This page is divided into the following topics:
Eicosanoids: Everyone has heard of hormones such as insulin, glucagon, testosterone and estrogens. These hormones can be measured with standard blood tests. However, most people have not heard of eicosonoids. Eicosanoids are the most potent biologic chemicals known. Eicosanoids are produced in low concentration by each cell in the body (except red blood cells). Since there are 50-75 trillion cells in the body releasing these chemicals, their control over the body is very powerful. In fact, eicosanoids control all hormonal systems and every physiologic function in the body. For this reason, eicosanoids are considered "superhormones". Eicosanoids effect the cardiovascular system, reproductive systems, central nervous system and the immune system. These chemicals last only a few seconds to a few minutes and do not enter the blood stream. Eicosanoids are considered "local harmones" because their effect extends only close to where they are produced. Eicosanoids exert hormone-like effects on the cells that produce them (autocrine effects), as well as on neighboring cells (paracrine effects).
Eicosanoid biosynthesis begins when a cell is activated by mechanical trauma, cytokines (discussed later), growth factors or other stimuli. (The stimulus may even be an eicosanoid from a neighboring cell; the pathways are complex.) These stimuli cause the release of a phospholipase at the cell membrane. The phospholipase travels to the nuclear membrane. There, the phospholipase catalyzes ester hydrolysis of a phospholipid or diacylglycerol. This frees a 20-carbon essential fatty acid from the nuclear membrane. These free fatty acids are what eicosanoids are made from.
Eicosanoid is a general term for oxygenated derivatives of three different 20-carbon fatty acids:
Essential fatty acids: Essential fatty acids are fatty acids that we need but our body cannot synthesize. We must get them from our diet. There are two fatty acids that are essential fatty acids for us. These include alpha-linolenic acid (ALA) (an omega-3 fatty acid) and linoleic acid (LA) (an omega-6 fatty acid).
Dietary LA produces arachidinic acid (AA) and dihomo-gamma-linolenic acid (DGLA) in the body. AA can also be consumed directly from the diet. DGLA is not available from the diet. It is synthesized in the body. DGLA is produced from gama-linolenic acid (GLA) which is produced in the body from LA as well as available from the diet. Dietary ALA produces EPA in the body. EPA can also be consumed directly from the diet.
As mentioned above, all eicosanoids come from 3 fatty acids:
Each one of these three fatty acids is the starting point of a cascade of chemical reactions that produces different types of eicosanoids. The AA cascade produces 20 different eicosanoids that control a wide array of bodily functions, but especially functions of inflammation and the central nervous system. Most AA is derived from dietary linoleic acid which comes from vegetable oils and animal fats.
Dietary sources of AA
Some foods contain AA (meat and dairy) and other foods contain LA (vegetable oils consumed directly or used for frying) which is converted to AA in the body.
In the inflammatory response, the EPA and DGLA cascades parallel and compete with the AA cascade. EPA(20:5 omega-3) provides the most important competing cascade. As already mentioned, EPA is ingested directly from the diet or derived from dietary alpha linolenic acid. DGLA (20:3 omega-6) provides a third , less prominent cascade. There is no direct dietary source of DGLA. It is derived from dietary GLA (18:3 omega-6). The EPA and DGLA cascades soften the inflammatory effects of the AA and its eicosanoids. Low dietary intake of these less inflammatory fatty acids, especially omega-3s, is associated with a variety of inflammation-related diseases. There is strong evidence that diseases are ameliorated by increasing dietary omega-3 including psychiatric disorders.
Dietary omega-3 and GLA counter the inflammatory effects of AA’s eicosanoids in three ways:
Omega-3 and omega-6 fatty acids: Omega-3 fatty acids include alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), all of which are polyunsaturated. Our bodies cannot produce ALA. We must get it from our diet. The body can produce EPA and DHA but it synthesizes these from dietary ALA. Omega-6 fatty acids include linoleic acid (LA) arachidonic acid (AA) and dihomo-gamma-linolenic acid (DGLA). Our bodies cannot produce LA. We most get it from our diet. The body can produce AA and DGLA. Although necessary, excessive levels of omega-6 fatty acids will increase disease in the body. Modern western diets are too high in omega-6 fatty acids. Also, the metabolism of omega-3 fatty acids from alpha-linolenic acid within the body is competively slowed by the omega-6 fatty acids. Chronic excessive production of eicosanoids from omega-6 fatty acids is associated with heart attacks, thrombotic stroke, arrhythmia, arthritis, osteoporosis, mood disorders, inflammation, obesity, and cancer. Omega-3 fatty acids lower cardiovascular disease, decrease inflammation, improve immune function, prevent cancer, and improve brain health.
