1) Introduction to the lymphatic system
2) Lymphatic organs – thymus, spleen and lymph nodes
3) Lymphatic vessels and lymph
4) Overview of the immune system
Introduction to the lymphatic system
Lymphatic system consists of lymphatic vessels which contain lymph and lymphatic tissue – that can be arranged in lymphatic organs (thymus, spleen and lymph nodes) or it can co-create specialized areas of mucosa or submucosa (e.g. in alimentary canal or respiratory tract – mucosa-associated lymphoid tissue (MALT)). We distinguish primary (central) and secondary (peripheral) lymphatic organs (tissues). Under primary lymphatic organs we understand thymus and bone marrow whilst secondary lymphatic organs are spleen and lymph nodes as well as mentioned specialized areas of mucosa and submucosa.
Lymphatic system performs several significant functions:
1) As a part of immune system it participates in defence – it provides an ideal surroundings for creation, maturation and differentiation of lymphocytes
2) It removes excess lymph from tissues
3) It helps transport lipids from GIT to a body (chylomicrons)
Lymphatic organs – thymus, spleen and lymphatic nodes
Lymphatic organs are formed by specialized type of tissue, lymphatic tissue. The tissue consists of lymphatic cells (lymphocytes, their developmental forms and plasmatic cells) and macrophages stored in reticular fibrous tissue (type of specialized loose connective tissue that operates as an architectural framework). The exception is thymus where lymphatic cells are embedded in reticular epithelium, which associates with organ development (see below).
Thymus is a primary (central) lymphatic organ stored in an upper mediastinum. Unlike all other lymphoid organs that arise from mesenchyme (mesoderm), thymus has a double origin (mesenchyme and endoderm). It origins from the 3th and the 4th pharyngeal pouch. During the development, the endodermal epithelium invaginates caudally as a tubular structure which later loses connection with branchial epithelium. The lymphocytes of thymus are derived from mesenchymal cells.
Thymus is fully developed and functional at a birth. It persists as a large organ approximately till puberty when in this developmental period it significantly reduced the differentiation and proliferation of T-lymphocytes and lymphatic tissue is gradually replaced by an adipose tissue and a connective tissue (thymus is a subject of involution).
There is a connective tissue capsule on the thymus surface from which penetrate connective tissue septa dividing thymus into false lobules. There are efferent blood vessels (never afferent!), lymphatic vessels and nerves in fibrous capsule and septa. We tell apart peripheral (darker) zone in lobes, rich in small lymphocytes (= cortex) and a lighter stained zone underneath, rich in reticular epithelial cells (= medulla, marrow). In some histological slides can “lobular” arrangement of cortex and marrow resemble a lymphatic node with germinal centre (often confuses students). The other morphological characteristics (described below) help positive identification of thymus in histological slides.
Cortex of the thymus
The outer parenchymal portion – cortex – is highly basophilic in histological slides stained with hematoxylin-eosin (H&E). That is because of a large number of densely accumulated developing small T-lymphocytes with intensely staining nuclei. These lymphocytes do not reproduce lymphatic follicles.
Medulla of the thymus
Medulla stained with H&E is stained less intense in the histological slides than cortex because (as well as terminal centres of lymphatic node) it contains mainly large lymphocytes. These have weakly staining nuclei and quantitatively more cytoplasm than small lymphocytes. In addition of large lymphocytes there is also a large number of epithelial reticular cells. However the most characteristic structure of medulla are Hassall’s bodies, eosinophilic structure with an inner concentric arrangement. There are reticular epithelial cells which degenerate and fill with keratohyalin granules (this is not surprising feature for cells developing from oropharyngeal epithelium) which number and size increases with age of the individual. Their function is not exactly known but we know they produce IL-4 and IL-7 that are involved in differentiation and maturation of the T-lymphocytes.
The support for free cells of thymus (in cortex and medulla) is a reticular epithelium. Reticular fibres do not occur. A small amount of connective tissue in the thymus is found only along the blood vessels. Reticular epithelium originates from endoderm and star-shaped projections of the cells are joined by desmosomes, in cytoplasm containing tonofilament and some of them show signs of secretory activity. By all the attributes it varies from reticular tissue forming substrate for free cells in other lymphatic organs.
