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CHAPTER 16. Organs Of The Immune System

The microscopic study of animal tissues and cells

Overview – Organs of the Immune System
Plate 16-1. The Lymph Node, Part I 
Plate 16-2. The Lymph Node, Part II 
Plate 16-3. The Spleen, Part I: Red Pulp 
Plate 16-4. The Spleen, Part II: Venous Sinuses 
Plate 16-5. The Thymus

This chapter describes the microanatomy of the lymph nodes, spleen, and Thymus– the major organs of an extremely important network of cells, tissues, and organs called the immune system. The tissues and organs of the immune system are notable in that they make widespread use of reticular fibers. Each reticular Fiberconsists of a core of Collagen fibrils coated with an amorphous, Glycoprotein – rich substance similar to material found in the Basement Membrane. Recent electron – microscopic studies have shown that each reticular Fiber, unlike other connective tissue fibers, is covered by a thin cytoplasmic extension of the reticular cell itself. (Reticular cells are similar in many ways to the fibroblasts that secrete Collagen).

In order to understand the structure and function of the organs of the immune system, having a basic knowledge of the cells that participate in the immune response is helpful. Immunology is a highly complex and rapidly evolving field; a detailed discussion of immunology is beyond the scope of this atlas. The following discussion is intended to supply the basics of the subject in order to facilitate learning the histologic organization of the major organs of the immune system.

The immune system distinguishes “self” from “nonself”; it recognizes and destroys (or inactivates) foreign invaders. These invaders, called antigens, are potentially harmful bacteria, viruses, fungi, foreign macromolecules, or fragments of unwanted cells and tissues. Lymphocytes are central to the immune response; they recognize antigens. The recognition, moreover, is highly specific; each Lymphocyte recognizes one specific kind of Antigen. The cells directly involved in the immune response include, among others, lymphocytes, macrophages, and neutrophils. (Neutrophils, you recall, are also called polymorphonuclear leukocytes, or “polys”). Of these, the lymphocytes are the most difficult to understand. It is believed that all lymphocytes can trace their ancestry to the bone marrow. One major class of Lymphocyte, the T-Lymphocyte, travels to the Thymus to mature; the other broad class of Lymphocyte, the B-Lymphocyte, does not. The functions of the lymphocytes, outlined below, are quite different.

Let us imagine that an Antigen find its way into the body. If it encounters a T-Lymphocyte with receptors that specifically recognize that particular Antigen, that T cell will bind to the Antigen and become activated. Activated T cells can proliferate, thus producing more lymphocytes with specific receptors for that Antigen. They can then differentiate into large lymphocytes that release Lymphokines – factors that attract and activate macrophages, which then come and destroy the Antigen. The Antigen may, however, contact another type of T-Lymphocyte – a so-called Killer T Cell – that directly destroys the Antigen.

If the Antigen should encounter a B-Lymphocyte instead of a T-Lymphocyte, a different course of events will follow. The B-Lymphocyte has receptors that recognize one kind of Antigen. Upon contacting that Antigen, the B cell becomes activated, proliferates, and differentiates into a Plasma cell. Plasma cells make and release antibodies-highly specific macromolecules that bind to the Antigen, neutralize it, and form an AntigenAntibody complex. In certain cases, these AntibodyAntigen complexes are quite large and can impede blood flow. Consequently, their presence triggers a highly complex response in which a series of blood proteins called Complement is activated. Activated Complement attracts neutrophils, which become highly phagocytic and ingest the antigenantibody complexes.

For the immune system to be effective, then, the cells involved in the immune response must be present in many regions of the body where undesirable foreign antigens are likely to gain entrance. Consequently, the Reticuloendothelial System, a network of connective tissues that contains cells of the immune system, has evolved. The Reticuloendothelial System is widespread and diffuse, present along all areas of exposure to the outside world such as the Submucosa of the respiratory system and the Lamina Propria of the alimentary canal. In these regions, local invasion of Antigen often triggers local proliferation of lymphocytes. As a result, the presence of lymphoid nodules in regions such as the Lamina Propria of the gut are quite common and can serve as indicators of the health of the individual.