Note: DHA (docosahexaenoic acid) is an omega-3 fatty acid synthesized from EPA (see above diagram). DHA is the most abundant fatty acid in the brain and retina. 60% of the weight of a neuron’s plasma membrane is composed of DHA. DHA deficiency is associated with cognative decline, increase neural cell death, and depression.
Two families of enzymes catalyze fatty acid oxidation to produce the eicosanoids:
Three 20-carbon EFAs and the eicosanoids series derived from them
Two families of enzymes catalyze fatty acid oxidation to produce the eicosanoids:
Eicosanoids are not stored in the cell, but are synthesized as required. They are derived from the fatty acids that make up the cell membrane and the nuclear membrane.
The AA cascade (the main cascade that causes inflammation in the body)
The "classic eicosanoids" include leukotrienes (LT) and prostanoids (prostaglandins (PG), prostacyclins (PGI) and thromboxanes (TX))
NSAIDS (non-steroidal anti-inflammatory drugs):
The classical COX inhibitors (NSAIDS) are not selective and inhibit all COX enzymes. Newer NSAIDS selecticely inhibit COX2. COX2 inhibitors have been found to increase the risk of atherothrombosis even with short term use.
Physiological and biochemical effects of the most physiologically important eicosanoids. LT, leukotriene; PG, prostaglandin; TX, thromboxane
Body temperature regulation
Vasodilatation (arterial), vasoconstriction (venous)
Reduction of water or NaCl reabsorption, increase in rennin secretion
Reduction of activation
Reduction of apoptosis, reduction of cytokine production
Increase in osteoblast activity
Uterus contraction, inducing of ovulation
Uterus contraction, induction of ovulation
|Vasodilatation, reduction of platelet aggregation and activation Bronchodilatation|
|Vasoconstriction, platelet aggregation and activation Bronchoconstiction|
|Chemotaxis, superoxide anion generation, degranulation, increased expression of adhesion molecules, aggregation Increase in cytokine production Induction of T-suppressor cells, reduction of apoptosis in premature cells, increase in cytokine production|
|LTC4/LTD4||Vasculature Airways||Vasoconstriction, vascular leakage, plasma extravasation Bronchoconstriction, mucus secretion|
(I previously referred to this website on the
"Chronic Inflammation" and "Lipid" pages)
The following website gives important nutritional information for all foods. The information includes fatty acid content, inflammation factor and glycemic load:
The formula used to calculate the IF Ratings measures the effects of more than 20 different factors that determine a food’s inflammatory or anti-inflammatory potential, including:
The glycemic index (GI) is a numerical system of measuring how much of a rise in circulating blood sugar a carbohydrate triggers–the higher the number, the greater the blood sugar response. So a low GI food will cause a small rise, while a high GI food will trigger a dramatic spike. A list of carbohydrates with their glycemic values is shown below. A GI is 70 or more is high, a GI of 56 to 69 inclusive is medium, and a GI of 55 or less is low.
The glycemic load (GL) is a relatively new way to assess the impact of carbohydrate consumption that takes the glycemic index into account, but gives a fuller picture than does glycemic index alone. A GI value tells you only how rapidly a particular carbohydrate turns into sugar. It doesn't tell you how much of that carbohydrate is in a serving of a particular food. You need to know both things to understand a food's effect on blood sugar. That is where glycemic load comes in. The carbohydrate in watermelon, for example, has a high GI. But there isn't a lot of it, so watermelon's glycemic load is relatively low.
Glycemic Index and Glycemic Load Rating Chart
cytokines: small proteins that are secreted by cells of the immune system (white blood cells (WBCs)) and other cells that effect the behavior of other cells. Cytokines, such as interleukins, tumor necrosis factor (TNF), and interferons, control the growth, development, and function of connective tissue cells and inflammatory mechanisms. Cytokines have hormone-like effects on neighboring cells (paracrine effects) but unlike eicosanoids, cytokines can enter the blood stream and effect cells a distance away (endocrine effect).
Cytokines in wound healing
White Blood Cells (WBCs): Another name for WBCs is leukocytes and immune Cells
Unlike most cells in the body, leukocytes act like single-celled individual organisms. There are many different type of WBCs each having different functions.
This picture shows various WBCs surrounded by red blood cells (RBCS)
Antibodies are produced by plasma cells which is a white blood cell, (discussed later). Antibodies are also called immunoglobulins. Antibodies make it easier for WBCs to bind to pathogens and destroy them. Below is a picture of an antibody. The variable region of the antibody (see below) is where the antibody can bind to an antigen (pathogen). The effector cell (WBC) attaches to the Fc region of the antibody. The binding of the WBC to the antibody initiates the WBC to destroy the pathogen. In the example below, two antigens can bind on the two variable regions and only one WBC can attach to the one Fc region.
