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Introduction to Histology
Histology is the microscopic study of tissues and cells. It involves
studying their distinctive microscopic structure, which also helps to
elucidate functional and clinical significance of these structures. This
field of study is important in medicine as it links physiology,
biochemistry and molecular biology to the study of disease.

Histology is also used in understanding the pathogenesis and

diagnosis of various diseases. Diseased tissue samples, or biopsies,
from the affected organs are usually processed, stained and
observed under the microscope. Histology involves the use of both
light microscopy and electron microscopy, depending on the
requirements of the sample being visualized.

In recent years, the field of histology has greatly advanced, as new

practices like protein sequencing, molecular genetics and cloning
have provided a greater knowledge and understanding of the
ultrastructure of cells and tissues in the human body.

Contents

1. Introduction to Cells and Tissues

1. Cells
2. Tissues
2. Light Microscopy
1. Tissue Preparation
2. Wide-Field Microscopy
3. Confocal Microscopy
4. Differential Interference Contrasct Microscopy
3. Tissue staining
4. Electron microscopy
1. Transmission Electron Microscopy
2. Scanning Electron Microscopy
5. In Situ Hybridisation
6. Blotting Technique
7. Highlights
8. Related Atlas Images
Nervous tissue - histological slide
Introduction to Cells and Tissues
Cells

Cells are the tiny living units that form the tissues, organs and
structures within the body. In turn, the body is composed of different
types and varieties of cells to carry out specific functions, but they all
have the same basic structure. All cells contain cytoplasm and are
surrounded by a membrane, and contain the following structures of
organelles:

 Nucleus (contains DNA)

 Mitochondria (produce energy in the form of ATP)
 Ribosomes (synthesise proteins)
 Lysosomes (contain digestive enzymes)
 Endoplasmic reticulum (synthesises protein and lipids)
 Golgi body (folded membranes that process proteins from the endoplasmic
reticulum)
 Vesicles (transport materials within the cell)
 Peroxisomes (contain metabolic enzymes)
Satellite cells - histological slide
Tissues

Body tissues are collections of cells, grouped in the body according to

structure and function. In histology, human tissues are separated into
four distinct categories:

 Muscular: Muscle tissue is made up of long thin muscle cells called

myocytes. Their structure and arrangement allows for muscular contraction.
 Nervous: Nervous tissue forms the nervous system, and is made up of
specialised cells called neurons and neuroglial cells. Neurons conduct nerve
signals from one to another in the form of electrical impulses.
 Epithelial: Epithelial tissue comprises epithelial cells arranged together in
sheets. These sheets serve as protective layers, forming coverings like the
skin, and the lining of the small intestine.
 Connective: Connective tissue forms a connective web throughout the
body. It fills gaps and connects different organs and body parts, so that the
carefully arranged structure of the body can be maintained.
Adipose tissue - histological slide
Light Microscopy
Tissue Preparation

Light microscopes (or optical microscopes), use a combination of

visible light and lenses to create a magnified image. In histology, thin
sections of tissue are specially prepared and placed under the
microscope for observation.

To create thin sections, the tissue is usually first immersed in a

fixative solution, which acts as a preservative to stop the sample
from degrading. The thin section is then embedded in paraffin wax,
which firms it, so that it can be sliced into thin sections. The thin
sections are usually between 5 and 8 µm in thickness. The sections
are then mounted on a microscope slide, the wax is removed with
organic solvent, and then the thin section is rehydrated with diluted
alcohol. In instances where a fresh tissue sample is required, for
example in surgical biopsies, frozen samples are made. Samples can
be snap-frozen using liquid nitrogen, and then stored at -80 degrees
Celsius. They are finely sliced inside a freezer using a piece of
equipment called a cryostat. In light microscopy, tissue staining is
generally required.
Light Microscope

Wide-Field Microscopy

One type of optical microscopy is known as wide field microscope.

