THE ORGANIZATION OF THE LYMPHATIC SYSTEM
The lymphatic system is responsible for the immunological defense of the body. It protects the body from exogenous (foreign) and abnormal endogenous macromolecules and from viruses, bacteria, and other invasive microorganisms.
It includes all the lymphatic organs: thymus, tonsils, spleen, lymph nodes and hemal nodes, and the diffuse lymphatic tissue and lymphatic nodules present in many mucous membranes. The circulating lymphocytes, as well as the lymphocytes and plasma cells that are widely disseminated throughout the organism, also participate in this protective system.Two types of functionally distinct lymphocytes are recognized: T lymphocytes and B lymphocytes. Both result from antigen-independent proliferation and differentiation of stem cells in the primary lymphatic organs: T cells come from the thymus, and B cells come from the bursa of Fabricius in birds and the bone marrow in mammals. From the primary organs, both types of lymphocytes seed the secondary lymphatic organs, and within these, B and T lymphocytes undergo antigen-dependent proliferation and differentiation into effector cells that either attend to the disposal of particular antigens or provide the memory cells that become temporarily inactive. There is, in addition, a reserve population of undifferentiated lymphocytes.
The brief introduction to the system presented in Chapter 1 emphasizes the role of the lymphatic capillaries and larger vessels in returning an important fraction of the tissue fluid to the circulating blood. This role justifies the inclusion of these vessels, and of the nodes through which the lymph is passed, within the broad concept of a circulatory system (see Figure 1-34). The framework that supports the lymphatic nodules (germinal centers) contains phagocytic cells that remove particulate matter, including microorganisms on occasion, from the percolating lymph; this element must be included within the widely diffused macrophage or reticuloendothelial system that also includes the tissue macrophages and the endothelium of the hepatic, splenic, and bone marrow sinusoids.
The vital uniting theme is defense, both humoral and cellular, against foreign invasion of the body. Because some of these functions do not intrude on the scope of gross anatomy, the present account concentrates on the lymphatic vessels and nodes as drainage and filtration mechanisms.Before considering the topographical layout of the lymphatic system, mention must be made of the so- called lymphoepithelial structures comprising aggregations of unencapsulated lymph nodules within various mucosae. These are conveniently genetically termed tonsils, although the name is most often used specifically for those in the pharyngeal region where they guard against the passage of infection to deeper parts of the respiratory and digestive systems (Figure 7-47/2). Pharyngeal and palatine tonsils are mentioned on pages 116 and 117. Other tonsils are found in the mucosae of the larynx, intestine, prepuce, and vagina and other parts of the female tract. The common features that distinguish tonsils from lymph nodes are the absence of a capsule, the close relationship to a moist epithelial surface, and the position at the origin of a lymphatic drainage pathway.
In addition to the ordinary lymph node, a second variety of similar structure exists but is positioned athwart the bloodstream. These hemal nodes (Figure 7-48) are not found in all species and are most familiar in sheep, in which their dark color (due to the contained blood) contrasts them with the white fat in which they are commonly embedded. They are mainly found below the roof of the abdomen and thorax. A so-called third variety, the hemolymph node, is probably only a lymph node that contains red blood cells in its sinuses as a result of hemorrhage in its tributary field.
It is uncertain whether lymph vessels develop independently and later make secondary entry to veins, bud from existing veins, or arise by a combination of these methods. Both methods account for the existence of the lymphaticovenous connections between the major lymphatic trunks and the great veins at the entrance to the chest.
In some (nondomestic) mammals additional connections are described, often with renal veins. Such additional openings into the venous system can develop in later life when the normal flow is obstructed.Lymph nodes initially form as mesenchymal condensations placed along the lymphatic capillary plexus. They are later populated by lymphocytes that emigrate from the central lymphoid organ, the thymus. All lym-
Figure 7-47 Schematic drawing of the palatine tonsils of the dog (A) and cattle (B). The tonsils of the dog develop around a fossa but protrude into the oropharynx. Those of cattle surround the tonsillar sinus within the oropharyngeal wall. 1, Epithelium; 2,palatine tonsil; 7,efferent vessels (arrows); 4, tonsillar fossa; 5, tonsillar sinus.
phoid structures are especially well developed in juveniles.
As already mentioned (p. 28) there are important species differences in the disposition of the components of the lymph nodes. In most animals, the lymph nodules are located in the peripheral cortex close to where the afferent lymph vessels penetrate the capsule (Figure 7-49 and Figure 7-50). The central medulla consists of loose lymphoreticular tissue where the efferent vessels take origin to leave the node in the indented hilar region. In contrast, in porcine nodes, the “cortical” tissue is central where most nodules lie alongside the trabecular sinuses. The afferent vessels penetrate the capsule at one or more sites and follow the trabeculae to reach the centrally located nodules. The periphery of the node is largely occupied by loose lymphoreticular tissue (Figure 7-51), and it is from here that the efferent lymph vessels emerge.
Figure 7-48 Hemal node of sheep (HE) (70?). 1, Erythrocytes; 2, lymphocytes.