The Lymphatic System- What it is, and its Parts

The lymphatic system helps to maintain the proper fluid balance in the tissues and blood, to conserve protein, and to remove bacteria and other particles from the tissues. It consist of a network of fluid-filled vessels that open into the blood-vessel system, together with masses of lymphoid tissue, the structure and activity of which will be discussed below. The lymphoid tissue is, in part, intimately associated with the network of vessels, as in the vessels, as in the tonsils and thymus, or has no obvious connection with the vessel network, as in the spleen.

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The fluid in the lymphatic vessels, called lymph, is as a rule clear or faintly opalescent, except the lymph called chyle, which comes from the intestine. Chyle at time may be white and milky in appearance because of the minute globules of fat that in contains. The thin-walled vessels through which chyle flows are referred to as lacteals because of their milky appearance. Modern knowledge of the lymphatic vessels may be said to date from the observations of Gaspare Aselli (Gaspar Asellius), who gave (1617) the first description of these vesssels after observing the white lacteals in the mesentery (a fold of membrane such as that which attaches the small intestine to the dorsal wall of the abdomen) of a well-fed dog. Since these first observations of Asellius, it has become well established that the lymphatic vessels are constantly receiving fluid from the bloodstream, to which they ultimately return it. There is thus a continuous circulation of lymph.

The lymphocytes

Of the variety of cells in the lymph nodes, the most numerous are lymphocytes, which are also found in the lymph, in blood, and elsewhere. Lympocytes vary in size, but the smallest are about the size of red blood cells and consist almost entirely of nucleus with very little cytoplasm-the cell substance outside the nucleus. The nuclear chromatin (the most readily stainable substance in the nucleus) is disposed in dense clumps. The cytoplasm has no distinctive granules, such as are to be found in most of the other white cells (leukocytes) of the blood. Lymphocytes are capable of active movement and can migrate through the walls of blood vessels and through the various tissues. Until a few years ago they were thought to be incapable of growth of differentation. In the late 1950s it was shown that an extract of the red bean, which has since been termed phytohemagglutinin, is capable if ctimulating  the growth and subsequent division on small lymphocytes; these are now regarded as primitive cells capable of growth and proliferation if suitanly stimulated. Since this discovery, a number of substances have been found capable of stimulating the growth of small lymphocytes. Consequently, the small lymphocyte is now regarded as the temporarily inactive phase of a cell, which on appropriate stimulation can enter upon a period of great ectivity.In the inactive phase the lymphocyte has been compared to a cellular spore form since it can remain inactive for a long time.Lymphocytes exist in a number of places in addition to the lymph nodes-namely, the spleen, the lymphoepithelial tissues, the thymus, and the bone marrow-and large numbers of the cells are scattered throughout the connective tissues of the body. The lymphoepithelial tissues are masses of lymphocytes in close relation to the epithelium (covering) of the alimentary canal. At the upper end, they form both the tonsils and the tissues at the back of the nose that when enlarged are called adenoids. The appendix is made up largely of lymphoepithelial tissues; and throughout the intestine there are lymphoid nodules, arranged either singly or in groups, known as the aggregated nodules, or Peyer’s patches. The lymphoepithelial tissues are separated from the bacterial contents of the mouth or the intestine only by thin epithelial layers through which bacteria may pass; hence, the tonsils and the appendix are notoriously sucject to infection.The thymus is a lymphoid organ that in particularly well developed at birth and during the growth period. After puberty it undergoes progressive atrophy. The bone marrow contains a large pupulation of lymphocytes scattered among its other cells and not arranged in dense nodules. A considerable number of lymphocytes are to be found in the connective tissues all over the the body. It is now known that contrary, are integrated by the continual migration of lymphocytes between them. These migrations are described as cellular migration streams.

The lymphatic vessels

The lymphatic vessels commence as a network of closed microscopic vessels-capilaries-whose walls consist essentially of a single layer of cells, the endothelium. This resembles the endothelium of the blood capillaries but is much more freely permeable to large molecules such as proteins and even to particles such as India ink. The classical method of demonstrating lymphatics, interstitial injection, has depended upon the great permeability of lymphatic endothelium. A solution of a dye or a suspension of particles is injected into the tissue spaces or interstices and immediately enters the lymphatic capillaries by passing through their endothelium. This can be demonstrated in man by injecting a dye into skin, the color appearing as streamers in the lymph vessels leaving the site of injection. In general, with certain exceptions due to special local circulatory arrangements, however there are blood capillaries, lymphatic capillaries also are to be found.

The lymphatic capillaries join to form progressively larger vessels whose walls become somewhat thicker because of their small amount of muscle and elastic tissue. These larger collecting vessels usually accompany the veins, and, like the veins, they possess valves, but in far greater numbers. The compression of valved lymphatic trunks by surrounding muscles is an important factor in the propulsion of lymph; in addition, lymphatic vessels themselves may possess a rhythmic contractility. At the points at which valves are attached, the walls of lymphatic trunks are thicker than elsewhere, and, consequently, distended lymph trunks usually have a beaded appearance.The intermediate vessels join to form a series of main lymph trunks. The main lymph trunk draining the lower limbs enters the abdomen; after passing through lymph nodes in the lumbar region (the small of the back), it becomes known as the lumbar trunk and enters the cisterna chyli; the latter also recieves the large common intestinal trunk, which conveys the lymph from the greater part of the intestine. The cisterna chyli is a dilated sac lying at the back of the abdomen opposite the first lumbar vertebra, and from its upper end the thoracic duct ascends through the chest, or thorax; it inclines slightly to the left and ends at the base of the neck by entering the left innominate vein from formed by the junction of the great veins draining the left side of the head and neck and the left arm. The thoracic duct thus returns to the blood stream the lymph from the both legs, the pelvis and abdomen, the left side of the thorax, the left arm, and the left side of the head and neck-in other words, the lymph from the greater part of the body. Lymph from the right side of the head and the neck, the arm, and the right side of the the thorax returns to the blood through the right lymph duct, which opens into the great veins at the base of the neck on the right side. There are valves in both the thoracic duct and the right lymph duct, including a particularly well-developed valve close to their termination.

