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Introduction to histopathology
S210_3 Developing your health science practice
Introduction to histopathology
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Introduction to histopathology
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Introduction to histopathology
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Introduction to histopathology
Contents

Introduction

Learning outcomes

1 Pathological processes - inflammation and infection
 1.1 Infection


1.2 Acute inflammation

1.3 Chronic inflammation

1.4 Autoimmunity

1.5 Hypersensitivity

1.6 Scarring and fibrosis

1.7 Wound healing, angiogenesis and tissue regeneration
2 Pathological processes - neoplasia
 2.1 Hyperplasia, dysplasia and neoplasia



2.2 Metastasis
3 Pathological processes - cell death
 3.1 Apoptosis and necrosis

3.2 Thrombosis and embolism

3.3 Degenerative diseases and storage disorders
4 Photography and reporting
 4.1 Image acquisition

4.2 Magnification and scale bars

Conclusion

Keep on learning

Acknowledgements
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Introduction to histopathology
Introduction
Histopathology, the study of tissues affected by disease, can be very useful in making
a diagnosis and in determining the severity and progression of a disease.
Understanding the normal structure and function of different tissues is essential for
interpreting the changes that occur during disease. This course introduces the basic
principles that apply to the preparation of microscope sections. It also shows how to
identify a number of human tissues and interpret the changes that occur in disease.
This OpenLearn course provides a sample of level 1 study in Science
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Introduction to histopathology
Learning outcomes
After studying this course, you should be able to:



