Histopathology

Introduction to the Study of Microscopic Examination of Tissues

Histopathology is the study of microscopic examination of tissues that are affected by disease. This field of study is crucial as it is useful in:

·         Determining the progression of disease

·         Understanding the mechanism of the disease

·         Discovering the potential treatment of the disease

·         Knowing the outcome or progression of the disease

The most basic skill in histopathology is to first understand and learn what is the normal structure and function of the distinct types of tissues. Another fundamental skill is to learn the preparation of microscope sections as it allows the identification of tissues, changes, and disease.

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Pathological Processes of Inflammation and Infection

Through the microscopic examination of tissues, there are characteristic changes in the tissue that can lead to the diagnosis of diseases. Since there are many different diseases, histology alone may not be able to produce a diagnosis but may be definitive and crucial in certain cases. For example, if there are signs of tissue damage and inflammation in the brain, the pathologist might be able to tell that it is a viral infection but is unable to tell which virus is responsible for it. The identification of the virus would require a process called immunohistochemistry for the viral protein to confirm the symptoms of the disease. Another example is the appearance of “owl-eye” cells in the brain as a specific type of measles infection (subacute sclerosing pan-encephalitis (SSPE)). The following are some of the basic terminology used in histopathology and can guide you in your understanding towards the introduction of histopathology.

a)       Infection

Infection can affect any part of the body leading to damage and inflammatory conditions. In viral infections where their presence is too small to be seen through the light microscope, it can still be detected due to the changes it causes in the tissues. However, their identification still must be confirmed through immunohistochemistry, serology or molecular biology. Bacteria can be seen through the light microscope. Different strains of bacteria can be distinguished using different histological stains. In parasites, the diagnosis of parasitic infection is dependent on histological findings as the appearance of parasite infected cells is characteristic of the specific infection.

b)      Acute inflammation

This is a common response to any injury or infection that typically develops and resolves rapidly. There are 3 main characteristics of inflammation:

i)                    Increased blood flow or supply to the affected area causing the dilation of arterioles.

ii)                   Increased permeability of vessels where larger molecules such as antibodies can now enter the tissue.

iii)                 White blood cells (leukocytes) migrate from the blood into the tissues, a process that is triggered by the signaling molecules known as chemokines and are bound to the endothelial surface.

All the above causes the defense system of the body to be activated. The blood contains:

·         Proteins that stop bleeding

·         Proteins that help clear infection

·         Proteins that induce repair or regeneration of tissues

·         Different leukocytes present that deal with different infections

c)       Chronic inflammation

Chronic inflammation can last for a long time which is usually due to the persisting foreign pathogen/matter as it has been able to resist the body’s natural immune defenses. Once the infection resolves, residual damage may still be present. It can occur in autoimmune conditions where the body’s immune system targets its’ own cells, which means the stimulus for inflammation can never be resolved but may be controlled and improved.

d)      Hypersensitivity

This is also an immune response where the reaction to the antigen or pathogen is dramatic regardless if the initial intent was to do good. Hypersensitivity can be classified into 4 main groups:

i)                    Type 1: Typically develop quickly after exposure. Examples include hay fever and allergic asthma.

ii)                   Type 2: An example is the autoantibodies seen in pemphigus.

iii)                 Type 3: Caused by the deposition of antigen-antibody complexes in regions where filtration occurs especially the kidney.

iv)                 Type 4: Develops slowly in association with chronic inflammation and hallmarks include macrophages and T-lymphocytes. For example, tuberculosis infection.

e)      Scarring and fibrosis

This is seen when cells are damaged, killed, and regeneration of the normal architecture cannot occur. For example, in cirrhosis of the liver, the tissue is replaced by fibroblasts. Scarred tissue and fibroblasts do not carry out the normal function of the replaced tissue. Both scarring and fibrosis are the end stages of healing where normal tissue can no longer be regenerated and is patched up to limit further damage.

f)        Wound healing, angiogenesis, tissue regeneration

Wounds heal when tissue regeneration and angiogenesis occur.

Pathological Processes - Neoplasia, Dysplasia, Hyperplasia

Neoplasia describes the development of tumors or cancerous tissue. Hyperplasia occurs when there is cell proliferation and increasing cell numbers. It is normal when it is replacing old tissue. Uncontrolled, it loses their control on the number, appearance and functions. This leads to dysplasia which shows that there are underlying changes in the cells, which can signify a higher risk of cancer. It can be seen as a stage of cancer development.

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Conclusion

Histopathology is an interesting but complex subject. It is the understanding of processes of diseases and biological functions of cells and how it interacts with each other. There are many more terminologies such as metastasis, apoptosis, necrosis, and more. Interpretation of changes are crucial in the diagnosis and treatment of diseases. This article shows a brief insight into the subject of histopathology and the type of work these professionals do in this field.