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What is a biopsy?
A biopsy is a piece of tissue which is removed from a living body for
the purpose of examination for medical diagnosis. This is usually done
as a minor operation under local anaesthetic.
Why is a biopsy done?
A biopsy is taken to aid the consultant physician in obtaining a diagnosis.
A large range of other tests, often less invasive, are available, but
if these prove inconclusive, a biopsy sample is a direct way of examining
the cellular structure of the tissue.
What happens to the biopsy once it has been taken?
Biopsies are processed, using a variety of techniques, in the histopathology
laboratory. The choice of test to be performed on a biopsy is usually
made by the consultant pathologist who is dealing with the case. Responsibility
for particular types of analysis may be passed from the pathologist to
other specialists (e.g. electron microscopists), however, the consultant
pathologist retains overall responsibility for the final results.
There are a number of ways of preparing tissue, and the method (or methods)
chosen depend on the nature of the enquiry made by the consultant physician
in charge of the case.
- Histology - uses stains and dyes to look at the light microscopic
anatomy of the tissue and cells.
A piece of tissue is fixed in formalin to preserve the structure and
then the water in the tissue is gradually replaced with molten wax.
When the wax has hardened, thin sections of tissue are cut on a machine
called a microtome, and then they are stained with various dyes to highlight
different parts of the structure.
- Histochemistry - uses chemical techniques to look at histological
sections.
Small blocks of fresh tissue are frozen in liquid nitrogen (-196°C)
and very thin (4 -10µ) slices are taken using a machine called
a cryostat. (There are 1000µ in 1mm). The cryostat allows the
operator to taken sections from frozen tissue, without the tissue block
thawing. (Once thawed the tissue rapidly degenerates and is useless
for histochemical analysis). The sectioning chamber is usually kept
at about -20°C. Sections are collected on glass microscope slides.
Each section is stained using a different method to highlight a different
chemical feature of the tissue.
- Electron microscopy - uses very high magnifications to look
at the cellular structure and substructure.
Electron microscopy allows the examination of the structures within
individual cells, and in many instances can identify cell types where
light microscopy cannot. A thin piece of the tissue (no more than 1
or 2 mm wide) is preserved with a fixative, in a similar manner to that
described for histological samples above. Instead of replacing the water
in the tissue with molten wax, a liquid epoxy resin is used. Tissue
pieces are placed in small moulds filled with liquid resin. These are
placed in an oven for 24-48 hours to harden. This makes the tissue into
very hard blocks which can withstand considerable forces when being
sectioned.
In order to obtain an image of the material in the section in the electron
microscope, the section must be thin enough for the electrons to pass
through it. A special machine is used, called an ultramicrotome, which
can shave sections off the tissue block at thicknesses between 70 and
80nm. (There are 100,000 nm in a mm)
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