define all the terms given in bold
outline key features of a number of pathological processes
relate the histological appearance of affected tissues to the underlying
pathology
 recognise the histological appearance of a number of pathological
tissues
 understand how sections can be photographed, presented and reported.
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Introduction to histopathology
1 Pathological processes - inflammation and
infection
Histological examination of tissues can help diagnose disease, because each condition
produces a characteristic set of changes in the tissue structure. There are such a wide
variety of diseases that histology alone usually cannot produce a diagnosis, although
in some cases the histological appearance is definitive. For example, a pathologist
might see signs of a viral infection in the brain, because of tissue damage and
inflammation, but would be unable to tell what virus is responsible; to identify the
virus might require immunohistochemistry (IHC) for the viral protein or more likely,
the diagnosis would be confirmed by the symptoms or serology. Conversely, the
appearance of 'owl-eye' cells in the brain is diagnostic of a particular type of measles
infection (Figure 1). Normally histopathology reports only form one part of the
disease picture that the clinician is assembling.
Figure 1 Owl-eye bodies in infected neurons of a child with subacute sclerosing panencephalitis (SSPE) are
characteristic of this type of viral infection, which is produced by a variant of the measles virus.
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Although diseases are very diverse, the responses made by the body are more limited
and fall into specific categories. For example inflammation, may be seen in response
to an infection or as a result of physical damage or as part of an autoimmune disease,
where the immune system attacks components of the body. The following sections
outline some of the more common pathological processes and relate them to examples
which can be seen in the virtual microscope.
1.1 Infection
Infection can affect any tissue of the body, producing cell damage and inflammatory
reactions. Viruses are generally too small to be seen in the light microscope, but their
presence can often be inferred by the changes they produce in tissue, even if their
identity requires confirmation by immunohistochemistry, serology or molecular
biology. Bacteria can be seen in the light microscope using high magnification
objective lenses; however the numbers of bacteria that are present in a tissue can be
highly variable even in one disease. A classic example of this variability is leprosy,
where there may be very large numbers of bacteria in the skin (lepromatous leprosy),
or very few (tuberculoid leprosy). Distinguishing the type of bacteria in a thin section
of a lesion generally requires specialised histological stains, although the morphology
of the bacteria may also be informative (Figure 2). As with viral infection, the
histological findings are an adjunct to serology and microbiology in producing a
diagnosis.
Figure 2 Gas gangrene in muscle - the micrograph shows a colony of bacteria, stained with haematoxylin.
The bacteria have the characteristic shape and growth pattern of Clostridia. Scale bar = 20µm.
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SAQ 1
(a) What stain could you use to identify M. tuberculosis in a section of lung? (b) What
stain could you use to identify N. gonhorrea in a urethral smear?
View answer - SAQ 1
Identification of parasites is often difficult by serological methods; however, the
appearance of parasite-infected cells (e.g. malaria) or the parasites themselves is
absolutely characteristic of the particular infection (Figure 3). Consequently,
diagnosis of parasitic infections relies substantially on the initial histological or
haematological findings.
Figure 3 A blood smear from a patient with a malarial infection. Several of the erythrocytes are infected, and
the appearance is typical of the schizont phase of infection. Scale bar = 50µm.
1.2 Acute inflammation
Inflammation is a common response to tissue injury or infection. Acute
inflammation develops quickly and resolves within days, whereas chronic
inflammation can last for months or years, usually because of the persistence of the
initiating factor. The histological appearance of acute inflammation is quite different
from chronic inflammation and the distinctive features can point to the initiating
agent. For example, an infection of the skin with Staphylococcus aureus usually
produces an acute inflammatory response, whereas infection with Mycobacterium
leprae (leprosy) typically produces persistent infection and chronic inflammation.
There are three main components of inflammation (Figure 4):
1. An increase in the blood supply to the affected area, caused by dilation
of arterioles supplying the area.
2. An increase in the permeability of capillaries, which allows larger
serum molecules such as antibodies to enter the tissue.
3. Migration of leukocytes from the blood into the tissues - the cells
cross the endothelial cells, which line the venules, and then move out
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into the tissue. This process is mediated by signalling molecules