Blood-thymus barrier ensures a development of lymphocytes by preventing contact between developing lymphocytes and antigens circulating in blood. Blood-thymus barrier consists of three basic structures:
1) Capillary endothelium with a strong basal lamina, non-fenestrated blood capillaries
2) Perivascular connective tissue with dispersed macrophages
3) Proper reticular epithelial cells with their basal lamina
Thymus does not have afferent lymphatic vessels. Therefore does not form a filter for the lymph. Nerve fibres are myelinated and unmyelinated. They enter into the capsule and connective tissue septa. They are branches of n. vagus, plexus cardiacus, n. hypoglossus and nerve fibers from 1st thoracic ganglion.
Function of the thymus
A primary function of the thymus is a regulation of formation, maturation and differentiation of T-lymphocytes. Thymus provides suitable conditions for formation of functional T-lymphocytes capable to recognize foreign antigens. Both defective and autoreactive clones of T-lymphocytes are eliminated. Thymus is therefore the crucial organ for inducing tolerance to the body’s own antigens. Through the production of thymic hormones (a group of about 40 polypeptides, eg. thymopoietins, thymosins and thymuline) affects thymus other organs of the immune system (and most probably the neuroendocrine system as well).
Morphology and a function of spleen discusses special chapter.
A lymph node is encapsulated spherical organ located in the course of lymphatic vessels. It serves as a biological filter of the lymph. The surface the node is covered by sensitively innervated capsule formed by dense collagenous connective tissue blended with elastin and a tiny amount of smooth muscle cells. Into the parenchyma of the node extend connective tissue septa from the capsule (= trabeculae). The mechanical support for lymphocytes is reticular connective tissue in the lymphatic node. On the section of the node under the fibrous capsule we can distinguish cortex on the surface and the medulla inside the node. Most cells are formed by B- and T-lymphocytes. Their distribution in the node is not homogenous.
Cortex of lymph node
Cortex is formed by thick cell clumps of lymphatic follicles (nodules) populated mainly by B-lymphocytes and follicular dendritic cells. A primary follicle is equally inhabited by small resting lymphocytes. A secondary follicle contains germinal center with stimulated B-lymphocytes. Germinal centre is lighter in the histologic slide stained with H&E, because the activated B-lymphocytes are bigger and stain less intensively than rapidly dividing cells in the periphery of the follicle. The dark appearance of the periphery of the follicle results from the thick accumulation of small lymphocytes.
Medulla of lymph node
Medulla of a node forms mutually anastomosing strands of lymphatic tissue. B-lymphocytes dominate, macrophages are present (more than in the cortex), plasma cells and in a small extend granulocytes too.
Paracortical zone of lymphc node
There is a paracortical zone between the cortex and the medulla. Morphologically it can be difficult to recognize it from the cortex and the medulla but functionally differ significantly from them. It contains mostly small recirculating T-lymphocytes. Cells do not organize into follicles.
The term hilum refers to a part of a node (slightly invaginated) where nerves and arteries enter it and veins and usually one lymphatic vessel leave it (called vas efferens). The size of the node is between 1-25 mm. After the birth new lymph nodes develop only very rarely. Where necessary, it occurs to compensatory hyperplasia (enlargement) of already existing lymphatic nodes.
Knowing the location of lymphatic nodes and direction of the lymph flow of individual organs are important for diagnosing and prognosis of malignant diseases. Lymphatic circulation is next to blood circulation the most typical way of spreading malignant tumour. Sentinel nodes are particularly important, they are constantly present in a human body and their alteration indicates in time about a disease of a specific organ (e.g. node of Sorgius on the outer side of chest related to breasts). Not only tumour modified node (also inflammatory altered nod) has an impact on enlargement, change of consistency or pain. At inflammatory diseases the nodes appear painful, rather softer. Their swelling occur rapidly and are usually accompanied by other symptoms as fever, malaise, pain of the affected organs, changes of mucous membranes and so on.