Antibodies come in different varieties called isotopes. The type of heavy chain present defines the class (isotype) of antibody; these chains are found in IgA, IgD, IgE, IgG, and IgM antibodies.
Heavy chain in dark green, light chain in light green.
Each heavy chain has two regions, the constant region and the variable region. The constant region is identical in all antibodies of the same isotype, but differs in antibodies of different isotypes.
An Antibody can be a monomer, containing only one Ig unit, dimeric with two Ig units, tetrameric with four Ig, or pentameric with five Ig units. The same antibody isotype can have different complexes (e.g IgA can be dimeric or tetrameric).
To combat pathogens that replicate outside cells, antibodies bind to pathogens to link them together, causing them to agglutinate. Since an antibody has at least two paratopes it can bind more than one antigen by binding identical epitopes carried on the surfaces of these antigens. By coating the pathogen, antibodies stimulate effector functions against the pathogen in cells that recognize their Fc region.
Note: The paratope is the part of an antibody which recognizes an antigen
The engagement of a particular antibody with the Fc receptor on a particular cell triggers an effector function of that cell; phagocytes will phagocytose, mast cells and neutrophils will degranulate, natural killer cells will release cytokines and cytotoxic molecules; that will ultimately result in destruction of the invading microbe. The Fc receptors are isotype-specific, which gives greater flexibility to the immune system, invoking only the appropriate immune mechanisms for distinct pathogens.
Chemotaxis: Chemotaxis is the process of attracting cells to an area. Cells follow an increasing concentration of a chemical (chemokine (a type of cytokine)) to an area where they are needed.
Pathogen a molecule or microbe that causes damage to healthy tissue.
Opsonin: molecules that attach to a pathogen so it can more easily be destroyed by a white blood cell. Opsonins include antibodies, complement proteins and mannose-binding lectins (discussed later).
Granules secretory vesicles in a cell that contain antimicrobial cytotoxic molecules.
Granulocyte A WBC that contains vesicles or granules in their cytoplasm. Granulocytes include: basophils, eosinophils and neutrophils (also a phagocyte). Other cells that contain granules but are not considered granulocytes are mast cells, natural killer cells (NK cells) and cytotoxic T cells.
Degranulation Release of chemicals from the granules in a granulocyte. Degranulation can occur inside a cell after it engulfs a pathogen or it can occur outside the cell when granules release their contents extracellularly. These chemicals either kill microbes or signal other WBC to function a certain way. Examples of toxic materials produced or released by degranulation by granulocytes include:
Antigen a molecule that binds to an antibody or a MHC protein (more on this below) that is recognized by the immune system. This may be an intact pathogen or a piece of a pathogen or a non pathogen.
Cell Wall The cell wall is located outside the cell membrane and provides a cell with structural support and protection and also acts as a filtering mechanism. The cell wall is made of a mesh-like layer of Peptidoglycan, also known as murein. Peptidoglycan is a polymer consisting of sugars and amino acids. A major function of the cell wall is to act as a pressure vessel preventing over-expansion when water enters the cell. They are found in plants, fungi, algae bacteria and some archaea. Animal cells do not contain cell walls.
Cell (Plasma) Membrane – The cell membrane is a biological membrane that separates the interior of all cells from the outside environment.. The cell membrane is selectively-permeable to ions and organic molecules and controls the movement of substances in and out of cells.. It consists of the lipid bilayer with embedded proteins, which are involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signaling. The plasma membrane also serves as the attachment surface for the extracellular glycocalyx and cell wall and intracellular cytoskeleton.
The cell membrane surrounds the protoplasm of a cell and, in animal cells, physically separates the intracellular components from the extracellular environment. Fungi, bacteria and plants also have the cell wall which provides a mechanical support for the cell and precludes passage of the larger molecules. The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix and other cells to help group cells together to form tissues. The barrier is differentially permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be either passive, occurring without the input of cellular energy, or active, requiring the cell to expend energy in moving it. The membrane also maintains the cell potential.
Gram – and Gram + bacteria: Gram negative and gram positive bacterial differ in the arrangement of there cell wall and membrane.
Phagocyte: A WBC that engulfs a pathogen and destroys it.
Ligand: A ligand is a general term for a molecule (many varieties) that binds on a receptor located on a cell membrane.
