Histopathology refers to the microscopic examination of tissue in order to study the manifestations
of disease. Specifically, in clinical medicine, histopathology refers to the examination of
a biopsy or surgical specimen by a pathologist, after the specimen has been processed and
histological sections have been placed onto glass slides. In contrast, cytopathology examines
free cells or tissue fragments.
Collection of tissues Histopathological examination of tissues starts
with surgery, biopsy, or autopsy. The tissue is removed from the body or plant, and then
placed in a fixative which stabilizes the tissues to prevent decay. The most common
fixative is formalin. Preparation for histology
The tissue is then prepared for viewing under a microscope using either chemical fixation
or frozen section. Frozen section processing
The second method of histology processing is called frozen section processing. In this
method, the tissue is frozen and sliced thinly using a microtome mounted in a below-freezing
refrigeration device called the cryostat. The thin frozen sections are mounted on a
glass slide, fixed immediately & briefly in liquid fixative, and stained using the similar
staining techniques as traditional wax embedded sections. The advantages of this method is
rapid processing time, less equipment requirement, and less need for ventilation in the laboratory.
The disadvantage is the poor quality of the final slide. It is used in intra-operative
pathology for determinations that might help in choosing the next step in surgery during
that surgical session. Staining of the Processed Histology Slides
This can be done to slides processed by the chemical fixation or frozen section slides.
To see the tissue under a microscope, the sections are stained with one or more pigments.
The aim of staining is to reveal cellular components; counterstains are used to provide
contrast. The most commonly used stain in histopathology
is a combination of hematoxylin and eosin. Hematoxylin is used to stain nuclei blue,
while eosin stains cytoplasm and the extracellular connective tissue matrix pink. There are hundreds
of various other techniques which have been used to selectively stain cells. Other compounds
used to color tissue sections include safranin, Oil Red O, congo red, silver salts and artificial
dyes. Histochemistry refers to the science of using chemical reactions between laboratory
chemicals and components within tissue. A commonly performed histochemical technique
is the Perls' Prussian blue reaction, used to demonstrate iron deposits in diseases like
Hemochromatosis. Recently, antibodies have been used to stain
particular proteins, lipids and carbohydrates. Called immunohistochemistry, this technique
has greatly increased the ability to specifically identify categories of cells under a microscope.
Other advanced techniques include in situ hybridization to identify specific DNA or
RNA molecules. These antibody staining methods often require the use of frozen section histology.
Digital cameras are increasingly used to capture histopathological images.
Interpretation The histological slides are examined under
a microscope by a pathologist, a medically qualified specialist who has completed a recognised
training program. This medical diagnosis is formulated as a pathology report describing
the histological findings and the opinion of the pathologist. In the case of cancer,
this represents the tissue diagnosis required for most treatment protocols. In the removal
of cancer, the pathologist will indicate whether the surgical margin is cleared, or is involved.
This is done using either the bread loafing or CCPDMA method of processing.
In myocardial infarction
After a myocardial infarction, no histopathology is seen the first ~30 minutes. The only possible
sign the first 4 hours is waviness of fibres at border. Later, however, a coagulation necrosis
is initiated, with edema and hemorrhage. After 12 hours, there can be seen karyopyknosis
and hypereosinophilia of myocytes with contraction band necrosis in margins, as well as beginning
of neutrophil infiltration. At 1 – 3 days there is continued coagulation necrosis with
loss of nuclei and striations and an increased infiltration of neutrophils to interstitium.
Until the end of the first week after infarction there is beginning of disintegration of dead
muscle fibres, necrosis of neutrophils and beginning of macrophage removal of dead cells
at border, which increases the succeeding days. After a week there is also beginning
of granulation tissue formation at margins, which matures during the following month,
and gets increased collagen deposition and decreased cellularity until the myocardial
scarring is fully mature at approximately 2 months after infarction.
See also Anatomical pathology
Molecular pathology Frozen section procedure
Medical technologist Laser capture microdissection
List of pathologists References
^ http:www.scribd.com4448747/Perl Perls' Prussian blue original formula and uses. Accessed April
2, 2009. ^ Chapter 11 in: Mitchell, Richard Sheppard;
Kumar, Vinay; Abbas, Abul K.; Fausto, Nelson. Robbins Basic Pathology. Philadelphia: Saunders.
ISBN 1-4160-2973-7. 8th edition. External links
Virtual Histology Course - University of Zurich Histopathology of the uterine cervix - digital
atlas PathologyPics.com: An interactive histology
database for the Practicing Anatomic Pathologist as well as Pathology Trainees.
Histopathology Virtual Slidebox - University of Iowa