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Lymph nodes

Lymph nodes are a relatively recent evolutionary development and in their characteristic form are present only in mammals, through a rudimentary type of lymph node is found in birds. Lymph nodes are found in both superficial and deep situations. There is a well-marked node-or nodes- at the back of the knee, while there are a number of both superficial an deep nodes n the region of the groin. There is a large collection of nodes in the armpit. There are numerous nodes in the thorax and abdomen, especially in mesentery. It is distinctive of man and primates that lymph passes through chains of several small lymph nodes instead of through one or two large nodes, as in other mammals.

This is readily seen in the mesentery, where in man there are many scattered small nodes, while in the dog the nodes are all gathered together  in a large mass at the root of the mesentery. Asellius thought that this was a small pancreas, and it is still sometimes known as the pancreas of Asellius.

The human lymph node is enclosed in a fibrous capsule; in the interior are masses of cells, mainly lymphocytes, which in the outer part of the node-the cortex are in dense masses, whereas in the interior, the medulla, they form a series of branching cords are clearer spaces, the lymph sinuses. In some animals, through not in man, there is a well-developed system of partitions (traculae) extending into the node from the capsule and partially subdividing the node into segments. Nodes tend to be more or less bean shaped; lymph vessels bring lymph to the outer convex surface of the node, where they pierce the capsule. Immediately under the capsule there is a lymph channel termed the subcapsular (or cortical) sinus; from this sinus other sinuses extend radially into the node to reach the network of medullary sinuses.The medullary sinuses converge to the concave of the bean-shaped node and leave the node in the form of two or three lymphatic vessels that quickly join to form a single vessel. Vessels bringing lymph to the lymph node are termed afferent; vessels draining lymph away, efferent. Freshly formed lymph that has not passed through a node is called peripheral lymph. Lymph that has passed through only one node is called intermediate lymph. Lymph that has passed through all the nodes that it is going to traverse and is on its way to the blood without further intertuption is termed central lymph.

Pharipheral lymph contains a small number of lymphocytes, around 500 to 1,000 per cubic millimetre, whereas intermediate and central lymph contain no more, up to 40,000 or more per cubic millimetre. For many years this difference in cell content between peripheral and intermediate lymph was taken to mean that large numbers of cells were constantly being formed in the node and entering the blood via the lymph stream. This view found further support because of the presence in the cortex of the node of lymphoid nodules with a clear central area called the germinal center . Walter Flemming, the German biologist who first described the germinal center, observed that it contained numerous cells in the process of dividing. It was, in fact, one of the first animal tissues in which cell division was observed.

It is now known that the position is far from being as simple as was first thought. In addition to the new formation of cells, there is appreciable migration of lymphocytes through the node. A small number of these reaches the node through its afferent lymph, while a larger number reaches it via the bloodstream. Lymph nodes contain a special type of blood vessel, the postcapillary veins, through the walls of other blood vessels into the bloodstream again. In either case, there in extensive recirculation of lymphocytes in lymph nodes as well as new formation. Cells that leave the node thus may be newly formed, or they may have been derived from the afferent lymph or from the blood.

Cells that reach the blood via the lymph stream are termed indirect- entry lymphocytes, as opposed to the direct-entry cells that obtain access to the bloodstream directly by migrating through the walls of bloodvessels in lymphoid tissue without first entering the lymph. Considerable numbers of lymphocytes are of the direct-entry variety. The few cells reaching the node via peripheral lymph are derved from the blood, passing first from the blood capillaries into connective tissue and thence into peripheral lymph.The steady drift of a small number of lympohcytes through the connective tissues of the numbers suggest. Furthermore, under abnormal conditions the number of lymphocytes in peripheral lymph may be greatly increased.

The lymph sinuses are not,as a rule, clear channels but are broken up by a network (reticulum) of cells that are capable of engulfing bacteria and other foreign particles; i.e.,they are phagocytic. Foreign particles, whether inert, such as carbon, or living, such as bacteria or fragments of cancerous growths, may all be held up for a time in the lymph node, where they are mechanically trapped by the cells of the sinus reticulum. This led Rudolf Virchow, in the middle of the last century, to ascribe a defensive, or barrier, function to lymph nodes; these observations on lymph nodes led to assignment of a defensive function to lympoid tissue generally. It is now known that Virchow was to a large extend right, though for the wrong reasons. When bacteria reach a lymph node, they stimulate rapid proliferation of a small part of the total lymphocyte population of the node, and this proliferation accompanied by a gradual change of the lymphocytes into an other type of cell, the plasma cell, that forms antibodies to the bacteria. The plasma cells may be formed in large numbers and would quickly swamp the lymph node were it not that large numbers of young plasma cells leave in the efferent lymph to settle in various parts of the organism where to go on producing antibodies. In this way a relatively small amount of lymphoid tissue can give rise to a large number of plasma cells. At the same time, through the migration of plasma cells, the lymph node is cleared,as it were, for further action.

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