Lysis: Disruption of the cell membrane which causes the contents of the cell to be released into the extracellular space.
Cell lysis is similar to the poppung of a balloon
Cell lysis occurs when the outside membrane of a cell looses its integrity
Cell lysis caused by viral infection
There are two ways a cell can die, either by programmed cell death (PCD) or necrosis.
Programmed Cell Death: PCD is known as cell suicide. The process of apoptosis is the most common and extreme form of PCD. Apoptosis has very specific morphologic and biochemical steps that occur in every cell that dies this way (see diagram below).
The picture below shows actual cell death by apoptosis
Unlike necrosis (discussed in a moment), apoptosis does not produce an inflammatory response from our bodies because a cell dying of apoptosis does not release any free intracellular contents into the extracellular space. Instead they release apoptotic bodies which are membrane enclosed vesicles containing pieces of intracellular debri. In addition. the chemicals (eicosanoids) released from a cell dying of apoptosis are not inflammatory signals. These chemicals signal phagocytes to engulf and destroy the apoptotic bodies. Apoptosis is the natural way most cells in our body die before they are replaced with new cells.
Apoptosis can be triggered in a cell through either the extrinsic pathway or the intrinsic pathway.
The Extrinsic Pathway: In the extrinsic pathway, signal molecules (ligands) which are released by other cells, bind to transmembrane death receptors on the target cell to induce apoptosis.
Death receptors detect the presence of extracellular death signals. When the signals bind with the receptor it ignites apoptosis. Death receptors are made by genes of the tumor necrosis factor (TNF) receptor superfamily. Two cytokines (signals) that bind to the death receptors of animal cells are tissue necrosis factor (TNF) and FasL. FasL is a member of the TNF family. TNF is produced mainly by macrophages but is also produced by lymphoid cells, mast cells, endothelial cells, cardiac monocytes, adipose tissue, fibroblasts and neuronal tissue. FasL is produced almost exclusively by cytotoxic T cells (discussed later).
The Intrinsic Pathway: The intrinsic pathway is triggered by cellular stress that damages the cell membrane or DNA and triggers the release of intracellular apoptic signals. Cell stress is caused by factors such as heat, radiation, nutrient deprivation, viral infection, hypoxia and increased calcium concentration.
Apoptosis is particularly important with cells infected with viruses. If an infected cell dies by lysis, the virus will simply leak out and infect other cells. If the same cell goes through apoptosis the virus will die along with the cell.
The diagram below shows the life cycle of virus that infects a cell. The virus will eventually cause cell rupture and spread viruses to other cells in the body. If a cell infected with a virus dies of a necrosis (lysis, Cell membrane disruption) , the virus will still be released and spread to other cells. However, if the cell dies of apoptosis, the virus will die with the cell. To protect the host, some WBCs are able to kill by apoptosis as well as lysis.
In addition to apoptosis, which is considered the most extreme form of PCD, there are other forms of PCD which include:
Necrosis: Necrosis is cell death not caused by PCD. Death by necrosis is caused by factors external to the cell such as infection, toxins, and trauma. These factors disrupt the cell membrane and ultimately cause the release of contents from the cell by lysis.
The diagram below shows a hole punched into a cell membrane by a complement protein complex (discussed later). This causes a flow of extracellular fluid into the cell which causes the cell to burst open and die. Different external factors can damage the cell membrane and cause lysis.
Phagocytosis is the process of engulfing and neutralizing a pathogen. The pathogen may be a microbe (living) or non-microbe.
The body contains professional phagocytes and non-professional phagocytes. Phagocytosis is the main function of professional phagocytes. Professional phagocytes have receptors on their outer cell membrane that can detect harmful objects (pathogens). They also have receptors for opsonins that help them phagocytose pathogens. Phagocytosis is not the main function of non-professional phagocytes. Non-professional phagocytes do not have receptors for pathogens and limited receptors for opsonins. Most non-professional phagocytes do not produce reactive oxygen containing molecules which are used to destroy pathogens when they are phagocytized. Professional phagocytes include: neutrophils, monocytes, macrophages, dendritic cells and mast cells. Non-professional phagocytes include: lymphocytes, NK cells, epithelial cells, endothelial cells, fibroblasts, red blood cells.
Phagocytes recognize not only microorganisms foreign to the body but also dying self-cells (host cells), which they eliminate by engulfment. This phenomenon prevents dying cells from releasing potentially harmful or immunogenic intracellular materials.
Phagocytes have receptors that will bind directly to the microbe (via PAMPS (pathogen associated molecular patterns) on the outside of the microbe. Phagocytes can also bind to opsonins (complement, antibodies, other) that are bond to the microbe. Either way, the phagocyte then engulfs the icrobe, kills it and breaks it down.