Because the Reticuloendothelial System is so widespread, the question arises as to why specific organs of the immune system, such as the lymph nodes and the spleen, are necessary. The answer lies in the fact that humans, like most vertebrate animals, consist largely of water. Consequently, it seems logical that humans would make use of fluid transport systems to move nutrients and wastes throughout the body. Two of these fluid transport systems, blood and lymph, are encased within vessels that constitute two separate vascular systems the blood vascular system and the lymph vascular system.

Because many substances are harmful to humans, it is highly probable that harmful substances will find their way into the blood and lymph vascular systems. As a result, humans have evolved a separate filtration system for each vascular system. Circulating lymph is filtered by lymph nodes, and circulating blood, by the spleen. Lymph nodes and spleen are far more than simple filters, however. They are elaborate networks of cells and connective tissues that hold cells of the immune system directly in the path of circulating blood or lymph in order to maximize the possibility of contact between harmful substances – foreign antigens – with the very cells or antibodies that will destroy them.

Because both the lymph node and the spleen are connective tissue networks loaded with immune cells, their organization will, of necessity, be loose and free from distinct layers. This condition makes their histologic organization difficult to understand. Students of microanatomy tend to have more difficulty in understanding the histologic organization of the organs of the immune system than of any other system. The diffuse nature of the immune system, which is so effective in function, is difficult to visualize in sectioned material. The following set of light and electron micrographs should simplify that task.

Lymph nodes are large masses (up to 2.5 cm) of lymphoid tissue that take station at intervals along major lymph vessels. Lymph enters one side of the node by way of several small Afferent lymphatic vessels, percolates through a series of leaky lymphatic sinuses, and exits the other side of the node through a single Efferent lymphatic vessel. In its journey through a major lymph vessel, lymph must filter through many lymph nodes arranged at intervals along the length of the vessel.

Lymph nodes perform several important functions. Within a given node, lymphocytes may leave the general blood circulation through holes in the walls of leaky venules and enter the lymphatic circulation. Similarly, newly produced lymphocytes, born in the node, may enter the lymphatic circulation. These lymphocytes may eventually enter the bloodstream via the subclavian vein, the site at which the lymphatic system empties its contents into the bloodstream. In addition to these functions, lymph nodes harbor many macrophages that phagocytose foreign matter picked up by lymphatic capillaries from the Interstitial Fluid in the connective tissue spaces.

One of the major functions of the lymph node is to produce new lymphocytes. Under conditions of antigenic stimulation, special regions in the cortex of the node called primary nodules become activated. Each Primary Nodule has a pale-staining core, the germinal center. Within the Germinal Center, large stem cells called lymphoblasts multiply. Their progeny are the B-lymphocytes-lymphocytes destined to become Antibody-producing Plasma cells. B-lymphocytes thus produced in the lymph node commonly enter lymphatic vessels, are delivered into the blood vascular system at the level of the subclavian vein, and are carried by the bloodstream to various regions of the body, where they differentiate into mature Plasma cells. The antibodies made and released by Plasma cells are one of the major first lines of defense in the immune response.

The cortex of a monkey lymph node, shown by light microscopy, appears in Figure A. Here, several Afferent lymphatic vessels (AL) that bring lymph to the node are evident as they pierce the thin connective tissue capsule (CAP) that envelops the node. Beneath the capsule, out in the cortex, lies a Primary Nodule (outlined by a dotted line). Note that the Primary Nodule has a dark periphery and a relatively clear center. The clear central area is the Germinal Center. The dark periphery of the primary nodule contains B-lymphocytes ready for distribution to areas of need.

