The lymphatic system is vital to the defense against illness. If infectious agents manage to breach the mechanical barriers and gain entry to the milieu interieur, the cells which deal with the invasion are those which have arisen, developed, matured, and/or been stored in lymphatic organs. Changes in the histology of lymphatic organs are important diagnostic criteria.
For reasons that should be obvious, the lymphatic system is intimately related, both structurally and functionally, to the blood vascular system. Lymph starts as blood fluid that is "strained" into the tissues under the hydrostatic pressure of the pumping blood. It is drained from the body's organs and intercellular spaces by thin vein-like lymphatic vessels, and repenters the venous circulation.
Lymph itself is a clear, slightly yellowish and opalescent fluid derived from blood. It contains some white blood cells, chiefly lymphocytes. Certain specialized organs devoted to processing and modifying lymph and lymphoid cells are present in all normal mammals.
In this laboratory exercise we will examine the spleen; lymph and hemal nodes; the thymus; tonsils; the aggregated lymphatic nodules of the digestive tract (commonly called Peyer's patches) and their avian equivalent, the bursa of Fabricius.
Lymphocytes will be treated in the exercise dealing with formed elements of blood, but if you want to see what a circulating lymphocyte looks like, click here.
The nomenclature for lymphatic tissue is confusing. In an attempt to clarify it I'll use two basic terms: diffuse and nodular lymphatic tissue. Diffuse lymphatic tissue consists of any unorganized collection of lymphocytes. These can be found wherever localized conditions have attracted lymphocytes in large numbers, and vary greatly in size. Such aggregations are usually transient features. The diffuse lymphatic tissue found in lymphatic organs is present all the time, however.
Nodular lymphatic tissue is always found surrounded by diffuse tissue, and it's much more organized. The typical example of nodular lymphatic tissue is the germinal center, a highly ordered collection of B-lymphocytes found in some (not all) lymphatic organs. Not all lymphatic organs will contain germinal centers.
Do not, by the way, confuse the terms "nodular lymphatic tissue" and "lymph node" with each other. The lymph node is a specific anatomical organ you can dissect out of an animal. The node contains diffuse lymphatic tissue and in most cases, nodular lymphatic tissue--in the form of germinal centers--as well. Germinal centers never occur outside of those lymphatic system organs that can provide an appropriate environment for them. Thus, lymphatic tissue outside of specifically lymphatic organs is, by definition, diffuse in nature.
To see examples of diffuse and nodular lymphatic tissue, click here.
The spleen, strictly speaking, is really part of the circulatory system, but it's generally described with the lymphatic organs because of the very large population of lymphocytes found in it. The spleen is a flaccid bag that serves as a storage site for blood, a processing station for the scavenging of aged erythrocytes, and a few other things. It's one of the "dispensable" organs, because mammals get along quite nicely without a spleen. In the case of a traumatic injury that ruptures the spleen, the easiest thing to do is to take it out, and a splenectomy is commonly done. It's easier to remove the spleen and tie off its blood supplies than to try to repair it and risk renewed hemorrhage.
This is another of those troublesome terms that really refer to gross anatomy, but with which the histologist has to deal as well. On the basis of its gross appearance in fresh sections, the spleen is traditionally said to have the bulk of its parenchyma as red pulp, with isolated areas of white pulp interspersed through it. The red pulp gets its appearance from the formed elements of the blood (mostly erythrocytes) it contains. The white pulp consists almost entirely of lymphocytes, in a peculiar association with the arterial blood supply (see below). These two terms are quite logical when applied to gross specimens, but things become a little confusing when they're applied to microscopic sections.
Briefly, in terms of microscopic sections, "white pulp" is equivalent to the lymphocyte population of the spleen, in the form of the periarteriolar lymphocyte sheath or PALS (see below). "Red pulp" is everything else, which means the splenic cords and the sinuses between them.
For a low power view of the spleen, and areas of red and white pulp, click here.