Phagocytes engulf solid particles (microbes, minerals, dead tissue) by their cell membrane which forms an internal phagosome. A phagosome is a vacuole formed by the fusion of the cell membrane around the particle. Ultimately the phagosome fuses with a lysosome. A lysosome is a spherical organelle that contains enzymes and toxins that kill/degrade the contents of the phagosome.
There are two ways the contents of the phagosome are killed/degraded, oxygen-dependent killing and oxygen-independent killing:
Oxygen-dependant killing: When a phagocyte ingests bacteria (or any material), its oxygen consumption increases. This is called a respiratory or oxidative burst, which produces reactive oxygen-containing molecules that are anti-microbial. There are two types of oxygen dependent killing.
The first involves the enzyme NADPH oxidase. NADPH oxidase catalyses the formation of superoxide anions (O2–). The superoxide is converted to hydrogen peroxide (H2O2) and singlet oxygen (O2) by an enzyme called superoxide dismutase. Superoxides (O2–) also react with the hydrogen peroxide to produce hydroxyl radicals (OH•). NADPH oxidsae is also used to produce Nitric Oxide (NO) and Peroxynitrite (ONOO? ) derivatives. NO in high concentration is cytotoxic. Peroxynitrite derivatives are also cytotoxic
The second type of oxygen involves the use of myeloperoxidase. Another phagocytic enzyme, myeloperoxidase, catalyses a reaction between chloride ions (Cl?) and hydrogen peroxide (H2O2) to yield hypochlorite (ClO?). Hypochlorite is extremely toxic to bacteria. Myeloperoxidase is most abundant in neutrophilic granules. Myeloperoxidase contains a heme pigment, which accounts for the green color of secretions rich in neutrophils, such as pus and infected sputum.
Oxygen-independent killing: This is not as effective as the oxygen-dependent killing. There are four main types. The first uses electrically charged proteins which damage the bacterium's membrane. The second type uses lysozymes; these enzymes break down the bacterial cell wall. The third type uses lactoferrins, which are present in neutrophil granules and remove essential iron from bacteria. The fourth type uses proteases and hydrolytic enzymes; these enzymes are used to digest the proteins of destroyed bacteria.
WBCs that cannot phagocytize pathogens release chemicals extracellularly that neutralize the pathogen. Extracellular killing can be done by releasing toxins near the pathogen or may require binding of the WBC to the pathogen first. Whichever of these mechanisms is employed (chemical release or receptor binding then chemical release), the microbe will die of either induced apoptosis or necrosis/lysis.
Antibodies can attach to a pathogen or their toxins and the coated pathogen or toxin can be engulfed by a phagocyte.
In the absence of phagocytosis, apoptotic bodies (membrane sealed fragments of dead and dying cells) may lose their integrity and proceed to secondary or apoptotic necrosis (see picture below). Here, the term apoptotic necrosis describes dead cells that have reached this state via the apoptosis. The presence of necrosis tells us that a cell has died but not necessarily how death occurred.
Apoptotic cell recognition and removal by phagocytes is critical for the restoration and/or maintenance of normal tissue structure and function. Macrophages engulf apoptotic cells before they lyse, thus preventing release into the tissue of potentially toxic and immunogenic intracellular substances. In addition, the binding and/or uptake of apoptotic cells not only fails to induce macrophage secretion of inflammatory mediators, but actually inhibits their pro- inflammatory cytokine production following stimulation.
General points about cell death:
Apoptosis vs. Lysis (necrosis):
Antibody-Dependent Cell mediated Cytotoxicity (ADCC):
Antibody dependent cell mediated cytotoxicity (ADCC). Antibodies bind to organisms. WBCs attach to the antibody and kill these organisms not by phagocytosis but by release of toxic substances.
Cells that kill by ADCC are non-specific for antigen, the specificity of the antibody directs them to specific target cells.
WBCs that kill by phagocytosis (engulfing):
neutrophils, monocytes, macrophages, mast cells, dendritic cells. (Phagocytosis can occur without opsonins but opsonins make phagocytosis easier.)
WBCs that kill by Antibody-dependent cell mediated cytotoxicity (ADCC), extracellular release of toxins :
neutrophils, eosinophils, monocytes, macrophages, NK cells
WBCs that kill by cell mediated cytotoxicity. May be Ab dependent or Ab independent.:
cytotoxic T cells or NK cells.