Part of the field of view in Figure A is depicted by electron microscopy in Figure B. Here, the thin connective tissue capsule (CAP) is evident, as is part of an Afferent lymphatic vessel (AL) that enters the node. Small lymphocytes (L) occupy the periphery of the nodule. Large lymphoblasts (LB), the progenitors of the small lymphocytes, commonly reside in the germinal center of the Primary Nodule.

The lymph node is held together by a fine network of connective tissue fibers called reticular fibers. Difficult to see by light microscopy without special stains, the reticular fibers are secreted by slender cells called reticulocytes (R) that resemble the Collagen-secreting fibroblasts described earlier.

Immune system

Plate 16-1
Plate 16-1, Figures A and B. Matched pair of light and electron micrographs of serial thick and thin sections taken through the cortex of a squirrel monkey lymph node. AL, Afferent lymphatic vessel; C, capillary; CAP, connective tissue capsule covering lymph node; L, Lymphocyte; LB, Lymphoblast; R, Reticulocyte; V, venule; arrowhead, mitotic figure (Figure A); dotted line (Figure A), primary nodule. Figure A, 650 X; Figure B, 1,300 X

Figure A is an electron micrograph of the periphery of a squirrel monkey lymph node depicting the capsule (CAP) and the outer reaches of the cortex. The capsule consists of a thin, densely woven envelope of Collagen fibers (Co) that are secreted by fibroblasts (F). Immediately beneath the capsule lies a large lymphatic sinus, the Subcapsular Sinus (SS). The subcapsular sinus, also known as the Cortical Sinus or marginal sinus, receives the lymph brought to the node by the Afferent lymphatic vessels that pierce the capsule. As with most lymphatic sinuses within the lymph node, the wall of the Subcapsular Sinus is leaky, more a meshwork of cells and their extensions than a true “wall.” Unlike other vessels, which are lined exclusively by enclothelial cells, the walls of the subcapsular (and other) lymphatic sinuses are lined by reticulocytes (R) and some macrophages (M) in addition to endothelial cells (E). The endothelial cells found in the walls of lymphatic sinuses are different from the flat, attenuated, squamous endothelial cells found in the walls of arterioles and capillaries. Lymphatic sinuses contain cuboidal endothelial cells, sometimes referred to as littoral cells.

Reticular cells, or reticulocytes are connective tissue cells that manufacture and secrete reticular fibers in much the same way as fibroblasts make Collagen fibers. (These are different from the immature red blood cells, which are also called reticulocytes.) Unlike Collagen, which is often organized into sheets, reticulin is frequently found as thin strands of connective tissue (arrows, Figure B). These strands form the skeleton that supports the lymph node and are often found to extend across the lumen of lymphatic sinuses. When viewed at high magnification with the electron microscope, strands of reticulin appear to be surrounded by thin cytoplasmic extensions of the Reticulocyte.

Figure B is an electron micrograph of the cortex of the monkey lymph node. Several blood capillaries (C) are present, as are two postcapillary venules (V1, V2). Postcapillary venules in lymph nodes have wall structures different from those structures found elsewhere; they are lined by cuboidal (instead of squamous) enclothelial cells (E). These cuboidal endothelial cells are capable of separating from one another to let whole cells pass through the wall of the leaky venule. Consequently, red blood cells, lymphocytes, and other white blood cells are free to travel in and out between the bloodstream and the inner reaches of the lymph node. The lymph node in this micrograph was fixed by intravascular perfusion that was incomplete; as a result, some blood vessels (such as V1) retain erythrocytes, whereas others (V2, C) have none. Two “marginated” lymphocytes, cells stuck to the vessel wall, are evident in V2.

Between the vessels lies a rich array of cells. Macrophages (M) are abundant, as are reticulin – secreting reticulocytes (R). Numerous lymphocytes (L) are in evidence, as are a few red blood cells (*) that have escaped from the postcapillary venules.