Any description of the spleen's histology must necessarily be centered around the arrangements of its blood vessels. We will begin with slide 1023, a spleen from a small animal (probably a rat).
You can see that there is a distinct capsule of collagenous connective tissue, which is interspersed with smooth muscle. One function of the spleen is to act as a reservoir of erythrocytes, and the contraction of the capsule expels these into circulation when needed. The admixture of muscle is particularly pronounced in ruminants and horses. The capsule sends septa part way into the interior volume of the organ, and from these septa arise a delicate network of reticular fibers, to form a stromal network.
The septa also serve as routes for the arterial branches into the interior volume of the spleen. As in other organs the capsule is also the route by which blood supply enters and leaves the spleen. The main input, the splenic artery, ramifies in the capsule and sends branches deeper and deeper along the CT septa (see below). Similarly, blood leaving the organ is drained back through a series of veins in the septa and the capsule, eventually exiting via the splenic vein. Both the artery and vein are physically located at the hilus. Surgical removal of the spleen is simplified by this arrangement: all that is needed is to tie off the vessels, cut through them, and yank the spleen out.
To see the splenic capsule, click here.
As an arteriole leaves a septum and enters the interior volume of the spleen it immediately acquires a continuous coating of lymphocytes. This "sleeve" of lymphocytes is the periarteriolar lymphocyte sheath, or PALS. You can see it on slide 1023 quite well as basophilic areas with small blood vessels running through them. These central arteries are the first part of the arterial input into the spleen. All of the PALS collectively constitute what the gross anatomist calls the spleen's "white pulp."
The PALS is a continuous feature of the arterial supply, almost to the point where it breaks up into capillaries. The main blood vessel you see running through most profiles of the PALS is the central artery, and it continues to branch. Eventually, the branching reaches the point where the PALS is reduced to only one or two cells on the smallest portions of the arteries formed by subdividing the central arteries.
To see examples of the PALS, click here.
We're going to come back to the blood vessels and what happens to them, but for the moment let's look a little more carefully at the PALS Most of it is diffuse lymphatic tissue, but nodular tissue in the form of germinal centers are usually seen in it. Within the PALS, a loose stromal framework of reticular fibers and their reticulocytes is present. The resident lymphocytes of the diffuse portion of the PALS are supported by it. This sort of arrangement is typical of all diffuse lymphatic tissue.
Within the germinal centers, however, things are a little different. The germinal center is a special place. It's a clone of B-lymphocytes, and it has to be isolated so that these cells can develop properly. Consequently the stroma isn't fibrillar in nature. Instead, special cells of a different cell line than the fibroblasts form a stroma within the germinal center and support the B cells in it.
Due to the dense packing of the lymphocytes in the PALS you will not be able to make out the fibers in an H&E preparation, even in the diffuse parts. But special staining for reticular fibers will reveal it pretty easily.
The central arteries, like any self-respecting artery, break up into smaller ones eventually. Each one gives rise to a tuft of small vessels, the penicillar arteries (so called from their fancied resemblance to the bristles of a paintbrush--the Latin word for "brush" is penicillum). The PALS is almost wholly gone by the time this level of branching is reached.
To see a group of penicillar arteries, click here.
Upon reaching the red pulp and finally losing the last of the PALS, some of the penicillar arteries individually break up into a peculiar form of capillary. These are the sheathed capillaries, the name referring to the presence of a specialized region of the capillary wall.
While most blood vessels are lined with simple squamous epithelium, the endothelium of the sheathed capillaries is composed of fusiform cells, oriented parallel to the long axis. These specialized capillaries are surrounded by a sheath of reticulocytes and macrophages, bound together by reticular fibers. Not all of the capillaries arising from the penicillar arteries have such a sheath. Those that do have special properties, and the cells of the wall are phagocytic. You can often find them by looking for lipofuscin in the walls of the sheathed portions.
Past the sheath, the capillaries return to the normal configuration expected of these vessels. Sheathed capillaries can be found on slides 673 and 1023.
