Disease and Defense
Francisco G. La Rosa, MD
February 5th, 2007
CELL AND TISSUE INJURY
Reading – Recommended: Chapter 1 - "Cell Injury, Adaptation and Death", pp. 3-31 in
Kumar et al Basic Pathology, 7th Edition.
Learning Objectives:
1. Understand major causes (etiologies) of cell injury?
2. Understand how cell injury contributes to the pathogenesis of disease?
3. Be able to describe major mechanisms of cell injury.
4. How can the study of morphologic change caused by cell injury explain the whys and
wherefores of signs and symptoms of disease?
5. What are free radicals, how do they arise, and how do they produce cell injury?
6. Understand how ischemia/hypoxia creates a setting where free radical damage
becomes an important cause of cell injury.
7. What are examples of free radicals and how does the body get rid of them.
8. Understand how necrosis differs from apoptosis.
9. Understand how chronic injury leads to adaptation.
10. Be able to describe the major alterations in the cell membrane, mitochondrion and
nucleus that occur during cell injury.
11. Understand the four major types of necrosis seen in human disease.
12. Know which morphologic and biochemical alterations during hypoxic injury are
reversible and which are irreversible.
I.
Introduction
A.
Principle #1. All human disease stems from some form of cell and tissue
injury. The study of pathology (the study and diagnosis of disease, and study
of mechanisms of disease) is based upon the concept that every human
disease results from injury or death of the fundamental unit of the body, the
cell. The term “injury” refers to non-lethal, physical damage or alteration from
normal of one or more components of the structure of the cell. The damage
invariably perturbs normal physiology. The injured cell can not function at full
capacity—metabolize nutrients, synthesize needed products, and illness
results. Injury can occur “acutely”-producing effects in cells within seconds or
minutes, or “chronically”-resulting in cell stress and damage that can persist
days, months, or even years. [In practice, pathologists classify diseases as
acute or chronic based on the type of inflammatory cells (PMNs, acute;
lymphocytes/macrophages, chronic) identified in the injured tissue.]
B.
Principle #2. Although any cell and tissue in the body can be injured, most
human disease occurs from injury to epithelium. Re-call from histology that
the human body contains >200 different cell types. These arise by
differentiation from totipotent/multipotent stem cells during embryogenesis.
The different types of cells are organized into four basic tissues: epithelium
(surface/internal), muscle (cardiac, smooth, skeletal), nerve (CNS, PNS), and
connective tissue (bone, joint, fat, blood, bone marrow, lymph glands, etc).
The epithelium is the tissue that first encounters injurious agents and stimuli
from the environment, and it is not surprising that many important human
diseases occur from epithelial injury-the major killer of adults, atherosclerosis
occurs from injury to the epithelial cells (the so-called endothelium) that line
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arteries and >90% of all adult cancers, as another example, arise from
epithelia). On the other hand, there are some very important human
diseases that occur from injury to other tissue types-arthritis,
leukemia/lymphoma, AIDS, lupus, etc.
C.
Principle #3. Injury to one tissue invariably affects the other types of tissue
that are adjacent. For example, destruction of the stomach epithelium by
helicobacter pylori infection (i.e., ulceration of the stomach lining), often leads
to damage of the underlying connective tissue of the submucosa and smooth
muscle of the muscularis propria, with intense inflammation, destruction of
cells and scarring.
D.
Principle #4. Cell/tissue injury produces “morphologic” change.
While cell and tissue injury leads to 1) disease symptoms (complaints voiced
by the patient), and 2) disease signs (abnormal findings observed by the
physician), it almost always produces a characteristic change in the
appearance of the affected tissue that can be seen “grossly” (i.e. visibly by
the naked eye) and/or microscopically. It is the identification of this
“morphologic” change in the affected tissue that allows the pathologist to
diagnose the disease process.