WBC binding and receptors:
Depending on the leukocyte, it may attach to a pathogen one of 4 ways:
phagocyte has receptor for:
complement attached to antibody
-PAMPs: Pathogen Associated Molecular Patterns (Using Pattern Recognition Receptors (PRR))
Phases of wound healing:
1) hemostasis, 2) inflammation, 3) proliferative, 4) reorganization.
Hemostasis: Hemostasis is the first thing that happens after wounding. Hemostasis lasts 5-10 minutes. The injured blood vessel contracts to decrease blood flow and minimize blood loose. Vasocontraction occurs until a platelet plug is formed. Eventually a fibrin clot replaces the platelet plug. Vasocontraction is due to local neurally regulated reflexes, direct mechanical impact on the smooth muscle cells in the vessel and the release of vasoactive substances from the injured tissue.
Platelet plug and fibrin clot: Platelets are cell fragments found in blood. Under normal (physiologic) conditions platelets flow in the blood in a nonactivated (nonsticky) state. Platelets are derived from bone marrow cells called megakaryocytes.
A megakaryocyte can produce 5-10,00 platelets. Eventually the megakaryocyte is destroyed by macrophages in the lung tissue. The lifespan of a circulating platelet is 5-9 days. In the blood stream platelets remain inactive by chemicals release by endothelial cells that line blood vessels.
The chemicals released by endothelial cells include nitric oxide, endothelial-ADPase and PGI2. These chemicals prevent platelets from becoming activated (sticking together). Beside circulating platelets, a reserve of platelets are stored in the spleen and are released when needed.
When a blood vessel is broken during injury, platelets are exposed to the 1) basement membrane under the endothelial cells (see above diagram of blood vessel), 2) von Willebrand factor (vWF) which helps endothelial cells stick to the collagen in the basement membrane and 3) tissue factor (factor3) also in the basement membrane.
All three of these activate platelets and makes them stick to each other and to the vessel wall to from a platelet plug.
Formation of a platelet plug is called primary hemostasis. Secondary hemostasis occurs when the platelet plug is replaced by a fibrin clot. Fibrin is produced by a clotting cascade. Clotting factors are proteins in the blood plasma. Most blood proteins (not just clotting factors) come from the liver. There are two pathways in the clotting cascade. Ultimately, both pathways split prothrombin (a plasma protein) to thrombin which splits fibrinogen (a plasma protein) to fibrin which cross-links and forms a stronger clot.
chemicals released from platelets:
chemicals for hemostasis -growth factors (released from platelets after the form platelet plug): platelet-derived growth factor (PDGF): a potent chemotactic agent. TGF beta: stimulates deposition of extracellular matrix (ECM) fibroblast growth factor insulin-like growth factor 1 platelet derived epidermal growth factor vascular endothelial growth factor [growth factors stimulate cellular growth, proliferation and cellular differentiation.] - thomboxane-A synthase - Thromboxane-A synthase, an enzyme found in platelets, converts the arachidonic acid derivative prostaglandin H2 to thromboxane. Thromboxane is a vasoconstrictor and a potent hypertensive agent, and it facilitates platele aggregation. It is in homeostatic balance in the circulatory system with prostacyclin, a related compound. The mechanism of secretion of thromboxanes from platelets is still unclear.
Platelet activation is needed to produce thromboxane A2 in the Arachidonic Acid cascade . Thromboxane A2 is a vasoconstrictor and stimulates activation of new platelets as well as increases platelet aggregation.
The next phase of wound healing after vasocontraction and clotting (hemostasis) is inflammation. Inflammation allows the access of blood components and cells to the site of injury. Inflammation is the vasodilation of blood vessels which makes them more leaky and allows transport of substances and cells in and out of the blood circulation and tissue. Inflammation facilitates the removal of 1) damaged cells and tissues and 2) pathogens from the wound site.
Vasodilators of inflammation:
Histamine – Released from damaged mast cells (covered later). Mast cells reside in tissue not in blood. Complement proteins (covered later) also release histamine from mast cells and basophils (covered later). "
Serotonin – from activated platelets (platelets that clumped to form clot)
Bradykinin -formed from precursor floating in the blood (a plasma protein). Production of bradykinin starts with Hageman factor (factorXll) which is activated by contact with damaged vessel wall.
Fibrin split products- Plasminogen (a plasma protein) is converted to plasmin by tissue plasminigen activator (tPA) which is produced by endothelial cells. Plasmin splits fibrin in the clot. Urokinase is found in the blood and in the extracellular matrix of tissue. Urokinase and tPA both convert plasminigen to plasmin.