We have now followed the arterial supply as far as we can, and it's time to look at the venous side of the system.
The red pulp constitutes the bulk of the splenic volume. The supporting reticulum of the red pulp is continuous with the septa. Again, it consists of reticular fibers invested by reticulocytes.
If you look carefully at the red pulp, it will become clear that it is arranged into anastomosing splenic cords, separated by venous sinuses. The sinuses are blood spaces. They're lined with endothelium and contain circulating blood cells. Hence most of what you find in them is erythrocytes. The splenic cords are the masses of cells in between the sinuses. They contain a lot of erythrocytes, but they also contain many other cell types: macrophages most prominently. It's in the cords that the phagocytosis of the aged erythrocytes occurs, and consequently a stain for iron (such as that used on slide 674) will reveal the presence of hemosiderin in the cords very well.
To view the differences between cords and sinuses, click here.
In most spleen sections, this cord-and-sinus separation is very difficult to see, but if you can find an area of slide 1023 in which the density of cells is not too great, you should be able to decipher the cord-sinus-cord arrangement without too much trouble. One clue as to which is which on this slide is the ratio of erythrocytes to other cell types. In the sinuses, the erythrocytes are present in the same overwhelming proportion that they represent in the blood. In the cords, however, there are many more non-erythrocytes present, and the nuclear density is much higher than in the sinuses. Look also for the endothelium which separates cords from the adjacent sinuses.
Since the sinuses are blood vessels, they are lined by endothelium, the cells of which are fusiform in shape and oriented longitudinally with respect to the long axis of the sinus. The wall of the sinus, while it demarcates the cells in the cords from the blood flowing through the sinuses, is nevertheless compliant enough to permit passage of cells through it. Cells of the blood can and do move from a sinus to an adjacent cord, and vice versa, and the static impression given by microscopic sections is incorrect. The endothelium lining the splenic sinusoids is phagocytic. On slides 673 and 674 you will be able to see the boundaries of the sinuses by looking for small bits of lipofuscin in the lining cells.
The sinusoids eventually join together to form veins of the pulp, which in turn coalesce into larger veins in the septa. Drainage of the entire organ is through the large vein running parallel to the splenic artery at the hilus.
One function of the spleen is to destroy old erythrocytes, and this is the role of the resident macrophages of the organ. The buildup of hemosiderin in the spleen is considerable over long periods of time. Erythrocytes that have reached the end of their useful lives (90 to 120 days) develop cell surface associated "senescence antigens" which mark them as the legitimate prey of the macrophages in the red pulp. They are seized, phagocytosed, and the salvageable components of their compounds recycled, sort of like old cars in a junkyard. The remainder becomes hemosiderin.
The macrophages are easily visible on slide 673, which is a spleen from an elderly dog. The macrophages are filled with hemosiderin pigment, the indigestible residue of phagocytosed red blood cells.
To demonstrate to you that this is really hemosiderin and not lipofuscin, look at slide 674, a companion section from the same block, stained with the Prussian Blue reaction which deposits a blue precipitate (ferric ferrocyanide (Fe4[Fe(CN)6]3) at the site of iron containing hemosiderin.
To see the difference between hemosiderin and lipofuscin in splenic macrophages, click here.
Well, there is no avoiding it any more. Having accounted for the arterial flow and the venous drainage, it's necessary to deal with the question of the connection between the two sides.
The termination of splenic capillaries is a matter of debate. It's believed by some investigators that the capillaries of the spleen end by opening right into the splenic cords, and that blood thus released passes through the cords and into the adjacent sinuses. This would mean that there is no direct connection between the arterial and venous sides, and that the cords in essence act as "filters" through which the total volume of blood has to pass before returning to the venous circulation. This is the open theory of splenic circulation.
An alternative view is that the capillaries do in fact end by joining the sinuses directly, as is the case in other organs in which sinusoids are found. Self respecting arteries, after all, should end in capillary beds. Thus the circulatory loop is closed, and the cells in the splenic cords get there from the blood passing through the sinusoids. This is, of course, called the closed theory.