called chemokines, which are bound to the endothelial surface.
Figure 4 The three main features of inflammation are controlled at different places in the vasculature. The
diagram shows a longitudinal section through an arteriole, capillary and venule. Smooth muscle in the
arterioles controls blood flow into the site of inflammation. Exudation of serum molecules occurs in
capillaries as endothelial cells retract in response to inflammatory mediators. This allows antibodies and
molecules of the complement system to enter the site of inflammation. Migration of leukocytes takes place
in venules, partly because the shear force is lowest in venules and partly because signalling molecules are
present on the endothelium of the venules which attract leukocytes at this point.
All of these processes bring the defence systems of the body to the affected area. The
blood contains a number of proteins that stop bleeding, help clear infection and induce
repair or regeneration of the tissues. It also contains different types of leukocyte
(white blood cells), each of which has evolved to deal with different types of
infection. One of the key histological differences between acute and chronic
inflammation is seen in the sets of leukocytes that are present in the tissues. In acute
inflammation polymorphonuclear neutrophils usually predominate, whereas
macrophages and lymphocytes predominate in chronic inflammation. Eosinophils are
often prevalent in sites of helminth infections. Hence the characteristics of
inflammation are determined both by the tissue in which it occurs and by the initiating
agent and its persistence.
1.3 Chronic inflammation
Chronic inflammation is seen in diseases where there is persistent infection, usually
because the pathogen can resist the body's immune defences. If the infection is
cleared, chronic inflammation resolves, but residual damage may still be evident in
the tissues. Chronic inflammation also occurs in many autoimmune diseases; in
autoimmunity the target of the immune response is one of the body's own proteins or
cellular components, and consequently the stimulus for inflammation cannot be
cleared, although the condition may improve if the normal controls that prevent
autoimmune reactions are restored.
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1.4 Autoimmunity
The immune system normally recognises and tolerates all of the body's own tissues.
However, in some conditions the immune system reacts against 'self', resulting in
autoimmune disease. The targets may be individual molecules found in a specific
tissue, or antigens present in many tissues or in the extracellular matrix. Table 1 gives
some examples of autoimmune diseases and the target antigens. An example of a
tissue-specific autoimmune disease is Hashimoto's thyroiditis, in which lymphocytes
recognise thyroglobulin and a thyroid peroxisome antigen (Figure 5).
Table 1 Autoimmune diseases
Disease
Organ
Target antigens
Histological appearance
Hashimoto's
thyroiditis
Thyroid
Thyroglobulin
Destruction of thyroid
follicles with severe
inflammation
Thyroid
peroxisomes
Goodpasture's
syndrome
Kidney, lung
Basement
membranes
Damage to kidney
glomerulus
and/or lung alveolae
Myasthenia
gravis
Skeletal
muscle
Acetyl choline
receptor
Pemphigus
Skin, mucosa Desmosome
proteins in
keratinocytes
Separation of layers of
epithelium
Diabetes - type I
Islets of
Langerhans
Selective damage and loss of
cells of pancreatic Islets
with inflammation
Pancreatic beta
cells
Degeneration of the motor
endplate at nerve/ muscle
junction
Insulin , GAD
(enzyme)
Rheumatoid
arthritis
Joint
IgG antibodies,
cartilage
components.
Systemic lupus
erythematosus
Kidney, skin, DNA and
CNS
intracellular
antigens
Erosion of articular cartilage
by fibrous, inflammatory
tissue - pannus.
Type-3 hypersensitivity
reaction in kidney, damage
to glomerulus
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Figure 5 Hashimoto's thyroiditis is an example of an autoimmune disease, in which the thyroid follicles are
destroyed by an immune reaction against components of the thyroid, including thyroid peroxisomes and
thyroglobulin. Scale bar = 50µm.
The histological appearance of autoimmune disease depends on the nature of the
immune response and the target organ. However a characteristic of many organspecific diseases is that autoantibodies bind to the antigen within the tissue and recruit
inflammatory cells. In this case, direct immunofluorescence microscopy can be used
to identify the presence of antibodies, which goes a long way towards providing a
diagnosis of the disease (Figure 6). It is also possible to detect autoantibodies in the
blood of patients, using the same technique; the patient's serum is first incubated with
normal tissue to allow any autoantibodies to bind, and these are then detected, by
direct immunofluorescence or immunohistochemistry. Examination of the stained
sections can determine not just whether there are autoantibodies in the serum, but also
indicate what the target antigen might be, depending on where the autoantibodies are
located in the cells.
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Figure 6 Autoantibodies to Islets of Langerhans in the pancreas in type-1 diabetes may be demonstrated by
immunofluorescence (Courtesy of Dr B. Dean)
1.5 Hypersensitivity
