• Histology is the study of tissues and how these
tissues are arranged into organs
• Histo in Greek means tissue or web
• Tissues consist of cells and extra cellular
• The function of the tissue depends on the
interaction between cells and extracellular
• The small size of cells and matrix content make
the study of tissues dependent on microscope
and other advances in biological techniques
• The most widely used method of studying
tissues is using histological slides.
• The tissue in the slide must reflect the actual
nature of the tissue in the body
• To insure that, tissues to be studied must
pass through a series of steps before
• These steps are in sequential order
• Fixation is a complex series of chemical
events that differ for the different groups of
substance found in tissues.
Aim of Fixation:
1- To prevent autolysis and bacterial attack.
2- To fix the tissues so they will not change
their volume and shape during processing.
3- To prepare tissue and leave it in a condition
which allow clear staining of sections.
4- To leave tissue as close as their living state as
possible, and no small molecules should be
•Fixation is a reaction between the fixative and
proteins in the specimen which form a gel, so
keeping every thing as their in vivo relation to each
Is to embed the tissue in a solid medium firm
enough to support the tissue and give it
sufficient rigidity to enable thin sections
to be cut , and yet soft enough not to
damage the knife or tissue.
• Stages of processing:
• To remove fixative and water from the tissue
and replace them with dehydrating fluid.
• Delicate specimens are dehydrated in a
graded ethanol series from water through
10%-20%-50%-95%-100% ethanol to minimize
tissue distortion from diffusion currents.
• In the paraffin method, dehydration from
aqueous fixatives is usually initiated in 60%-
70% ethanol, progressing through 90%-95%
ethanol, absolute ethanol before proceeding
to the clearing stage.
Types of dehydrating agents
• Tissues may be held and stored indefinitely
in 70% ethanol without harm
• Replacing the dehydrating fluid with a fluid
that is totally miscible with both the
dehydrating fluid and the embedding
• Choice of a clearing agent depends upon
• Is the process by which tissues are
surrounded by a medium such as agar,
gelatin, or wax which when solidified will
provide sufficient external support during
• A substances added alone or in combination
to the wax to:
Decrease melting point
Improve adhesion between specimen and
A. paper boat mould
B. metal boat mould
C. Dimmock embedding
D. Peel-a-way disposable
E. Base mould used with
embedding ring ( F) or
cassette bases (G)
• A microtome is a mechanical instrument
used to cut biological specimens into very
thin sections for microscopic examination.
• Most microtomes use a steel blade and are
used to prepare sections of animal or plant
tissues for histology.
Hematoxylin and Eosin (H & E)
• H & E is a charge-based, general purpose stain.
• Hematoxylin stains acidic molecules shades of
• Eosin stains basic materials shades of red, pink
• H & E stains are universally used for routine
histological examination of tissue sections.
• Deparaffinize and hydrate to water
• If sections are Zenker-fixed, remove the mercuric chloride
crystals with iodine and clear with sodium thiosulphate
• Mayer's hematoxylin for 15 minutes
• Counterstain with eosin from 15 seconds to 2 minutes
depending on the age of the eosin, and the depth of the
• Dehydrate in 95% and absolute alcohols, two changes of
2 minutes each or until excess eosin is removed
• Clear in xylene, two changes of 2 minutes each
• Mount in Permount or Histoclad
• Many tissue components can not be stained
with (Hematoxylene and Eosin).
• Other dyes are used to specifically stain
certain tissue components
Resorcin-Fuchsin for elastic fibers
Silver stain for reticular fibers and
Periodic-Acid Schiff (PAS) Reaction for