Function of the lymph nodes
Lymphatic nodes work as a filter flowing lymph. It must always pass at least one node before it gets into the blood circulation. After the circuit several lymphatic vessels enter the node, called vasa afferentia. There is an uptake of antigens in nods from the lymph and their presentation. This activates B-lymphocytes that are than transformed into plasma cells synthesizing specific immunoglobulins (antibodies). Some B-lymphocytes are transferred to efferent vessels by which they get into a blood circulation and then deposit in tissues. There are so-called memory cells in a lymphatic node capable of setting off quick immunoglobulin production when confronted with relevant antigen.
T-lymphocytes localized in paracortical zone of the lymphatic node do not produce antibodies but directly destroy pathological cells.
Lymphatic vessels and lymph
Lymphatic vessels begin in the connective tissue as lymphatic capillaries. Lymphatic capillaries have unlikely blood capillaries bulky and irregular lumen. They are stuffed with endothelium of flat cells that have on their basal surface numerous small protrusions and they exhibit pinocytic activity. The fixation of endothelial cells into the surrounding tissue provide so called anchoring ligaments. Lamina basalis is missing here and there which facilitates and accelerates transport of substances from the connective tissue into lymphatic capillaries. The lymphatic vessels conduct the lymph into the venous system.
The lymphatic vessels are found in almost all tissues of the body. An exception is the nerve tissue, some parts of eye (cornea, lens, vitreous humor) or nails.
Lymphatic vessels of small and medium calibre
We distinguish three layers:
1) Tunica intima: is composed of endothelium and a thin layer of elastic subendothelial connective tissue
2) Tunica media: made up of smooth muscle (circular arranged) and elastic fibres
3) Tunica adventitia: the thickest layer which contains smooth muscles inserted into collagen tissue, we can find elastic fibers here as well
Duplicatures of tunica intima are called valves. The valve is covered with endothelium, a thin lamina basalis and a thin layer of collagen connective tissue.
Large lymphatic vessels (ductus thoracicus and ductus lymphaticus dexter)
We can distinguish three layers:
1) Tunica intima: fibrous elastic layer with a small amount longitudinally organized smooth muscle cells, with endothelium on the surface. A thin elastic membrane separates it from tunica media.
2) Tunica media: the thickest layer of the wall, contains more muscle than the veins of the same calibre – smooth muscle is arranged circularly
3) Tunica adventitia: smoothly flows into the surrounding tissue, it contains longitudinally arranged bundles of smooth muscle cells
The lymph is a clear to yellowish liquid with a similar composition as blood plasma. The main difference is lower concentration of protein in the lymph (50-60% of plasmatic concentration) with the content of fibrinogen the lymph can participate. Levels of ions do not differ significantly. After passing through lymphatic organs the lymph contains immune cells (the vast majority is formed by lymphocytes). The exact composition of the lymph depends on the site of its formation (which tissue it originate from) for example:
1) lymph formed in liver has a bigger protein concentration
2) lymph originated from a small intestine (chyle) contains more fat (including fat-soluble vitamins) and has therefore milky appearance
Movement of the lymph
In our lymphatic system circulates approximately 1 litre of the lymph. In tissues it forms about 2-2,5 litres of lymph from tissue fluid daily. The lymph travels through lymphatic vessels, in which progress are located lymphatic nodes, which work as a biological filter. Small lymphatic vessels are gradually merged into vessels with a bigger calibre until the origin of ductus thoracicus and ductus lymphaticus dexter, which converge into a venous system (the flow through the ductus thoracicus is about 1ml/min). Lymphatic system is therefore in compare to blood circulation an open system.
The movement of the lymph is ensured and directed by:
1) contraction of smooth muscle of lymphatic vessels with a frequency 4-7/min
2) contraction of surrounding arteries and muscles
3) negative thoracic pressure
4) negative pressure in venous system
5) formation of tissue fluid
6) valves of the lymphatic vessels (prevent backflow of the lymph)
If the lymph circulation slackens and thus the removal excess tissue fluid, we develop a condition called lymphedema. It is most often caused by a blockage of lymphatic vessels and lymphatic nodes by tumour cells or after a surgery. Very rarely occurs congenital hypoplasia of lymphatic vessels.