Hypersensitivity is defined as an immune response, where the reaction is out of
proportion to the damage caused by the antigen or pathogen and does more harm than
good. Autoimmune diseases are by their very nature a type of hypersensitivity
reaction; however, there are many instances where the immune reaction against an
antigen or a pathogen is out of proportion to the damage that it causes. A simple
example is hay fever or asthma induced by pollen, where the pollen itself is clearly
harmless, but the inflammatory reactions, especially in the lung, can be lifethreatening. In some infectious diseases, such as M. tuberculosis, a significant
component of the pathology is the collateral damage caused to lung tissue by the
ongoing immune reaction against the bacteria. Obviously the bacterial infection is
itself potentially damaging, but the severity of the disease in different individuals is at
least partly due to the variability in their immune responses. Diseases such as multiple
sclerosis are even more complex. In this case, it is suspected that there is an
autoimmune reaction, although the target antigen is unclear, and there is clearly a
hypersensitive response taking place in the brain. The fact that this immune response
is particularly damaging is partly related to the nature of the CNS, which is delicate
and normally shielded from immune and inflammatory reactions.
Hypersensitivity reactions can be classified into four main types depending on the
type of immune response that causes them. Although the causes of hypersensitivity
are beyond the scope of this course, the histological appearance of the different types
of hypersensitivity reactions is often distinctive and can aid in diagnosis. Referring to
the examples given above, hay fever and allergic asthma are examples of type-1
hypersensitivity reactions, which develop rapidly following exposure to antigen. They
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are characterised by neutrophils and eosinophils in mucosal and submucosal tissues of
the respiratory tract; basophils are also common in the bronchial wall in asthma. In
contrast, tuberculosis is an example of a type-4 hypersensitivity reaction, which
develops slowly, in association with chronic inflammation, and is characterised by
macrophages and T-lymphocytes. The other types of hypersensitive reaction are due
to antibodies in tissues. For example the autoantibodies seen in pemphigus (Figure 7)
are an example of a type-2 reaction, whereas type-3 reactions are caused by the
deposition of antigen-antibody complexes from the circulation in organs where
filtration occurs, particularly the kidney.
Figure 7 Autoantibodies in pemphigus bind to components of the desmosome, a cellular structure that
connects cells to their neighbours. The antibodies disrupt inter-cellular adhesion causing blistering of the
skin. The section is stained with a fluorescent antibody which detects bound auto-antibodies (IgA) of the
patient. (Courtesy of Dr R. Mirakian and Mr P. Collins)
1.6 Scarring and fibrosis
Scarring and fibrosis are seen when the cells of a tissue are damaged or killed and
regeneration of the normal tissue architecture cannot take place. For example, in
cirrhosis of the liver, the normal hepatocytes are damaged and do not regenerate
effectively. The tissue is repaired and replaced by cells such as fibroblasts, which lay
down extracellular matrix components including collagen, which can be seen by
appropriate histological stains.
SAQ 2
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How does collagen appear in H&E staining? What stains show collagen more
effectively?
View answer - SAQ 2
The cells which carry out the repair vary from one tissue to another. For example,
following damage to the CNS, a group of glial cells called astrocytes replace damaged
neurons, forming a glial scar. Obviously this scar tissue cannot carry out the normal
function of nervous tissue, but it also can actively prevent the tissue from regenerating
- neurons do not regrow their axons through glial scars. Similarly scar tissue in the
skin will usually lack characteristic features of normal skin, such as hair follicles and
sweat glands.
Fibrosis also occurs in some infections, particularly if the infectious agent cannot be
cleared, fibroblasts lay down areas of extracellular matrix, which walls off the
infection. For example, schistosomiasis (a worm infection) in the liver often results in
areas of fibrosis surrounding the individual parasites.
Fibrosis and scarring are end-stages of a pathological process in which the body is
unable to regenerate normal tissue and does the best it can by patching up the
remaining tissue to limit further damage.
1.7 Wound healing, angiogenesis and tissue
regeneration
In many cases cells can divide and regenerate the tissue, restoring it to virtually
normal. For example the basal cells of the skin epidermis can divide to cover a scratch
or a graze, provided that it does not extend over too great an area. Epidermal cells
from hair follicles can contribute to the regeneration, provided that the damage has
not gone too deep. In this case there is a balance between regeneration from the
epidermis and repair from the dermal layers, the outcome of which will determine
whether a scar is formed or not. The process of normal tissue regeneration can be
