I. DEFINITION
II. INJURY
III. VASCULAR RESPONSE
IV. CELLULAR RESPONSE
V. MEDIATORS OF INFLAMMATION
VI. CELL GROWTH CONTROL
VII. REPAIR
VIII. INFLAMMATION V. INFECTION
IX. INFLAMMATION GONE AWRY
X. PATTERNS OF INFLAMMATION
OBJECTIVES:
KEY WORDS:
Define these words and describe their function in inflammation and repair.
leukocytes |
myeloperoxidase |
neutrophil |
plasmin |
monocyte and macrophage |
Fibrin split products |
phagocyte |
Hageman factor (Factor XII) |
eosinophil |
leukotrienes |
basophil and mast cell |
thromboxanes |
lymphocyte |
prostacyclins |
exudate |
EGF |
transudate |
PDGF |
serous |
FGF |
pus |
VEGF |
purulent exudate |
IL-1 and TNF |
fibrinous |
TGF-beta |
vasodilation (synonym: vassodilatation) |
granulation tissue |
histamine |
angiogenesis |
bradykinin |
neo-vascularization |
stasis |
fibroblast |
margination |
myofibroblast |
adhesion |
lymphangitis |
selectins |
abrasion |
integrins |
laceration |
diapedesis |
incision |
chemotaxis |
ulcer |
colloid osmotic pressure |
abscess |
Hydrostatic pressure |
scar |
opsonin |
granuloma |
complement |
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Inflammation (in'' flah-ma'shun) [Latin. inflammatio; inflammare; to set on fire]
The inciting injury may be physical (a laceration, a splinter, a crush injury),
thermal (heat or cold) or radiation (X-ray, ultraviolet). Physical and thermal
insults generally initiate inflammation by triggering mast cells to release
histamine. Radiation injury triggers inflammation through vascular damage
leading to leaking vessels. A vascular response and a cellular response
follow. Plasma factors activated at the time of injury initiate the repair
phase however this is not immediately evident. Keep in mind that the idealized
goal of inflammation is to contain and eradicate local injury then initiate
repair of the damage. In reality, the end result of inflammation is usually
repair (complete resolution with mild fibrosis) but harmful side effects
do occur (see below).
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The next requirement for the inflammatory response is to get the inflammatory
cells (leukocytes) to the site of injury. Vascular dilatation increases
the volume of blood to the tissue site but also changes the flow characteristics
within the vessel. The cells are normally contained in the central or axial
part of the blood column. Dilatation increases cross sectional area of the
vessel and decreases the net flow rate per unit area. This causes cells
to fall out of the central region of the vessel; they begin to tumble along
the epithelial surface. Specialized receptors known as cellular adhesion
molecules (selectins, integrins and immunoglobulins) facilitate binding
between endothelial cells and leukocytes. Chemo-attractants (chemotactic
factors) can now do their job. Leukocytes will migrate along chemical gradients
of certain mediators of inflammation (see below). Leaking proteases (e.g.
C5a complement fragment) will attract the leukocytes. If the lesion is infected,
for example with bacteria, this is also a potent chemotactic factor. More
sophisticated chemotactic factors such as cytokines (lymphokines, monokines)
and leukotrienes will sustain the reaction once the inflammatory cells arrive
on the scene.
The cells present (inflammatory infiltrate) in an area of inflammation determine
how we classify it. If the infiltrate is composed of neutrophils and macrophages,
we call it acute inflammation. If the infiltrate is composed of macrophages,
lymphocytes and/or plasma cells, we call it chronic inflammation. Acute
and chronic have some relation to time in that neutrophils will precede
macrophages and lymphocytes in an "ideal" inflammatory response.
However, there are many exceptions. A small skin laceration will have a
chronic infiltrate in 3 to 5 days. A post-traumatic bacterial osteomyelitis
will have an acute infiltrate months after the injury. Acute infiltrates
generally mean that an injurious stimulus is still present.