Overview of the immunity
The immune system gives our body an ability to resist pathogenic bacteria, viruses, protozoa, fungi or our own tumour-transformed cells. The range of the training text does not allow us to discuss this issue in detail. We only get a basic outline. For more detailed information we refer you to textbooks of immunology.
Non-specific (non-adaptive) defence mechanisms
Unlike specific defence mechanisms the non-specific defence mechanisms do not distinguish pathogen (its antigens) against which they act. At every meeting with the antigen they reply by the same process, they do not have a memory. These mechanisms are evolutionarily older and provide a quick reaction. The activation occurs almost immediately (seconds to hours).
Among the non-specific immunity we classify:
1) Intact cover of the organism (mucous membrane and skin), that prevents our body from the contact with pathogens
2) Antimicrobial substances produced on the outer and inner surface of the body – such as an hydrochlorid acid produced in the stomach, lysozyme contained in saliva and tears
3) Cellular component – phagocytes (neutrophils granulocytes and macrophages), eosinophil and basophil granulocytes or NK-cells (“natural killer cells”) – see respective subchapter
4) The humoral component – the complement, acute phase proteins and cytokines (e.g. interferons)
Lysozyme is an enzyme that breaks certain beta -1,4-glycosidic cellular bounds found in cellular wall of bacteria (it has the effect of mainly gram-positive bacteria, attacked component is called murein-peptidoglycan). Lysozyme therefore has a strong antibacterial effect. It occurs in saliva, tears, blood plasma and white blood cells (neutrophils).
Proteins (reactants) of acute phase (RAF)
RAF is a group of proteins forming part of non-specific immune response of the body in reaction on function of stressor. Inflammatory cytokines (IL-1, IL-6 and TNF-..) stimulate their production in liver. Amongst RAF belong e.g.: CRP, alpha-1 antitrypsin, haptoglobin, coagulation factors or components of complements.
Among complement befit more than twenty serum and membrane proteins whose cascade activation cause lysis of cells as well as modulation other immune processes. The main component of complement represent nine serum proteins marked C1-C9. The main functions of complement:
1) Lysis of pathogenic cells by acting so called membrane attack complex (MAC) composed of components C5b-C9
2) Opsonization of bacteria leads to more effective phagocytosis, phagocytic cells have a receptor for the C3b component
3) Chemotaxis (movement of cells in a certain direction) attracts immune system cells by components C3a and C5a
Specific (adaptive) defence mechanisms
The specific immunity is represented by evolutionary younger mechanisms capable of specific reaction on individual antigens (antigenic specificity). Its activation is rather slower but has a “memory” that allows it more effective reaction on repeated encounters with the same antigen (pathogen). The ability is taken from so called memory cells. The specific immunity consists of cellular and humoral components.
The cellular component represents B- and T-lymphocytes. B-lymphocytes transform after activation into plasma cells protecting the body by producing antibodies (immunoglobulin). T-lymphocytes evince direct cytotoxicity (so called cytotoxic T-lymphocytes, CD8+) as well as play a key role in modulation of individual components of the immune system (so called helper T-lymphocytes, CD4+ or suppressor T-lymphocytes).
Humoral component is represented by antibodies and cytokines. Antibodies synthesized by plasma cells ease the phagocytosis (opsonization), trigger cytotoxic reactions (e.g. so called classical way of complement activation) or prevent adhesion of pathogens. Their structure is created by two pairs of heavy and two pairs of light chains mutually connected disulphide bonds Depending on heavy chains we distinguish five kinds of gamma globulins: IgA, IgG, IgM, IgD and IgE.
Some areas of our body are colonized by symbiotic microorganisms (eg gastrointestinal tract or skin). Their presence prevents the overgrowth of other (pathological) bacteria that could cause illness (eg diarrhea). We can say that our symbiotic bacteria “compete” with these pathogens and thus protects us from infections.
Subchapter Authors: Martina Šajdíková and Josef Fontana