Complement proteins- Cause histamine release from mast cells. Eicosanoids: Arachidonic Acid (AA) is precursor molecule produced on cell membranes of most tissues. Prostaglandins and Leukotrienes are produced from AA. Prostaglandins are vasodilators. During inflammation neutrophils and macrophages are most important source of prostaglandins and leukotrienes.
Platelet Activating factor (PAF): generated by activated inflammatory cells, endothelial cells, and injured tissue cells. This enhances release of seratonin from platelets.
Nitric Oxide: produced by endothelial cells.
Neuropeptides: produced by nerve endings.
Oxygen derived free radicals: produced by endothelial cells.
constituents of lysosomal granules of phagocytes
Cytokines: Such as IL1 and TNF
After clotting and inflammation, the next phase of wound healing is the proliferative phase followed by the remodeling phase.
Proliferative phase: Before describing the proliferative phase of wound healing we need to discuss the extracellular matrix (ECM) of the body. The ECM is the tissue that supports the cells throughout the body. It also performs other functions in the body as well. The ECM includes the basement membrane and the interstitial matrix (see below).
In the diagram above, the ECM is composed of basement membrane and connective tissue with interstitial matrix.
There are three basic types of connective tissue:
Fibroblasts produce ECM in dense and loose connective tissue. Chondrocytes produce ECM in cartilage. Osteoblasts produce ECM in bone.
The molecular components of the ECM include:
In the case of soft tissue, about 2-3 days after the wound occurs, fibroblasts enter the wound site, marking the onset of the proliferative phase even before the inflammatory phase in the entire wound has ended. Fibroblasts normally reside in the extracellular matrix (ECM) (see above diagram). As the fibroblasts enter the wound, endothelial cells migrate to the wound as well. Fibroblasts and endothelial cells come from the surrounding tissue. Fibroblasts produce extracellular matrix (ECM) and endothelial cells produce new blood vessels. The resulting tissue is called granulation tissue. It is rudimentary tissue that is rapidly produced until the entire wound is covered. Granulation tissue will contain ECM, blood vessels and white blood cells (WBCs) or leukocytes. WBCs will remove damaged tissue and protect the wound from infection. At the end of the granulation phase, fibroblasts begin to die off converting the granulation tissue from an environment rich in cells to one that consists mainly of collagen. Granulation tissue allows for the re-epithelialization phase to take place. Epithelial cells are skin cells that cover the wound. Contraction is the final phase of the proliferative phase. Contraction shrinks the wound.
Remodeling phase: During this phase the collagen becomes more organized and stronger. Type 3 collagen is replaced with stronger type 1 collagen. The shrinking wound finally becomes a scar. The scar becomes about 80% as strong as the normal tissue it replaced. Tissues can either regenerate or scar. Scars (also called cicatrices) are areas of fibrous tissue (fibrosis) that replace normal skin (or other tissue) after injury or disease. A scar results from the biologic process of wound repair in the skin and other tissues of the body. Thus, scarring is a natural part of the healing process. With the exception of very minor lesions, every wound (e.g. after accident, disease, or surgery) results in some degree of scarring. An exception to this is animals with regeneration, which do not form scars and the tissue will grow back exactly as before.
As stated above, inflammation is necessary for proper wound healing. As the wound is healing the immune system is also working. It is during the inflammatory phase of wound healing that the immune system really “kicks in”. White blood cells that remove damaged tissue and pathogens have the greatest access to the injured site during inflammation, when the blood vessels are porous and leaky.
The immune system is divided into:
The innate immune system comprises the cells and mechanisms that defend the host from infection by other organisms, in a non-specific manner. This means that the cells of the innate system recognize and respond to pathogens in a generic way, but unlike the adaptive immune system, it does not confer long-lasting or protective immunity to the host. Innate immune systems provide immediate defense against infection, and are found in all classes of plant and animal life.
The innate immune system includes: 1) surface barriers, 2) inflammation, 3) the complement system, 4) cells.
|Skin||squamous cells, sweat||desquamation, flushing, organic acids|
|GI tract||columnar cells||peristalsis, low ph, bile acid, flushing, thiocyanate|
|Lung||tracheal cilia||Mucociliary elevator, surfactant|
|Mucus, saliva, tears||flushing, lysozyme|
|Surface barriers also include the normal flora of the body which prevents colonization of pathogenic bacteria.|
Surface barriers also include the normal flora of the body which prevents colonization of pathogenic bacteria.
Already discussed above.
The complement system is a group of proteins produced by the liver, tissue macrophages (a white blood cell), blood monocytes (a white blood cell) and epithelial cells (lining cells) of the genitourinal tract and gastrointestinal tract. They are released into the blood stream. Complement proteins are directly or indirectly involved in many processes that fight infection.