The arguments for and against both theories rest on theoretical and empirical grounds. The Open Theory proponents point out that if one cannulates the splenic artery, and pumps in dyes, the dyes will be found in both the cords and the sinusoids. But if one cannulates the vein, the dyes are only found in the sinusoids. Therefore, they argue, there is no direct connection, as otherwise the cords and the arteries leading into them would be full, too.
The adherents of the Closed Theory reply that there would be more or less constant hemorrhage if the Open Theory were correct, as the ending of arteries in parenchymal tissue could not result in anything else, in the absence of venous drainage capable of handling the large volumes of blood the spleen holds. To the evidence of the dyes, they respond by saying that splenic veins are so delicate that it is impossible to retro-perfuse them without damage, and that a failure to get dyes into the arteries simply means the connections between the two vessel types was broken by the pressure of perfusion.
As you probably expected, there is a third theory holds that both types of ending exist simultaneously. That at times the circulation is closed, and others open; that in normal times the open system operates, but when there is need for those reserve erythrocytes, the connections between arteries and veins takes over, the system is closed, and the blood is pumped out.
Lymph nodes and hemal nodes share some similarities of structure, but really are fundamentally different organs both qualitatively and quantitatively. The lymph node is found along larger lymphatic vessels. The lymphatic vessels, remember, drain tissue fluid back towards the venous circulation. They originate as "blind" structures, i.e., there is no closed loop as there is for the blood circulation. Flow of lymph through these vessels isn't under a continuous pressure the way the blood is; it's "squeezed" back by contraction of muscles around the vessels.
The lymph nodes are stationed along these routes to act as "filters" for lymph as it passes through. Lymph is pushed through from the periphery of the node to its center, and then continues on its way back to join the venous circulation. Since lymph nodes are pretty sizable organs (most of them are about the size of a pea or larger, some as big as a large broadbean) they must have blood vessels and circulation of their own. Blood vessels serve each lymph node, entering and leaving them without releasing blood into the volume of the node, any more than blood is released into other organs.
Hemal nodes are different. They are formed by partitioning off part of a forming blood vessel, and they really are sort of cul de sacs in vessels of the blood circulatory system. They can be likened to lay-bys or pullover points on highways. They contain large numbers of erythrocytes. They don't have their own internal circulatory subsystem, and they don't receive lymph at all, though they do have lots of lymphocytes (as well as erythrocytes) inside them.
Both lymph and hemal nodes contain diffuse lymphatic tissue, and in most cases have nodular tissue in the form of germinal centers, as well.
The lymph node is the most organized of the lymphatic organs. Grossly, they are shaped somewhat like beans, with a depression on one side, the hilus. Blood vessels enter and leave the lymph node at the hilus, but lymphatic vessels enter at the periphery, and exit at the hilus. More will be said about the flow of lymph through the node below.
To see a lymph node at low power, click here.
A real and reasonably typical lymph node can be found on slide 672, and you'll find the features described below on it. To delineate details of the stroma, use slide 29, which is stained with Wilder's reticulum method. It will clearly show you the fine fibrils that make up the supporting scaffold of this organ.
Lymph nodes have a discrete CT capsule which sends trabeculae deep into the volume of the organ. The capsule acts as an overall envelope for the node, and also as the route of entry for lymphatic vessels. It's composed of dense irregular collagen with a few elastic fibers as well. In ruminants there is a considerable admixture of smooth muscle.
A delicate CT stroma arises off the trabeculae to support the very cellular parenchyma, consisting almost wholly of lymphocytes. The fibers of the stroma rise from and are anchored to the trabeculae and to the inner face of the capsule. The stroma consists of reticular fibers and the cells which make them, the reticulocytes, which are a form of fibroblast. At one time it was thought that the reticulocytes were phagocytic, but it's now known that this is not the case.