Figure A. Electron micrograph of the capsule and cortex at the periphery of a squirrel monkey lymph node. CAP, collagenous capsule, Co, Collagen fibers; E, endothelial (littoral) cell; F, Fibroblast; L, Lymphocyte; M, Macrophage; R, Reticulocyte; SS, subcapsular (cortical) sinus. 2,300 X

lymph node

Plate 16-2

Plate 16-2, Figure B, Electron micrograph of a region deep within the cortex of the same lymph node shown in Figure A. C, capillary; E, endothelial cell; L, Lymphocyte; M, Macrophage; R, Reticulocyte; V1, venule with red blood cells; V2, venule with no red blood cells; *, erythrocytes within lymphoid tissue; arrow, reticular Fiber. 2,400 X

The spleen is a complex filter placed in the path of the bloodstream. It brings foreign antigens in the blood in direct contact with various cells of the immune system that remove, destroy, or otherwise neutralize those unwanted bits of matter, be they bacteria, cells, or harmful macromolecules. The spleen also removes old erythrocytes from the bloodstream, digests them, and recycles their components for use elsewhere in the body. These functions are accomplished by portions of the spleen grossly described as the red Pulp. In addition to acting as a filter in the bloodstream, the spleen is a lymphoid organ active in the production of new lymphocytes. These lymphocytes, produced in the region known as the white Pulp, migrate into the red Pulp, where they enter the bloodstream through the walls of leaky venous sinuses.

Figures A and B at right, a matched pair of light and electron micrographs of serial sections taken through the spleen of the monkey, illustrate some of the major splenic structures. At the top of the figures lies the connective tissue capsule (C) that surrounds the spleen. The capsule, coated with a layer of mesothelial cells (M) continuous with the lining of the body cavity, is made of Collagen fibers, elastic fibers, and a few Smooth Muscle cells. The capsule sends into the depths of the spleen a network of Trabeculae that provides structural support for the organ. Reticular cells associated with the Trabeculae send out a fine feltwork of reticular fibers that anchor the loose cells of the spleen. Beneath the capsule lies a portion of the red Pulp that contains profiles of several venous sinuses (S). These venous sinuses, quite unlike other blood vessels encountered thus far, are central to the workings of the spleen.

The splenic venous sinuses (S) are extremely leaky, highly modified blood vessels endowed with large lumens and discontinuous walls. Unlike other blood vessels, they are not lined by a thin layer of endothelial cells. Instead, each Venous Sinus is lined by a discontinuous layer of cuboidal cells that, although traditionally referred to as endothelial cells, really resemble modified Smooth Muscle cells in shape and fine structure. These endothelial cells (E) are fusiform, measure about 100 Â µm long and are oriented with their long axes parallel to the axis of the Venous Sinusitself.

The profiles of several venous sinuses are shown by light microscopy in Figure A and by electron microscopy in Figure B. These images clearly show the large lumen of the sinuses (S) and the cuboidal nature of the lining cells (E), and they give an impression of the irregular nature of the wall that makes each sinus so leaky. Because the sinuses are leaky, the tissue between them contains representatives of all the cells found in circulating blood, including erythrocytes, platelets, and white blood cells. The tissue between the venous sinuses is commonly referred to as the Splenic Cords (also called the Cords of Billroth). In addition to the formed elements of the blood, the Splenic Cords contain reticular cells, macrophages, and Plasma cells. As a result of this configuration, any Antigen that strays out of a venous sinus will be subject to direct immunologic attack. In addition, all of the cells of the Splenic Cords have ready access to the lumen of the Venous Sinus. Reticular cells send reticular fibers into and across the sinus lumen, creating a cobweblike mesh that entraps passing objects. Macrophages send pseudopodial extensions of their Cytoplasm into the sinuses to capture material to be phagocytosed.

The venous sinuses, their contents, and the Splenic Cords of tissue between them constitute the red Pulp. Arteries and the lymphoid tissue associated with them constitute the white Pulp. It should be noted that Figures A and B at right show red Pulp only. The structure of the red Pulp will be examined in more detail in the following plate.