Complement proteins have the following basic functions:
The complement system is activated by three pathways, 1) the Classical Pathway, 2) the Alternative Pathway and 3) the Lectin Pathway.
All three pathways produce C3 protein which ultimately leads to direct killing of microbes, phagocytosis and production of inflammatory mediators. The classical and lectin pathways require molecules to bind with pathogens which takes time. Since the alternative pathway does not require this extra binding it can work more quickly. The alternative pathway is activated by plasmin which is already circulating in the blood because of the initial tissue injury.
Complement proteins bind to:
microbes (opsonization for phagocytosis (WBCs have receptors for complement) and directly killing by lysis with MAC (membrane attack complex)) (alternative pathway).
mannose binding lectins (MBL) which are attached to microbe membrane (lysis by MAC (lectin pathway). Mannose binding lectin (MBL) are sugar binding proteins. Lectins are produced endogenously and acquired from our diet. MBLs bind with sugar groups (mannose) on the cells membranes of pathogens (viruses, bacteria,fungi protozoa). This binding causes two things:
-direct phagacytosis by a white blood cell
-binding of complement to MBL which forms the MAC which lyses the cell.
-fc portion of Ag-Ab immune complex (more specifically Ag-IgM or Ag-IgG) (phagocytosis and direct killing with MAC (classical pathway) (from products of above pathways)
-phagocytes and other cells (degranulation for extracellular killing of microbes, release of mediators mast cells (e.g. vasodilation) ) (from products of above pathways)
Anaphylatoxin (fragment C3a,C4a and C5) binds to nonpathogen cells. Each of the 3 proteins is an anaphylatoxin. They cause degranulation of endothelial cells, mast cells and phagocytes, which produce local inflammatory response. So the proteins indirectly cause:
Most of our WBCs are part of the innate immune system. White blood cells (WBCs) are known as leukocytes. WBCs form in bone marrow from hematopoietic stem cells.
Unlike most cells in the body, leukocytes act like single-celled individual organisms. Leukocytes of the innate immune system include:
These cells are involved in dilation of blood vessels, recruitment of other WBCs into the area, destruction of pathogens and removal of dead and damaged cells and tissue.
Major Histocompatibility Proteins:
Before discussing the individual cells of the innate immune system, we need to discuss major histocompatibility proteins. Most cells in the body have a genome (area on their chromosomes) called the major histocompatibility complex (MHC) that produces proteins which are displayed on the outer membrane of the cell. These proteins allow immune cells (WBCs) to recognize the cell as part of the body ("self" antigens). Cells that do not display these MHC proteins are recognized as “non-self” cells and destroyed by the immune system. Cells infected by viruses are missing MHC proteins and are considered "missing self" cells. There are three classes of MHC proteins:
natural killer cells: Natural killer cells (NK cells) do not kill invading microbes. Rather, NK cells destroy compromised host cells, such as tumor cells and virus-infected cells. NK cells come from a lymphoid progenitor cell like T and B cells. NK cells are cytotoxic; small granules in their cytoplasm contain proteins such as perforin and proteases known as granzymes. Granzymes are serine proteases that are released by cytoplasmic granules within cytotoxic T cells and natural killer cells. Their purpose is to induce apoptosis within virus-infected cells, thus destroying them. Perforin is a cytolytic protein found in the granules of CD8 T-cells (cytotoxic T cells) (discussed later) and NK cells. Upon degranulation, perforin inserts itself into the target cell's plasma membrane, forming a pore. Despite recognizing target cells in very different ways, cytotoxic T cells and NK cells both utilize perforins, and a battery of serine proteases as a principal means of inflicting cell death by apoptosis. Perforins can kill the target cell by:
Given their strong cytolytic activity and the potential for auto-reactivity (destroying host cells), NK cell activity is tightly regulated.
To control their cytotoxic activity, NK cells possess two types of surface receptors: activating receptors and inhibitory receptors.
These inhibitory receptors recognize MHC class I protein, which could explain why NK cells kill cells possessing low levels of MHC class I molecules.
mast cells: Reside in connective tissue and mucous membranes. When activated, mast cells release histamine, heparin, hormonal mediators and chemotactic cytokines into the environment. Histamine dilates blood vessels and recruits neutrophils and macrophages. Mast cells are produced in the bone marrow and move into the blood as an immature cell. When they settle in tissue, they mature. Mast cells can be stimulated to degranulate if: 1) they are damaged, 2) complement protein binds to them, or 3) immunoglobulin E (IgE) on their receptors cross-link with antigen (see below).