The relationship of the reticulocytes and the reticular fibers is very intimate. The fibers are not directly exposed to lymph: instead, they are completely enveloped by processes of the reticulocytes, which thus lie over and around the fibers they have elaborated. The fibers and their covering reticulocytes form a complex webwork that crisscrosses the greatest part of the volume of the node, especially in the cortical region. These fibers also traverse the subcapsular space and the sinuses.
At the periphery and in the region around the outside of the node you should be able to locate at least one afferent lymphatic vessel which courses through the CT and adipose tissue around the node, and appears to end at the capsule. Many if not most of these will have valves in them. Valves are a feature of most of the larger lymphatic vessels, and are closer together than in veins.
Passage of lymph through the larger vessels and towards a node is accomplished by contractions of the adjacent musculature and movements of the viscera. Compression of the very thin walls forces lymph through the valve, and then to and eventually through the node.
Flow of lymph through the node is from the periphery towards the hilus. Immediately beneath the capsule is a large subcapsular sinus. This space is in communication with the rest of the volume of the node and offers free passage to the incoming lymph. Entering lymph drains into the subcapsular space, and from there it trickles through the cortex into the more open sinuses of the medulla. The drainage of the node is via a coalescence of the sinuses, which form an efferent lymphatic vessel at the hilus. This vessel leaves the node in company with the blood vessels.
To see the capsule and an afferent lymphatic vessel, click here.
The valve in the afferent vessel will be visible on slide 672 as a small flap, (actually they are paired flaps) connected to the walls of the vessel. Like the valves of veins, the valves of lymphatic vessels are designed to permit only one-way flow. This situation is analogous to rest areas on an interstate highway:traffic must enter and leave going in one direction. The flow of lymph through the node is from the periphery towards the center.
To see a lymph vessel with a valve, click here.
The parenchyma of a lymph node is packed with lymphocytes. It's divided into a cortex and a medullary region, with the boundary between them rather indistinct. Lymphocytes fill the bulk of its volume. They're supported by the complex 3-D latticework of the stroma. Macrophages are present scattered throughout the parenchyma. They can be identified by their content of lipofuscin.
The cortical and medullary regions of the parenchyma are really the result of irregular distribution of the lymphocytes. The cortex is densely packed with cells, and usually shows nodular lymphoid tissue as highly organized, discrete locations where lymphocyte proliferation is taking place in germinal centers.
In the cortex germinal centers can be seen pretty easily, and this region is densely packed with lymphocytes. The less-dense medulla represents the rest of the node's volume. In here the cells are arranged into cords, with open lymph sinuses in between them.In the medulla, the parenchyma takes on the structure of anastomosing cords of cells, separated by lymph sinuses. The sinuses are in communication with the subcapsular sinus peripherally, and with the efferent vessels at the hilus.
To view the medullary region of a lymph node, click here.
The flow of blood through the node enters via an arteriole at the hilus, is distributed throughout the volume of the node, and leaves via a vein, also at the hilus.
Entering blood vessels run through the CT septa peripherally, towards the outer region of the node, where they break up. They supply blood to the medullary cords and the cortex, both of which have capillary plexuses. Areas of nodular lymphatic tissue will be enveloped individually by dedicated capillary plexuses.
In the return circuit, there are special regions of the postcapillary venules in the cortex. The postcapillary venules are devoid of a muscular tunic, and have specialized endothelial lining cells. These are the site at which small lymphocytes carried in the blood enter the node. The lining epithelium of the postcapillary venules is cuboidal rather than squamous, and apparently has recognition antigens on it so that the lymphocytes can find it. Lymphocytes are capable of insinuating themselves between the endothelial cells in this region, and entering the parenchyma. Look for what appears to be a small blood vessel with a high cuboidal lining.
Note that the direction of migration is the reverse of that in the thymus: lymphocytes brought to the node migrate out of the blood and into the node's volume. They will return to the circulation via the lymphatic vessels. In the case of the thymus, lymphocytes that are maturing in the organ migrate out to the blood.