Usually the first cell on the scene after injury. Recognized by its multi-lobed nucleus (2-4), pale cytoplasm and neutral to basophilic (blue in H&E stains) granules within the cytoplasm. The granules are lysosomes and are of two varieties: specific and azurophilic (blue). The granules contain primarily enzymes and the azurophilic granules contain the extremely important H2O2-myeloperoxidase-halide system (oxygen dependent bacteriocidal mechanism), see figure 3-22, p 72. Neutrophils degenerate after 24-48 hours in tissue.
These cells are more "durable" than neutrophils. They arrive on the scene a little later and last longer. Eosinophils may be thought of as a marker of sub-acute inflammation. They are also present in allergic reactions and reactions to parasites. Recognized by its bi-lobed nucleus and orange-red (eosinophilic) granules within the cytoplasm.
Usually a marker of chronic inflammation (exception: viral infections). Recognized by its single nucleus and scant cytoplasmic rim. The nucleus is approximately the size of a red blood cell in tissue section. There are several varieties of lymphocytes to be discussed further in the section on immunopathology (Path 301) and hematopathology (Path 302). They are B-cell lymphocytes, T-cell lymphocytes and non-B, non-T lymphocytes (natural killer, etc). B-lymphocytes differentiate to plasma cells and produce antibodies in response to antigenic stimulation (humoral immunity). T-lymphocytes produce cytokines (cyto=cell, kine=kinetic) or mediators in response to antigenic or chemical stimulation. T-cells are major players in cellular immunity.
These are large, mononuclear phagocytic cells; hence the term macrophage. When a macrophage is in tissue it has traditionally been called a histiocyte and the terms are used synonymously. Recognized by its single, large nucleus and abundant cytoplasm, sometimes containing dark staining granular debris. Macrophages are derived from blood monocytes which have left the circulatory system. They are early recruits in the inflammatory response and constitute the majority of cells by 48 hours. Macrophages may fuse to form giant cells in response to indigestible micro-organisms or foreign material, see page 82.
These are terminally differentiated B-lymphocytes which are producing antibody. They are seen later in the inflammatory response (chronic inflammation) and indicate that an antigenic stimulus is present in the injury. Recognized by their single nucleus with a "clock face" (peripherally clumped chromatin) and eccentrically distributed amphophilic (purple) cytoplasm with a peri-nuclear clear zone (golgi apparatus).
Neo-vascularization of an idealized tissue defect occurs approximately 3 days into the inflammatory response (repair stage). This vascularization is a primary component of granulation tissue and involves angiogenesis. Endothelial cells are recognized by their spindle shape with dense cytoplasm and oval nuclei. They are generally arranged around small caliber, irregular lumens.
These cells manufacture collagen. Recognized by their spindle shape nuclei and eosinophilic, filamentous cytoplasm. When stimulated, fibroblasts not only make collagen, they take on properties similar to smooth muscle cells and are called myofibroblasts.
This is a rapidly evolving area in the field of experimental pathology.
We will cover this in detail only where it is directly applicable to classic
acute and chronic inflammation and repair (pp 64-72 and pp 40-41). These
two sections are built upon later in the book and you will want to review
them and refer back to them, but for now, concentrate on the effects mediators
have on the inflammatory process. The tables and summaries are helpful on
pp 73-74. Here is a short summary.
Prostaglandins and leukotrienes (including thromboxane)-- these "local, short-range hormones" (autocoids) act primarily as effectors of vascular permeability, vascular constriction or leukocyte chemotaxis. See table 3-3 and figure 3-18 on page 69 for a summary.
These polypeptide products were initially described in immunologic responses but have since been shown to play a role in the inflammatory response. Figure 3-20 on page 70 gives a good summary. The two principle players in inflammation are IL-1 (interleukin-1) and TNF (tumor necrosis factor). Among other things, they also stimulate both collagen and collagenase production by fibroblasts. The cytokines play important roles in neoplasia as well via autocrine (act on same cell), paracrine (act on cell in proximity) and endocrine (act on distant cell) effects.
