledge of the lymphatic system to determine whether a given part of a carcass should be condemned. An enlarged node may indicate infected or cancer­ous tissue in the region of the body draining to the node and necessitate condemnation of all or part of the carcass.

Hemal nodes are small dark red or black nodes in cattle and sheep, usually in the dorsal parts of abdominal and thoracic cavities. They resemble lymph nodes but are found on the

Table 16-2. Selected Lymph Nodes (Lymphocenters) of Cattle

course of small blood vessels and have blood within their sinuses.

Spleen

The spleen is a lymphoid organ associated with the circulatory system. It is attached to the stomach either directly by connective tissue, as in the ruminants, where it adheres closely to the rumen, or by the gastrosplenic ligament.

The parenchyma (substance) of the spleen consists of red pulp and white pulp (Fig. 16-4). The red pulp has a dark red appearance because it is engorged with blood. The white pulp is lighter colored, as it is composed largely of lym­phatic nodules, which are constructed much like the follicles of lymph nodes. Both B and T lymphocytes are found in abundance in the white pulp. The association of blood capillaries with the white pulp ensures that blood will be exposed to populations of immune cells.

In addition to important immunologic func­tions, the spleen functions as a storage area for red blood cells, so the size of the spleen varies from time to time even within a given individ­ual, as well as from species to species, depend­ing on the number of red blood cells in the spleen at a given time. The spleen is also an important site where senescent (old and worn- out) red blood cells are removed from the cir­culation, broken down, and their iron stored. These blood-related functions are associated with the red pulp of the splenic parenchyma.

Figure 16-4. Basic internal anatomy of a spleen. (Reprinted with permission of Wiley-Blackwell from Reece, W.O. Functional Anatomy and Physiology of Domestic Animals, 3rd edition. Baltimore: Lippincott, Williams & Wilkins, 2005.)

Although the spleen is a useful organ, it is not essential in the adult, as all of its functions can be carried on by other organs. The spleen can be removed (splenectomy) without significant impairment to a mature animal.

Thymus

The thymus is an organ of immature animals, undergoing involution at puberty, although never completely disappearing.

Tonsils

In the most traditional sense, a tonsil is an unencapsulated aggregate of lymphatic nodules associated with the pharyngeal mucosa. These aggregates lack afferent lymphatic vessels, instead relying on their proximity to the epi­thelial surface to make contact with antigens. Many tonsils are characterized by deep invagi­nations on their epithelial surfaces called crypts, which presumably increase the surface area for contact with lymphatic tissue. Although the word tonsil is usually reserved for the lymphatic organs associated with the pharynx, identical histological elements are found in the mucous membranes of the prepuce and vagina and in the submucosa of the intestinal tract.