Plate 16-3
Plate 16-3, Figures A and B. Matched pair of light and electron micrographs of serial thick and thin sections taken through the spleen of the macaque. C, capsule; E, endothelial cell lining Venous Sinus; M, Mesothelial Cell; S, Venous Sinus; arrow, line formed by dense, Actin-like material in foot processes of endothelial cells. Figure A, 1,300 X; Figure B, 2,000 X

Venous sinuses are large, leaky vessels located in the red Pulp that are essential to proper splenic function. Previous discussion of the spleen in this chapter indicated that the venous sinuses are unique in at least two respects. First, the Endothelium that lines them is cuboidal; second, large spaces frequently occurring between adjoining endothelial cells permit passage of whole blood, including red and white blood cells, in and out of the sinus itself.

Figure A, a cross section through several venous sinuses (S1 S2, and S3) in the spleen of the macaque, illustrates the special nature of the endothelial cells (E) that line the sinuses. The endothelial cells are long, slender, fusiform cells. They average 100 Â µm in length and lie with their long axes parallel to the long axis of the Venous Sinus. Consequently, a cross section through a Venous Sinus will display cross-sectional images of the endothelial cells that line the sinus. When thus viewed, as in Figure A, the endothelial cells are seen to have a Nucleuslocated close to the lumen. The basal portion of the cell is slender and has a foot-process filled with fine filamentous material that lies at the perimeter of the sinus (arrow). The fine filaments in the foot-process resemble the Actin filaments observed in Smooth Muscle cells. When viewed by electron microscopy, the endothelial cells bear a close resemblance to smooth muscle cells.

The presence of contractile elements within the wall of the Venous Sinus would, it seems, favor the movement of cells and other blood-borne materials in and out. The sinuses labeled S1 and S3 at right reveal that several red blood cells (Es) are literally caught in the act of escaping, or being squeezed out, through the walls of the Venous Sinus into the surrounding tissue, the splenic cords. Once out of the vessel and into the Splenic Cords, macrophages (M) await the now-extravascular red blood cells (Ex), eager to engulf them, digest them, and recycle their contents as bilirubin (a component of Bile) and Hemosiderin (bearing iron for use in new erythrocytes). In addition to macrophages, Plasma cells (P) lie in the Splenic Cordsjust outside the boundaries of the venous sinuses. Plasma cells secrete antibodies that combat antigens brought by the blood into the spleen.

The microanatomy of the venous sinuses is made even more apparent when viewed in longitudinal section, as shown in Figure B. Figure B is an electron micrograph of a thin section that caught a curving Sinusoid in both longitudinal section (LS) and cross section (XS) at different points along its length. Here, the longitudinal orientation of the endothelial cells (LS) is apparent. The filament-packed foot-processes, cut lengthwise, resemble thin electron-dense strips (arrowheads). The Sinusoid is supported by a framework of spirally wound thin strips of Basement Membrane-like material (arrows) that surround it like the coils of a spring. The actincontaining endothelial cells, acting in concert with the associated skeleton of Basement Membrane material, may, through cycles of contraction and relaxation, forcibly promote the exchange of materials through the open spaces in the wall of the Venous Sinus.