Nitric oxide synthetase (NOS) produces nitric oxide (NO) from L-arginine in the presence of oxygen and NADPH. It is a free radical produced by endothelial cells and macrophages. It causes smooth muscle relaxation in the vessel wall (vasodilation) and reduces platelet activation and aggregation. In macrophages, the free radical nature of NO is toxic to microbes.
The major growth factors of importance are EGF (epidermal growth factor), PDGF (platelet derived growth factor), FGF (fibroblast growth factor), VEGF (vascular endothelial growth factor), TGFbeta (transforming growth factor, a growth inhibitor), and the cytokines (TNF and IL-1), see pages 79-80, 40-41, table 2-1 and table 3-10. These mediators are involved in the proliferation and production of vessels and collagen (granulation tissue) during the repair phase of inflammation.
Cells are dividing, quiescent or non-dividing. Non-dividing cells are nerves, skeletal muscle and cardiac muscle. These cells cannot be recruited into the cell division cycle. If there is injury to these cells, other cells must take over their function. The brain can be "re-wired" to some extent after injury and muscle cells can hypertrophy. Examples of stable (quiescent) cells can be found in almost all the glandular organs of the body. If needed, and under the proper stimuli, these cells can be recruited into the cell cycle. Examles of continuously dividing cells include the epithelial lining of the digestive tract and squamous cells of skin. The fisrt step in the cell cycle is G1, or gap 1. The cell is busy producing all the necessary proteins, enzymes and molecules to replicate the DNA. When this is complete, the cell enters the synthetic phase (S-phase or S). The DNA is replicated. At the beginning of S-phase, the cell has two copies (2C) of DNA (maternal and paternal derived). At the end of S-phase, the cell has four copies of DNA (4C). When DNA synthesis is complete, the cell must degrade the excess enzymes and proteins produced in G1 and make the necessary proteins and enzymes for mitosis, this is called G2, or gap 2. The DNA content is 4C during G2. When G2 is complete, cell division, or mitosis (M) occurs and two daughter cells result. Quiescient cells are said to be in a resting state, or G0.
The surfaces of cells have receptors that bind growth factors that are usually polypeptides. Polypeptides binding to receptors is generically called ligand-receptor binding. Some growth factors prepare a G0 or G1 cell for DNA synthesis; these are called competence factors. Other growth factors initiate the DNA synthesis process after a cell has been rendered competent; these are called progression factors. The prototype sequence of events for initiation of cell growth is as follows: the growth factor binds with a surface receptor; this causes a conformational change in the receptor, usually dimerization of two receptor molecules; this leads to phosphorylation of a tyrosine kinase on the inner surface of the cell membrane (growth receptors span the cell membrane and have extracellular and intracellular regions or domains); the kinase can then "activate" a protein within the cell by enzymatic cleavage; the activated protein can then act on the nucleus or the nuclear membrane to initiate DNA synthesis. The activation of intracellular proteins is called signal transduction and the activated proteins are called second messengers. A summary of this process can be found in figure 2-3, page 38. Important enzymes in this process are phospholipase C, G-proteins, raf-1 and the ras proteins. The precise mechanisms by which DNA synthesis is initiated are not known, but the ras proteins play a central role by activating MAP kinases. Eventually, growth regulating genes such as fos, jun and myc are activated (they code DNA transcription factors). A group of proteins known as cyclins and enzymes known as cdc kinases are involved in regulation of the cell cycle after the transcription factor genes have been activated.
The vascular response triggers the cellular response. The cellular response
is primarily responsible for neutralizing the injury or injurious agent
then cleaning up the debris. Once this is accomplished, there will be a
tissue defect that needs to be reconstituted. The common themes to repair
are generation of new blood vessels at the periphery of the injury (angiogenesis)
and secretion of collagen by stimulated fibroblasts. The combination of
abundant new blood vessels and immature collagen admixed with plump fibroblasts
(myofibroblasts) is called granulation tissue. Granulation tissue will fill
the defect and is the common pathway in repair. Eventually, many of the
blood vessels regress and all that is left is collagen. The excess collagen
is recognized as a "scar" or "scar tissue". The human
body has a limited capability for regeneration and the amount of excess
collagen depends on the extent of tissue damage, the regenerative capacity
of the injured tissue as well as an individuals tendency to produce excess
collagen.