favoured by closing wounds with stitches, or skin grafts. Conversely, if the damage
persists or the area of damage is large, fibrosis and scarring prevail.
The ability to regenerate varies greatly between cell types. For example, neuronal
cells have a very limited capacity to regenerate (regrow) their axons if they have been
severed, and virtually no capacity to replace themselves by cell division. By contrast,
hepatocytes have enormous potential for division, which can be seen following
removal of a portion of the liver, following surgery; the remaining cells can divide to
fully restore the liver to its original size.
In tissue such as skeletal muscle, regeneration is characterised by an increase in the
thickness of myofibres (hypertrophy), but without significant increase in their
number. The same effect is seen with adipocytes, which increase or decrease in size
(i.e. the volume of the lipid-filled vesicle) in response to fasting or over-eating rather
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than by changes in cell number. In such tissues, the histological appearance can give
an indication of tissue damage that has taken place a long time previously.
Angiogenesis is the process by which new blood vessels grow into tissues, forming
capillaries.
SAQ 3
Under what circumstances would you expect new vessels to grow into tissues?
View answer - SAQ 3
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Figure 8 Ischaemic regions of tissue release angiogenic cytokines, including VEGF (vascular endothelial cell
growth factor). The cytokines and locally released enzymes cause the breakdown of the vessel walls of
arterioles and venules, and sprouting of cells, including pericytes in the vessel wall. Endothelial cells
proliferate and migrate out of the vessel into the tissue. They reorganise to form capillaries which
interconnect (anastemosis) and link to venules, thereby forming a new capillary network.
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2 Pathological processes - neoplasia
2.1 Hyperplasia, dysplasia and neoplasia
Cell division is normally a highly regulated process. The numbers of cells in any
tissue is usually fairly constant, although some tissues can respond to physiological
demand by an increase in cell number.
SAQ 4
What process occurs as mountaineers acclimatise to high altitude? Why?
View answer - SAQ 4
Other types of cell may increase in numbers in response to appropriate stimuli. For
example, in a guitar player, the basal cells of the epidermis in the fingertips can
proliferate to produce hard pads of keratin (calluses) caused by repeated contact with
the strings. Cell proliferation and the consequent increase in cell numbers seen in
these two examples is called hyperplasia. It is a normal physiological response to
demand placed on a tissue. The numbers of each cell type are controlled specifically.
For example, the numbers of erythrocytes in the blood is controlled by a hormone,
erythropoietin; an increase in erythrocyte numbers does not produce any concomitant
increase in leukocyte numbers, since leukocyte subsets are each subject to their own
controls on cell number.
If cell division becomes poorly regulated, cells may lose some of their morphological
characteristics and/or functions. The tissue becomes disordered in appearance, often
with an increase in the numbers of immature cells, and greater variability between
cells. This appearance is called dysplasia. It should be emphasised that dysplasia does
not necessarily show that the cells have become cancerous; however, it does suggest
underlying changes in the cells, which may predispose to cancer. In this sense
dysplasia may be a stage on the way to cancer development. For example, when
histologists screen cervical smears, they are particularly looking for changes in the
normal morphology of the cells which indicate pre-cancerous changes.
Neoplasia is the term used to describe the development of tumours or cancerous
tissue. The development of a tumour requires a series of changes in the biology of the
cell, with progressive loss of the controls that limit cell division. Even a cell which is
undergoing uncontrolled proliferation will not necessarily be malignant. Malignancy
typically arises when the dividing cells invade the normal tissue and move away from
their site of origin. Because of the great variety of different tumours, it is impossible
to generalise. Nevertheless it is very important for a pathologist to be able to
distinguish between a benign tumour and a malignant cancer, since the treatment
required will usually be radically different. Consequently, pathologists often grade
tumours according to how malignant/invasive they are. Histologists can get some
impression of the rate of cell division within a tissue according to the number of
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mitotic figures - the number of cells with the nucleus showing the characteristic
pattern of separating chromosomes, seen as the cell divides (Figure 9). Invasion of
tumour cells within the tissue can be estimated by observing where the cells are in
relation to their normal position and in relation to other cells in that tissue, and this
forms an important element in the pathological report on a tumour.