Morphologic change is seen readily for example following several types of
injury to the liver. Recall that the liver is constructed of millions of hexagonal
lobules that are each tethered by portal tracts (each containing branches of
the portal vein, hepatic artery, and bile ducts). Blood from the portal vein and
hepatic artery filters from the portal tract along the hepatic sinusoids towards
the center of the lobule and drains into the central vein. In the normal liver,
this lobular architecture is seen easily, the hepatocytes lining the sinusoids
appear cuboidal, uniform in shape, typically showing a round, centrally placed
nucleus surrounded by abundant eosinophilic cytoplasm. Normally, there are
no inflammatory cells, such as lymphocytes or PMNs. Following injury, we
can identify specific morphologic changes microscopically. In alcoholic
hepatitis (inflammation “-itis” in the liver caused by alcohol consumption), we
see swollen hepatocytes, death of occasional hepatocytes, an infiltrate of
PMNs, and a ropy, eosinophilic material within the cytoplasm of some
hepatocytes called “alcoholic hyaline” (representing aggregates
of
cytokeratin filaments).
These histologic changes are usually so
characteristic, the pathologists can report a diagnosis of alcoholic hepatitisand rule out other causes for the patient’s liver problems. In other cases, the
microscopic exam may only show large accumulations of fat (lipid) within
hepatocytes-a morphologic change called “fatty change”. This change is less
specific and many potential causes need to be considered (drugs, ethanol,
starvation, pregnancy, etc). In summary, the study of the morphologic and
macromolecular change induced by injury allows the pathologist to classify
and diagnose disease, and to study pathogenesis.
II.
Outcomes and consequences of injury to the cell/tissue.
A.
The outcome following an injurious insult depends upon several obvious
factors, including the type, severity, and duration of the injury, and the type of
cell being injured (a skeletal muscle cell can survive many hours without
oxygen, while a cardiac myocyte and neuron die after only a few minutes).
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The ability of a tissue to survive also is a function of the tissue’s ability to
undergo repair/replacement. Some tissues undergo continual proliferation
(e.g. GI/bone). This facilitates replacement of irreversibly damaged cells.
Others can be induced to undergo new cell proliferation when necessary (eg
liver). Yet other tissues (eg heart/brain) contain cells that are “post-mitotic”
and presumably “never” divide. This prevents replacement of irreversibly
damaged cells and such cells are effectively lost forever. In addition, it
follows that the ability of the cell/tissue to survive injury also depends upon
whether the blood supply to the tissue is compromised, and whether the
population of stem cells present in the tissue had survived.
B.
C.
Early, commonly seen changes seen in injured cells. Although different
injurious agents may damage the cell in their own specific ways, we often see
common patterns of injury with a wide variety of different agents. This is
because some compartments of the cell are especially vulnerable. When
these structural compartments are damaged, it is not surprising that changes
in the cell’s morphology develop.
1.
Cell membranes. The cell membrane is perhaps the most important
target for both reversible and irreversible injury. The outer cell
membrane because it surrounds the cell and directly interacts with the
environment is usually the first cellular component to be damaged. In
addition, the lipid within the membrane is easily oxidized and supports
an oxidative chain reaction called lipid peroxidation. Damage to the
membrane may physically break the membrane or inactivate the ion
pumps that control the ionic concentrations in the cytoplasm. It is
therefore not surprising that cell swelling is a morphologic change
commonly seen in nearly all types of injury. Recall that in the normal
cell there are impressive Na+, K+ and Ca++ concentration gradients
across the membrane that the cell requires. Outside the cell, the Ca++
is approximately 10-3M while within the cytoplasm, the Ca++ is
approximately 10-7M. In the injured cell, the accumulation of Na+
leads to an increase in H20 and cell swelling.
2.
Mitochondria. Mitochondrial swelling, due to the accumulation of H20
in the matrix compartment, is a morphologic change that occurs very
soon after many types of cell injury, especially in those cases where
the supply of oxygen to the cell is interrupted. This swelling results
from a decrease in the 02- dependent synthesis of ATP required to
fuel the ion pumps of the mitochondrial membrane.
3.
Endoplastic reticulum. In many types of acute injury, the cisternae
of the endoplasmic reticulum are also distended and the
polyribosomes detached from the rough ER. This causes a decrease
in the ability of the cell to synthesize new protein.