The repair process can be dichotomously divided into replacement of damaged
tissues and regeneration of damaged tissues. Replacement has been described
in the preceeding paragraph--the damaged tissue is replaced by collagen:
a scar. Regeneration is when the damage is repaired with no scarring--the
regenerated tissue is indistinguishable from the original tissue. What determines
whether an injury will be repaired by regeneration or replacement? It appears
that if basement membranes and the underlying extracellular matrix are intact,
regeneration occurs. The more damage to the basement membrane and extracellular
matrix, the more replacement by collagen occurs. In most injuries, there
is a combination of regeneration and replacement.
Inflammation is the generalized response to injury. When inflammation is
complicated by microbes (bacteria, viruses, fungi, protozoa, helminths)
we call it infection. In fact, most of the "inflammation" we all
commonly observe is infection. Examples include the common cold, typical
skin "cuts" which are always secondarily contaminated by skin
flora and gastroenteritis (viral or bacterial). Our bodies have a broad
range of defenses against microbes including physical barriers such as the
skin itself or the mucus covering of non-squamous epithelia. Viral defenses
are in the realm of immunology and you are covering this in other courses.
Here we will briefly review bacterial defense mechanisms.
This is accomplished by neutrophils and macrophages. The philosophy is surround, entrap and kill. Bacteria are phagocytosed and the phagosome fuses with a lysosome to form a phago-lysosome. Killing can take place through oxygen dependent and oxygen independent mechanisms. The oxygen dependent mechanisms are more toxic and efficient. The first oxygen dependent mechanism is enzymatic production of oxygen free radicals (O2-) via oxidase. The second is enzymatic production of hydroxide-halide (HOCl.) radicals via myeloperoxidase and the third is production of hydrogen peroxide (H2O2) via dismutase. Oxygen independent mechanisms include secretion of hydrolytic enzymes. See page 72-73. When bacteria are inefficiently killed or not killed, granulomas may result.
The intentions of the engineer who designed the inflammatory response system were certainly benevolent; however, the consequences of inflammation are not always predictable nor desirable. When neutrophils and macrophages are digesting bacteria some of the free radicals and hydrolytic/proteolytic enzymes may leak from the phagosome causing local destruction of tissue and cells (e.g. pulmonary emphysema, osteoarthritis). Recruitment of inflammatory cells and secretion of growth factors may cause over-exuberant granulation tissue with subsequent scarring. The inflammatory system may also be an unwitting accessory in processes such as fistula formation (e.g. inflammatory bowel disease), atherosclerotic plaques, autoimmune diseases, immune hypersensitivity responses (e.g. anaphylaxis, allergies), etc.
(lab sessions and Dr. Winn)
It is important to understand that there exists a continuum between acute
and chronic inflammation and that the distinction between the two may seem
arbitrary in some cases despite the seemingly straight forward definitions
of acute and chronic inflammation given above. The inflammatory response
may be proceeding at different rates in different areas of the same histologic
section; this is important to recognize as you study the slides. You may
appreciate different stages of inflammation in a lesion but should also
realize that the overall process can be classified as primarily acute or
primarily chronic. The "vascular" and "cellular" response
in inflammation are primarily exudative (cells and fluids leaking from vessels
and accumulating in interstitium), while the "repair" process
is primarily proliferative (fibroblasts, neo-vascularization). The "repair"
process frequently overlaps the late stages of the cellular response (chronic
infiltrate) and some authors (including your text) consider the repair process
as part of chronic inflammation when in reality, "repair" is the
end result of all aspects of the vascular and cellular response.
A word about cells in tissue sections...
It is often very difficult to point to a cell and answer the question "what
type of cell is this?" Cells in tissue sections are often distorted
and not definitively classifiable. It is much better to look for cells you
can classify and then ask "what types of cells are present in this
process?" and "how does this help me classify the process?"
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Questions?
Comments? Send a message to the CATS guru: jkessler@salus.uvm.edu
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