Figure 9 A mitotic figure in a carcinoma of the breast (arrowed) indicates cell division. The number of
mitoses, together with other factors, are used to grade the tumour.
2.2 Metastasis
Tumours can also move away from their original tissue by invading blood vessels or
lymphatic ducts and being carried to distant sites. This process is called metastasis.
Tumour cells that are carried through lymphatics will usually metastasize to local
lymph nodes - this is the reason that surgeons may remove lymph nodes as well as the
original tumour to treat a cancer. Tumours that metastasize via the blood must first
invade a blood vessel at the initial tumour site, and then exit the blood vessels in a
different organ to establish a new tumour site. Such an event is relatively rare for any
individual tumour cell; nevertheless, metastasis accounts for 90% of cancer-related
deaths, so identification of metastatic tumours is important both for prognosis and
treatment. Pathologists recognise metastatic tumours, because the affected organ
contains clumps of cells which are completely uncharacteristic. In some cases the
primary tumour-type can be recognised because it has retained some distinctive
characteristics of the original cell-type. However, as noted above, the original identify
of tumour cells is not always self-evident and this is particularly true of metastatic
tumours. Hence, it may be possible to observe a metastatic tumour in a tissue, but be
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unable to identify the primary cell-type and hence the original site of the tumour, at
least by H&E staining. In this case additional staining, particularly
immunohistochemistry is valuable to identify the original cell type, because it can
provide an important guide for patient-scanning, further surgery, radiotherapy and
drug treatment.
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3 Pathological processes - cell death
3.1 Apoptosis and necrosis
When cells die, they do so in two main ways: by apoptosis or necrosis (Figure 10).
Apoptosis is programmed cell death; the cell dies as part of its normal programme of
development, or it may be lacking in growth factors, or it may be instructed to die by
cells of the immune system, because it has become infected. Even pre-cancerous cells
may be propelled into apoptosis, by the normal cellular controls that check the
development of tumours. In all cases, apoptosis is a highly ordered process. If it
occurs as part of a developmental process, it does not induce inflammation - the dead
cells are quietly removed by phagocytes within the tissue. Hence, it is often very
difficult to identify apoptotic cells within tissues, since they are usually individual
cells, with small condensed nuclei and little cytoplasm. Cell death in degenerative
conditions (e.g. Alzheimer's disease) appears to occur by apoptosis. Although the loss
of individual cells is histologically undramatic, the cumulative loss of cells in such
degenerative conditions can cause major loss of function in the affected tissue.
Moreover, cell loss may be accompanied by the accumulation of products of tissue
breakdown, which are histologically evident.
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Figure 10 Schematic diagram comparing the events that occur in cells undergoing death by necrosis (left) or
apoptosis (right). The first events in necrosis are irregular condensation of chromatin, swelling of the
mitochondria and breakdown of membranes and ribosomes. The cell is eventually disrupted, releasing its
contents and inducing an inflammatory reaction. In contrast, a cell undergoing apoptosis shows condensation
of the nucleus into fragments and shrinkage of the cell. The nucleus and cytoplasm break up into fragments
called apoptotic bodies, which are phagocytosed by mononuclear phagocytes.
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In contrast necrosis is wholesale unregulated cell death caused by lack of nutrients or
infection. For example the failure of the blood supply to an organ due to thrombosis
(see below) will cause massive cell death due to lack of oxygen (ischaemia). A large
area of cell death caused by ischaemia is called an infarction. Another example of
cell necrosis is seen in severe viral infections with cytopathic viruses (e.g. polio).
Necrosis is an uncontrolled process and the dying cells release their contents. Areas of
necrosis are characterised by infiltration with inflammatory cells; macrophages and
neutrophils enter the area over a number of days and weeks in order to clear the dead
cells and associated cellular debris. Such large areas of cell loss and inflammation are
frequently easily seen in pathological specimens, even without microscopic
examination (Figure 11).
Figure 11 An area of necrosis in the lung caused by a thromboembolism is visible on the left of this section.
The area on the right includes surviving lung tissue, which is thickened and has areas of fibrosis. Scale bar =
2mm.
3.2 Thrombosis and embolism
Blood clots may form in vessels for a variety of reasons. A blood clot is called a
thrombus, and the process by which it forms is thrombosis. Embolism occurs when
something is carried through the circulation from one site to another. When a
thrombus breaks away and is carried through the circulation, it is referred to as a