4.
Nucleus. In most types of reversible injury, there are alterations in
the appearance of the nucleolus. These changes are not well
characterized but there is probably some effect on the synthesis of
rRNAs, again causing a decrease in protein synthesis.
Complete recovery from injury. The cell swelling, and mitochondrial and
ER swelling and dysfunction just mentioned are all potentially reversible, if
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the injurious stimulus is removed. Cells have a remarkable ability to repair
themselves. Damaged organelles can be degraded via autophagosomes,
damaged membranes replenished and repaired via new membrane and lipid
synthesis, denatured proteins removed via ubiquitination and proteasome
degradation, and damage to DNA chromatin repaired via a large assortment
of DNA repair enzymes. Once membranes are restored, intracellular ion
concentrations return to normal and cell and organelle swelling abates.
D.
E.
Adaptation to the injury. Some forms of cell/tissue injury and stress while
not lethal, persist for long periods of time. In order to maintain tissue
“homeostasis” the tissue adapts. Four classic types of adaptation are
hypertrophy, atrophy, metaplasia, and hyperplasia.
1.
Hypertrophy - an increase in the size of the cell secondary to an
increase in cell function. There is typically an increase in the number
of mitochondria and ER, etc. Example: enlargement (hypertrophy) of
the left ventricle secondary to severe, longstanding hypertension.
With hypertension, each myocyte works harder and this causes the
cell to produce more organelles.
2.
Atrophy - decrease in the size and functional capacity of the cell.
Example: shrinkage (atrophy) of skeletal muscles following motor
neuron loss from infection by poliovirus.
3.
Metaplasia - replacement of one type of tissue with another in
response to an injury. Example: chronic reflux esophagitis leads to
replacement of the stratified squamous epithelium along the distal
esophagus by a columnar type of intestinal epithelium.
Or
replacement of the pseudostratified columnar epithelium of the
bronchus by stratified squamous epithelium in response to thermal
injury from tobacco smoke.
4.
Hyperplasia - an increase in the number of cells of a tissue in
response to a stimulus or injury. Example: increase in the number of
adrenal cortical cells secondary to a tumor that produces an ACTHlike polypeptide.
Cell death (necrosis). Cell/tissue does not survive the injury or adapt: the
cell/tissue dies. There are two basic types of cell death: necrosis and
apoptosis.
1.
Classic cell/tissue necrosis. This is the type of cell death usually
seen following ischemia. Ischemia is hypoxic injury (meaning injury
from too little oxygen) caused by a problem with the vascular blood
flow to the tissue. Cell death from ischemia is called “ischemic (or
coagulative)” necrosis (see below). Typically in ischemic necrosis,
large portions of tissue, containing thousands of contiguous cells, die
all at once. The transition from reversible to irreversible hypoxic injury
is still poorly defined. Probably reflects progression to critical changes
in the cell membrane that cannot be reversed, associated with an
elevation in the intracellular concentration of Ca+2, shut down of
mitochondrial ATP synthesis, release of lysosomal hydrolases. Ca+2
leaks across the plasma membrane into the cell and is released
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internally from storage depots of Ca+2 in the ER and mitochondria.
Once this occurs, Ca+2-dependent proteases and lipases become
activated; the mitochondrial membrane permeability transition pore
(MTP) is opened, with a loss in the ability to make ATP. In this
“classic” type of cell necrosis, the cytoplasm of the dead cell is
swollen, the mitochondria and ER appear dilated, there is prominent
blebbing of the plasma membrane, and loss of membrane integrity.
This type of cell necrosis occurs from ischemia/anoxia (see hypoxic
cell injury below), and from exposure to toxins/chemicals. Analysis of
the chromosomal DNA from the necrotic cell by agarose gel
electrophoresis reveals a nonspecific degradative smear of DNA
fragments.
2.
III.