After the region of the postcapillary venules, the venules enter the medullary cord and regain a normal lining; small venules coalesce to form the outflow from the node at the hilus.
Hemal nodes are characterized by the presence of large numbers of erythrocytes, and are most common in sheep. One can be seen on slide 502.
Like the lymph node, the hemal node has a cortex and a medulla, and germinal centers are frequently seen. There is a distinct capsule, and hemal nodes are served at the hilus by an afferent arteriole.
Within the hemal node, there are capillary plexuses as well as the peculiar postcapillary venules with specialized endothelium. A vein leaves the node at the hilus.
Unlike lymph nodes there are no afferent or efferent lymphatic channels. Hemal nodes are found along major blood vessels in the cranial portions of the body, and in development are derived from them. They range in size from barely noticeable to pea-sized, and are considered by some investigators to be accessory spleens. In splenectomized animals, it's normal for the hemal nodes to enlarge (and/or increase in number) and take over some of the functions of the missing spleen.
The thymus is the primary lymphatic organ of mammals, and its presence and functionality is required for immunocompetence to be fully established. In one strain of mice, nude, the thymus is congenitally absent, and these animals must be housed in a sterile environment, as they have no functional immune system.
Structure of the Thymus
Begin with slide 117, a thymus from a young pig. The organ is covered by a thin connective tissue capsule, and lobulated by septa originating from it. The thymic lobules are clearly divided into a dark staining cortex and a lighter medulla by the differential density of the cells in each region. The cell types are the same but their distribution within the lobules is not uniform.
To see examples of the thymus, click here.
The septation of the thymus is not complete. Serial sectioning would easily show that the different lobes and lobules are continuous with each other through bridges of parenchymal material.
Blood vessels enter the thymus through the capsule, and travel along septa to the corticomedullary border, at which point they enter the parenchyma. You can trace parts of this route by looking for erythrocytes confined in capillaries, and by identifying the capillary walls. Arterioles entering the thymus send capillaries to the cortex, which branch at the periphery and return. At the corticomedullary junction, postcapillary venules are found, which represent specialized sites of transit of matured lymphocytes into the blood.
The bi-lobed thymus is relatively largest (in relation to body weight) very early in life, but continues to grow in absolute size for a while after birth. Nevertheless, in the normal course of aging, the thymus undergoes involution, by which the total volume of active tissue is reduced. It's replaced by connective tissue and adipose tissue for the most part, but some functional tissue remains throughout life. Many stimuli (such as overexposure to radiation and some chemicals) can cause accidental involution, and in these cases the organ is usually capable of regeneration if the offending stimulus is removed.
To see what an involuted thymus looks like, click here.
The thymus has three principal cell types: thymocytes, i.e., lymphocytes of the thymus; a few macrophages; and the specialized epithelio-reticular cells, stellate cells which provide a supporting framework (the stroma) for the parenchyma.
Most of the lymphocytes in the thymus are T-cells. In order to acquire immunocompetence, these cells must reside in the thymus for a period of time. Macrophages are interspersed among these, and can be seen as larger cells with vacuolar nuclei. Frequently coarse granules of undigested material may be present in the macrophages' cytoplasm.
The epithelio-reticular cells are almost impossible to demonstrate in H&E preparations, being obscured by the lymphocytes that sit on them. Hence you will not see them in your specimens as such, even though they are indeed present.
The thymic stroma isn't fibrillar in nature. It is wholly cellular. The epithelio-reticular cells are branching and tree-like in shape, and put forth processes, supported by intracellular cytoskeletal elements. These processes come into contact with each other, and are held together at the points of contact with desmosomes. The processes thus form a completely cellular scaffolding--sometimes called a cytoreticulum to distinguish it from other, truly fibrillar, reticula--to support the rest of the parenchyma.