Plate 16-4
Plate 16-4, Figure A. Electron micrograph of cross sectioned venous sinuses within the spleen of the macaque. E, endothelial cell; Es, Erythrocyte escaping from sinus; Ex, extravascular Erythrocyte that has escaped from Venous Sinus; M, Macrophage; P, Plasma cell; S1, S2, and S3, venous sinuses; *, Erythrocyteengulfed by Macrophage; arrow, foot-process of endothelial cell tipped with cluster of Actin-like filaments; arrowhead, Basement Membrane wrapped spirally around sinus. 2,000 X

Figure B. Electron micrograph of a macaque’s splenic Venous Sinus cut in longitudinal and cross section at different points along its length. E, endothelial cell; Ex, extravascular Erythrocyte; LS, longitudinally sectioned region; M, Macrophage; N, Neutrophil; XS, cross sectioned region; *, cross section through foot-process of endothelial cell; arrow, cross section through spirally wound strip of Basement Membrane; arrowhead, longitudinally oriented foot-process of endothelial cell. 2,100 X

Of all the organs of the immune system, the Thymus remains the most mysterious. Although much remains to be learned about the functions of the Thymus, we do know this; once the stem cell precursors from the bone marrow have taken station in the Thymus and have differentiated into thymic lymphocytes, they undergo intense proliferation in the cortex of the lobules of the Thymus. Having proliferated, the thymic lymphocytes – still functionally inert – migrate from the cortex to the Medulla of the lobules. There, they enter the bloodstream and leave the Thymus. The Medulla of the Thymus is populated by leaky postcapillary vermles, similar to those in lymph nodes, that permit passage of lymphocytes through their discontinuous walls. Once in the bloodstream, the lymphocytes that have left the Medulla of the Thymus acquire immunocompetence, become known as T-lymphocytes (or T cells), and travel through the bloodstream to peripheral lymphoid organs such as the spleen, lymph nodes, appendix, tonsils, or Peyer’s Patches of the ileum of the gut. Once placed in these various outposts of the immune system, T cells become active in the various phases of cell-mediated immunity that so effectively protects the body from foreign invaders.

The microanatomy of the Thymus of the squirrel monkey is illustrated at right. Figure A is a light micrograph taken at low magnification. Figure B, an electron micrograph of a serial section taken through the same Thymus, illustrates the area enclosed by the rectangle in Figure A. In Figure B it is evident that the Thymus is surrounded by a capsule (CAP) of loose connective tissue. A number of connective tissue septa (S) extend inward from the capsule; these septa subdivide the Thymusinto a series of lobules. Each Lobule has an outer, darkly staining cortex (CO) and an inner, pale Medulla (ME). The Thymus contains many different kinds of cells, including lymphocytes (L), macrophages, and epithelial reticular cells (E). The epithelial reticular cells are unique; of endodermal embryonic origin, they are usually stellate. They have a large, clear cell body that sends out numerous protoplasmic extensions that contain tonofilaments for structural support. Despite their misleading name, the epithelial reticular cells are not associated with reticular fibers. Their cytoplasmic extensions, which completely line the septa, are intimately associated with lymphocytes.

Lymphocytes-large, medium-sized, and small-are the most numerous cells in the Thymus. Large lymphocytes divide and give rise to the small lymphocytes. Many of the small lymphocytes born in the cortex degenerate and die; others migrate to the Medulla (ME), wherein they enter the bloodstream. Several dividing cells (arrows) and degenerating cells (D, Figure B) are present in the micrographs at right.

Within the Medulla of the lobules of the Thymus are structures named Hassall’s corpuscles. Hassall’s corpuscles, illustrated in the insets, consist of conspicuous concentric arrays of squamous epithelial cells. Not found elsewhere in the body, Hassall’s corpuscles, whose function remains unknown, provide convenient histologic landmarks by which the Thymus may be readily identified in sectioned material.

The Thymus – Histology

the Thymus

Plate 16-5
Plate 16-5, Figures A and B. Matched pair of light and electron micrographs of serial sections taken through the Thymus of the squirrel monkey. CAP, capsule; CO, cortex; D (Figure B),degenerating Lymphocyte; E, epithelial reticular cell; F, fat cell; L, Lymphocyte; ME (Figure A), Medulla, S, connective tissue Septum; arrow, mitotic figure of dividing Lymphocyte. Inset; Hassall’s Corpuscle. Figure A, 380 X;

Figure B, 900 X; insets, 4,400 X