Apoptosis. There is a second type of cell death-apoptosis or
programmed cell death. Apoptosis, unlike necrosis, tends to affect
scattered, individual cells rather than a large area of tissue. In
contrast to classic cell necrosis, apoptosis is highly regulated,
switched on by the binding of extracellular ligands to specific cell
surface receptors (eg Fas ligand and its receptor). Also important are
signals coming from the mitochondrion-(cytochrome c/apoptosis
inducing factor AIF) that activate caspases (a class of protease) and
an apoptotic-specific nuclear DNA endonuclease, etc.
In the
apoptotic type of dead cell, the cytoplasm is not swollen but shrunken,
there are large plasma membrane blebs, and the nuclear DNA appear
uniformly compacted and very dense. Eventually, the cell breaks up
into small membrane bound vesicles. These are taken up by
macrophages. Analysis of the chromosomal DNA by agarose gel
electrophoresis reveals a degradative pattern that reflects
internucleosomal (nucleosomal ladder) breakage of the DNA.
Morphology of "necrosis". The recognition of dead cells by light microscopy can
provide critical information for the pathologist and is often essential in order to make
certain diagnosis. Over the years, pathologists have learned to identify four classic
types of necrosis.
A.
Coagulative necrosis - dead cell remains a ghost-like remnant of its former
self.
Classically seen in the heart following a myocardial infarction
("infarction" means "necrosis" secondary to vascular insufficiency). The
necrotic myocyte has a cytoplasm which is more eosinophilic than normal.
The nucleus shrinks and the chromatin becomes deeply basophilic as it
clumps. This pattern of chromatin staining is called "pyknosis". The pyknotic
nucleus may then fragment, an appearance known as "karyorrhexis".
Eventually, the pyknotic clumps are broken down and disappear - a process
called "karyolysis".
B.
Liquefactive necrosis - dead cell dissolves away as lysosomal hydrolases
digest cellular components. Commonly seen in the brain and spleen, and
with acute infection.
C.
Caseous necrosis - seen only in tuberculosis. The central portion of an
infected lymph node is necrotic (attributed to toxic affects of mycobacteria)
and has a chalky white appearance, not unlike the milk protein casein.
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D.
IV.
V.
Fat necrosis - refers to necrotic adipose tissue typically following acute
pancreatitis or trauma. In fat necrosis, fats are hydrolyzed into free fatty
acids which precipitate with Ca++ producing a peculiar chalky gray material
characteristic of "fat necrosis".
Causes of cell and tissue injury:
A.
Physical agents - trauma, heat, etc.
B.
Chemical and drugs - drug toxicity, poisoning, etc.
C.
Infection - pathogenic bacteria, virus, fungi, protozoa, etc.
D.
Immunologic insults - anaphylaxis, autoimmunity, etc.
E.
Genetic derangement-phenylketonuria, cystic fibrosis, etc.
F.
Nutritional imbalance - atherosclerosis, protein and vitamin deficiency, etc.
G.
Hypoxia - cells receive too little 02. Multiple causes - primary lung disease,
heart failure, shock, arterial or venous thrombosis, etc.
Hypoxia leads to ischemic injury, probably the single must important type of injury
seen in clinical medicine. Different cells vary in the ability to tolerate hypoxia.
Neurons can tolerate only 3 to 5 minutes of hypoxia while fat cells and skeletal
muscle cells survive many hours.
A.
B.




Reversible changes:
1)
2)
3)
4)
ATP
Na pump (cell swelling)
glycolysis,  pH
protein synthesis
Irreversible changes:
1) activation of lysosomal enzymes
2) DNA, protein degradation
3)  Ca 2+ influx
Hypoxia - some of the injury simply stems from the cells inability to make
sufficient ATP that is required to maintain viability. A second type of injury is
as important. This injury stems from the production of oxygen radicals that
follow 02 therapy, acute inflammation and reperfusion of hypoxic tissue.
1.
02 therapy - high levels of 02 are needed acutely to keep the patient
alive. However, high levels of 02 radicals are also produced and have
toxic effects on cells - especially in the lung.
2.
Acute inflammation - PMNs have enzymes such as
myeloperoxidase which produce oxygen radicals. Many hypoxic
tissues are infiltrated with PMNs.
