3 rd Lecture
 LM, TEM and SEM
 Histology
 4 types of tissue in human body
 Tissue preparation
 Fixation , Dehydration, Clearing,
Embedding, Sectioning, Staining
 Fixation
 Aim of fixation
 Types of fixatives
Histology is the study of tissue biology structure & function of each
tissue type, how these tissues are combined to form the organs &
systems of the body, & how these combinations function together.

Levels of organization
Cells, Tissues, Organs, Organ systems, Organism
Cell is defined as the smallest basic structure of an organism capable
of independent existence.
Tissues are groups of cells of similar structure, function and origin that
form functional units within the multicellular organism. Tissues are
made of cells & extracellular matrix, 2 components that are intimately
related functionally.
The organs represent an even greater measure of complexity and are
composed of various tissues. Most organs are formed by an orderly
combination of several tissues.
At an even higher level of organization there is the organ systems
composed of several organs.
Thus the body can be seen to be formed of different levels of
organization, with increasing levels of complexity & each of which
plays important roles in the physiological homeostasis of the body
 Basic Tissues:
The
body
is
composed
1.
Epithelium Tissue
2.
Connective Tissue
3.
Muscle Tissue
4.
Nerve Tissue
of
four
basic
tissues:
Epithelium:
Tissue organized as attached sheets of cells which line or cover all
organs & body cavities & form tubular structures within many
organs
2.
Connective:
Tissues which provide structural & metabolic support .Connective
tissue contains few cells separated by extracellular matrix
molecules secreted by those cells
3.
Muscle:
Electrically excitable tissue which contracts in response to
specific stimuli (such as signals from nerve tissue), & moves
tissues to which it is attached.
4.
Nerve:
Electrically excitable tissue which receives stimuli, processes them,
& transmits signals to target tissues to integrate the functions of
the whole body
1.
Tissue preparation for microscopic examination occur in the following
order:
(Fixation , Dehydration, Clearing, Embedding, Sectioning, Staining)
Fixation :
When tissues are removed from the body they undergo a series of
degenerative changes known as autolysis and putrefaction. Autolysis
is the breakdown of cells and tissue components by the body’s own
enzymes and putrefaction is the degenerative change brought about
in tissue as a consequence of bacterial action.
Fixation should arrest autolysis and putrefaction and preserve the cells
and tissue components in as ‘lifelike’ a state as possible.
The fixative should not cause excessive shrinkage, swelling or
hardening of the tissue and should stabilize the tissue against the
rigors of processing. Finally, the fixative should complement and
enhance subsequent histological staining, immunohistochemical and
molecular biology procedures. This is broadly the remit of an ‘ideal’
fixative. However, in reality fixation is a compromise of these various
requirements.
 Simple fixatives:
 are single-chemical solutions, e.g. methanol, ethanol,
glacial acetic acid and formaldehyde, which have no
additives and which, used on their own and may
produce some of the artefacts mentioned previously.
 Compound fixatives:
 are mixtures of simple fixatives which have been
formulated to offset and minimize fixation artefacts, e.g.
Carnoy’s fluid (ethanol– chloroform–acetic acid), which
is recommended for the fixation of nucleic acids.
Simple and compound fixatives react with proteins in two ways:
1.
2.
Coagulants, as suggested by their name, coagulate tissue protein, e.g. ethanol.
Non-coagulant fixatives, e.g. formaldehyde, form cross-links with tissue protein.
There are five major groups of fixatives, classified according to mechanism of
action:
1.
Aldehydes
2.
Mercurials
3.
Alcohols
4.
Oxidizing agents
5.
Picrates
Thank you