Most importantly to normal thymic function, these cells form a physical, cellular boundary between the parenchymal thymocytes and the rest of the body. At the periphery of the cortex, subjacent to the capsule, and around all blood vessels, the processes form a continuous cellular layer constituting the blood-thymus barrier, to limit exposure of the lymphocytes to blood borne antigens. The reason for this arrangement is that the thymus has to be maintained as an immunologically protected site;the cells here are undergoing maturation and specialization, and to subject them to the influences of the outside world may mean they "learn" to respond incorrectly. You will recall that a similar "cytoreticulum" arrangment is present in the germinal centers where B-lymphocytes proliferate. Like germinal centers, the thymus is a special place and it's isolated from the possibly malign influences of the outside in the same way.
Epithelio-reticular cells may undergo degeneration, and organize themselves into concentric eosinophilic whorls of material, called thymic corpuscles (or Hassall's corpuscles, after Arthur H. Hassall, 1817-1894, an English physician). These are of unknown function, and appear very early in life. They appear to be nothing more than debris
You can see examples of thymic corpuscles by clicking here.
A few of what might be called "miscellaneous lymphoid organs" are scattered in various places in the body. While unorganized masses of diffuse lymphatic tissue can be found in almost any CT or any epithelial organ, in these "miscellaneous" organs it has a regular organization into nodular lymphoid tissue, often showing germinal centers.
One such place is the tonsils. Slide 116 is a section of palatine tonsil, and the presence of these organized structures is obvious.
Tonsils are found in association with the oral cavity, and can easily be identified by their surface covering of stratified squamous epithelium. The gross structure of a tonsil is what you might get if you pushed your extended fingers into modeling clay: deep crypts, with lymphoid tissue filling the spaces between them.
To see an example of a tonsil, click here.
Tonsils are well encapsulated by CT on the side away from the oral cavity, making them easy to remove if they become infected. The cell types are the same as those seen in other lymphoid organs, including lymphocytes and macrophages and plasma cells. Germinal centers are common.
Slide 849 is a longitudinal section of the ileum segment of the small intestine. In the submucosal parts of this organ (the CT layer separating the lining epithelium from the outer muscular coat) there are large aggregates of lymphoid tissue. These are the aggregated lymphatic nodules, classically called Peyer's patches (after Johann K. Peyer, 1653-1712, a Swiss anatomist).
These structures are large enough to be visible with the naked eye as whitish areas on that side of the intestine opposite to its mesenteric attachment. In ruminants, they have germinal centers present in them almost from the time of birth. They're the site of maturation and development of the B-cells in mammals )as the thymus is for T-cells). Another choice example of this organ can be seen on slide 41.
To see an example of Peyer's patches, click here.
Tonsils and Peyer's patches, along with all the diffuse lymphatic tissue in the gastrointestinal tract and respiratory system, collectively are labeled gut associated lymphatic tissue, or GALT. The respiratory system arises in embryonic life as a diverticulum of the gut, and its lining cells, like those of the digestive tract, are derived from endoderm. The GALT is considered to be a lymphoid organ in its own right, which processes immunological challenges encountered in those systems.
The bursa of Fabricius, seen on slide 675, is an organ peculiar to birds. (It's named for Giralamo Fabrizi, 1533-1619, an Italian anatomist, who, as was customary among men of science in those days, Latinized his name). This is a blind, sac-like structure on the caudodorsal side of the cloaca. The term "bursa" is the Latin for "purse" and implies the blind "pocket book" gross structure of this organ.
Like the tonsil, it is a number of blind, crypt-like invaginations, and some references actually call it a "cloacal tonsil," which isn't so far off the mark. Its appearance is at first glance similar to the thymus, and in some ways it performs a thymus-like function; it's involved in conferring immunocompetence on the avian equivalent of B-cells in birds. The aggregated lymphatic nodules of the ileum are the mammalian equivalent of this organ.
The free surface of the avian bursa is covered with stratified columnar epithelium, not stratified squamous, which makes it easy to distinguish from tonsils or the thymus. Most of the cells in it are lymphocytes.
To see examples of the avian bursa, click here.