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3.
C.
Reperfusion. In hypoxia, xanthine dehydrogenase is proteolytically
converted to xanthine oxidase. Once the hypoxia is corrected, the
xanthine oxidase produces activated oxygen species.
Oxygen free radicals. At present, there is a lot of interest in how these
radicals are formed and how the cell protects itself from this injury.
A free radical is a chemical species with an unpaired election. Important
species include 02, and 0H. These free radicals can chemically damage
proteins, DNA, RNA and trigger lipid peroxidation in cell membranes. Free
radicals are generated by intrinsic oxidases (present in the ER of all cells and
in PMNs) and radiation especially in the setting of high p02.
e.g.
02

oxidase
02 (superoxide)
Superoxide can be removed by superoxide dismutase (SOD)
202
2H+  H202 + 02
However, if catalase is not sufficiently active the H202 can be converted to highly
reactive 0H (the hydroxyl radical) via:
1)
ionizing
radiation
H202 
0H + 0H
2)
Fe++ + H202  Fe+++ + 0H + 0H (Fenton reaction)
3)
H202 + 02
 0H + 0H + 02 (Haber-Weiss reaction)
Fortunately, antioxidants (uric acid, vitamin E, etc.) catalase, and glutathione
peroxidase serve to eliminate these radicals.
catalase
 02 + 2 H20
a)
2 H202
b)
2 0H + 2 GSH

2 H20 + GSSG
glutathione
peroxidase
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Important Causes of Cell & Tissue Injury
I.
Injury from temperature extremes. Humans must maintain a temperature no lower
than about 30C, nor higher than about 42C.
A.
Burns. Burns cause about 5000 deaths annually in the U.S. Mortality and
morbidity from a burn injury depend upon:
1.
total surface area affected.
2.
depth of burn injury. In a “partial thickness” burn, the dermis and
dermal appendages survive. Partial thickness burns typically show
blistering. In a “full thickness” burn, there is total destruction of the
epidermis and dermis.
3.
whether there is thermal injury to the lungs.
4.
effective treatment.
Complications of serious burns include:
a.
neurogenic shock and large losses of fluid.
b.
infection-especially with Pseudomonas aeruginosa and Staph.
Infection of the burn can lead to endocarditis,
sepsis, septic shock, and renal failure.
c.
hypermetabolic state - the metabolic rate may double with
severe burn injury.
d.
anemia-bone marrow production is suppressed.
B.
Hyperthermia -elevated body temperature.
1.
Exertional heat stroke - (i.e. marathon runners): hot, dry skin, usually
(not always) there is cessation of sweating. Usually lactic acidosis.
May lead to rhabdomyolysis (breakdown of skeletal muscle fibers),
necrosis of renal tubules (ATN), widespread intravascular coagulation
(DIC), multi organ failure.
2.
C.
Classic heat stroke - young, elderly, obese in hot humid weather.
Hot, dry skin. No lactic acidosis, but respiratory alkalosis.
Hypotension, coma. ATN and DIC are very uncommon.
Hypothermia. Injury from prolonged exposure to the cold. Slowing of
metabolic processes, especially in the brain, may lead to coma and death.
1.
Freezing of cells and tissues - results in an increase in local
concentrations of salt as intracellular water crystallizes. Proteins may
denature, cell organelles may be injured.
2.
may cause a decrease in blood flow form vasoconstriction and from
an increase in blood viscosity. Organ damage, loss of digits, toes,
etc. become evident once blood flow is restored.
II.
Electrical injury. In addition to burn injury, the exposure to electric current may
cause sudden disruption of neural impulses and lead to cardiac arrest. Outcome is a
function of tissue conductance, the amount of heat generated, and the intensity of the
current. Dry skin is fairly resistant. However, wet skin is a good conductor. Exposure of
wet skin to even household levels of current (120 or 220 Volts) can trigger ventricular
fibrillation. Exposure to AC (alternating current, which is used in the US) can cause tetany
and prolong the contact with the electrical source. Spasm of chest wall muscles can cause
asphyxia.