thromboembolism. Other examples of emboli are tumour cells or air-embolism,
where air is accidentally introduced into the circulation by a physician.
Thromboembolism can block the downstream blood vessels; emboli formed in veins
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pass through the heart to block arteries in the other side of the circulation, while
thrombi formed in arteries can block vessels in the organ where they form. Exactly
which vessels may become blocked also depends on the size of the embolism, and the
site determines what damage may follow.
SAQ 5
(a) If a thrombus is formed in the veins of the leg, where is it likely to end up? (b) If
thrombi form on the tricuspid valves of the heart (leading to the aorta), where might
the emboli end up?
View answer - SAQ 5
3.3 Degenerative diseases and storage disorders
Cell loss occurs in many tissues with age; the effects are particularly notable in tissues
that have a limited capacity for regeneration, such as nerves in the central nervous
system, the retina of the eye and the sensory cells of the inner ear. Histologically, it is
more difficult to identify something that is not there than a change in the structure of
the tissues. In diseases such as Alzheimer's disease there is progressive loss of
neurons and shrinkage of the brain, which may be more evident in the gross
pathology, although counting the relative numbers of cells within an area can also
give some histological indication of the cell loss. For example, the relative numbers of
neurons relative to glial cells falls in areas affected by Alzheimer's disease. More
evident are characteristic accumulations of proteins. Degenerating neurons leave
tangles of fibres (neurofibrillary tangles) produced be degenerating components of the
cytoskeleton. In addition there are extracellular accumulations of 'amyloid' within the
brain. It is debated whether these deposits are the cause or consequence of the disease,
or both.
Amyloid is an extracellular insoluble deposit of protein, and amyloidosis refers to the
diseases in which amyloid occurs. In some cases production of amyloid is a primary
event, and in others it is secondary to infection or a tumour. The actual protein type
varies, depending on the cause of condition. In some cases it affects individual organs,
such as the brain in Alzheimer's disease, but in the so-called 'systemic amyloidoses'
many organs may be affected, including the lung, kidney, heart and spleen.
SAQ 6
What stain can be used to identify amyloid deposits in tissues?
View answer - SAQ 6
There are a number of hereditary conditions in which the person lacks enzymes that
break down particular macromolecules; they are collectively called storage diseases
because the components that cannot be degraded within lysosomes accumulate and
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form insoluble deposits. This is particularly noticeable in the brain, where they are
often associated with neurodegenerative diseases (Figure 12).
Figure 12 Protein aggregates in brain cells associated with human degenerative diseases. Arrows highlight
(a) extracellular plaques in prion disease, (b) extracellular plaques (blue) and neurofibrillary tangles (yellow)
in Alzheimer's disease, (c) nuclear aggregates in Huntington's disease, (d) cytosolic aggregates known as
'Lewy bodies' in Parkinson's disease, (e) nuclear aggregates in amyotrophic lateral sclerosis and (f)
accumulations of sphingolipids in the distended neurons of a patient with Niemann-Pick disease.
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4 Photography and reporting
4.1 Image acquisition
Digital photography has superseded the use of film for obtaining images of
histological sections and many microscopes have a digital camera attached. An image
obtained from a slide generally only includes a tiny proportion of the section, however
microscope systems are now available that can scan entire slides, providing very large
images. Such images can be transmitted electronically, so that a pathologist can 'view'
a section from a distant location. Such systems are being increasingly used, although
they are still very much the exception to the standard practice where the pathologist
observes and records their observations at their own hospital.
Images are not usually obtained for routine work. Since the sections are stored for
many years it is always possible to return to them later. However, for presentations,
images are essential, and there is some skill in selecting suitable areas of the section to
illustrate a point. Journals require a minimum of 300dpi for histological images,
usually in jpg or tiff formats. If you are preparing images for publication, it is
essential to generate images of acceptable quality and format, by checking the
requirements on the journal website beforehand.
4.2 Magnification and scale bars
When you see micrographs in older text-books, a magnification is usually stated in the
legend (e.g. x100). Strictly, this should mean that the magnification of the illustration
in the book is 100-fold larger than the original item. However, there is occasionally
some ambiguity. For example, the statement can mean that the picture was taken
using a microscope set with a 100x magnification (10x objective, 10x eyepiece).
Since the light path to the camera is not the same as the light path to the eye (which
passes through the eyepieces), these magnifications are not meaningful. Moreover
publishers may increase or decrease the size of a micrograph to fit the available space.
The stated magnification in the legend should then be corrected, but often it is not.
For these reasons, the use of scale-bars has replaced a statement of magnification. A
scale bar, corresponding to a convenient unit of length, is added to the image taken by
the camera, and is then an integral part of that image. If the image is increased or
reduced in size thereafter, the scale bar changes in proportion, so that it is always
possible to see the correct size of the cells or tissue.
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Conclusion
Pathological processes leave their imprint on the tissue. Interpreting these changes can
give key information about a disease and aid diagnosis. Some of the changes are very
subtle, whereas others are easily seen. In all cases it is important to distinguish natural
variations from pathological changes, and it requires many years of experience to be
able to recognise the different diseases that can occur - even within a single type of
tissue. This course should have given you some insight into the subject of
histopathology, and the type of work that is done by specialists in this field.
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Acknowledgements
Course image: Larry Darling in Flickr made available under Creative Commons
Attribution-NonCommercial-ShareAlike 2.0 Licence.
The material acknowledged below is Proprietary and used under licence (not subject
to Creative Commons licence). See terms and conditions.
Grateful acknowledgement is made to the following sources:
Figure 1: Brostoff, J. et al. (1991) Clinical Immunology. Gower Medical Publishing
Figures 2, 3, 5 and 11: David Male
Figure 6: Dr D. Bean
Figure 7: Dr R. Mirakian and Mr P. Collins
Figure 9: Woo, E. K. et al. (2005) 'Myoepithelial carcinoma of the breast: a case
report with imaging and pathological findings', British Journal of Radiology, vol. 78,
May 2005. The British Institute of Radiology
Figure 12a-e: Claudion, S. (2003) 'Unfolding the role of protein misfolding in
neurodegenerative diseases', Nature Reviews Neuroscience, vol. 4, 2003 © Nature
Publishing Group
Figure 12f: Riezman, H. (2002) 'The ubiquitin connection', Nature, vol. 416, 28
March 2002. Nature Publishing Group.
Every effort has been made to contact copyright holders. If any have been
inadvertently overlooked the publishers will be pleased to make the necessary
arrangements at the first opportunity.
Don't miss out:
If reading this text has inspired you to learn more, you may be interested in joining
the millions of people who discover our free learning resources and qualifications by
visiting The Open University - www.open.edu/openlearn/free-courses
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SAQ 1
Answer
(a) Ziehl Nielsen stains mycobacteria red; their identification is aided by the bacterial
morphology - mycobacteria are rod-shaped. (b) Gram stain can help distinguish
Neisseria, which are gram-negative streptococci from other streptococci and
staphylococci which might also be found in the specimen.
Back
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SAQ 2
Answer
In H&E staining collagen is pale pink and often difficult to differentiate from support
cells embedded within it. Masson's trichrome stains collagen blue. Van Gieson stains
collagen red/pink.
Back
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SAQ 3
Answer
An increase in the requirements for oxygen or nutrients stimulate angiogenesis. It may
be due to an increased metabolic activity of the organ, e.g. in a muscle following
training. Regenerating tissues also require a new blood supply, and angiogenesis is
frequently a critical requirement for tumour development.
The process of angiogenesis involves new capillaries sprouting from the side of
arterioles and extending as blind-ended tubes in the tissue. Eventually they connect up
(form anastemoses) with venules to complete a capillary loop (Figure 8).
Regenerating tissue often contains numbers of these developing capillaries; in the skin
the base of scars has characteristic pink spots, which are the newly sprouting
capillaries.
Back
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SAQ 4
Answer
The number of erythrocytes in their blood increases. The fall in the level of oxygen in
the air at altitude means that the capacity of the blood to carry oxygen increases in
order to compensate. There is a progressive increase in the numbers of erythrocytes
over a period of weeks as the bone marrow responds by increasing production.
Back
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SAQ 5
Answer
(a) Thromboemboli formed in leg veins will usually pass through the heart to end up
in the pulmonary arterial circulation, causing damage to the lung. (b) Thrombi formed
on the tricuspid valves will pass into the systemic circulation and are particularly
damaging if they enter the cerebral or carotid arteries, as they can then damage the
brain (stroke).
Back
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SAQ 6
Answer
Congo red, as mentioned in Histology (S120_2).
Back
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