This type of microscopy immerses the specimen in light from either a
xenon or mercury source, and the image can be viewed directly.
However secondary fluorescence can be produced which may
interfere with the resolution of the image.
Confocal Microscopy

Another type of optical microscopy is confocal microscopy. This is

often referred to as confocal laser scanning microscopy (CLSM).
CLSM is used to increase resolution and contrast. This is achieved by
adding a spatial pinhole at the confocal plane of the lens. This
removes any out of focus light that might be present. CLSM allows for
reconstruction of 3D structures from images taken at different
depths.

Differential Interference Contrasct Microscopy

A further type of optical microscopy is differential interference

contrast microscopy. This is used to improve contrast in unstained or
clear samples. This form of microscopy is fairly complex, and uses a
process known as interferometry to obtain information regarding the
optical path length of a sample. This allows the visualisation of
structures that would otherwise be invisible to our eyes.

Tissue staining
As cells are generally colourless, they need to be stained so that they
can be easily viewed under the microscope. There are four types of
stains used in histology. Empirical stains are the most common, but
histochemical, enzyme histochemical and immunohistochemical
stains can also be used. Details of each stain are listed below:

 Empirical: These are simple stains used since many years and
discovered by trial and error. They cause differential coloration of
the various components of the cells and tissues, allowing their
 ultrastructure to be viewed more clearly. Common examples of
empirical stain are trichrome stains and van Gieson’s stain.
Trichrome uses mixture of three different dyes to stain different
components of tissues while van Gieson colour muscle tissue
yellow, and collagen pink.

Cirrhosis of the liver (Trichrome stain)

 Histochemical: This method involves using stains that are specific
to a particular molecule within the sample, allowing the chemical
components of cells and tissues to be observed under the
microscope.

 Enzyme Histochemical: As its name suggests, this technique uses a

staining agent to identify and locate activity of specific enzymes.
For the enzymes to be active, this method requires the use of fresh
tissue, which is generally incubated together with the substrate
specific to the enzymes being observed.

 Immunohistochemical: This uses antibodies to detect specific cell

molecules within tissues. The antibodies are often attached to
enzymes or immunofluorescence, that will induce a colour change
at the site of interest. This method is also important in identifying
abnormal cells, such as cancer cells.
Speckled HEp-2 cells (Immunofluorescence)
Electron microscopy
Electron microscopy is a more modern form of microscopy than light
microscopy and provides much higher magnification. Electron
microscopes work by emitting parallel beams of electrons onto the
sample being observed. The high magnification capabilities of
electron microscopes provide high resolution images, meaning that
the tiniest ultrastructures can be seen. Fixation is very important in
this type of microscopy, and glutaraldehyde is normally used as the
fixative agent. There are two types of electron microscopy;
transmission electron microscopy, and scanning electron microscopy.
Electron microscope

Transmission Electron Microscopy

Transmission electron microscopy uses very thin sections of tissue.
Emitted electrons are absorbed by some parts of the tissue and pass
through others. The electrons that penetrate through the tissue
sample hit a phosphorescent screen, forming an image. Staining the
tissues with chemicals like osmium tetroxide can be used to
emphasise the differences in electron uptake of the different
structures being visualised.

Scanning Electron Microscopy

Scanning electron microscopy does not require thin sections, instead

it uses larger pieces of tissue. One key benefit of this type of
microscopy is that it can produce 3-dimensional images. To prepare
the tissue, it must first be fixed, before being dried and coated with
gold. The specimen is then scanned by a beam of electrons, which
creates a 3-dimensional image of the surface structure.

In Situ Hybridisation
In situ hybridization is a technique used to label DNA, RNA, and
probes. It enables us to identify the location and density of specific
sequences within a tissue sample. In situ hybridisation uses a probe
which is labelled (using either radioactive, fluorescent, or antigen-
labeled bases), and hybridizes to a target sequence at high
temperatures. Hydrolysis takes place, and then excess probe is
washed away. The labelled probe is localised and quantified using
microscopy, autoradiography, or immunohistochemistry. This
technique is very useful in Neuroscience.

Blotting Technique
Blotting technique, specifically Western Blotting, is a form of
immunostaining technique that uses artificial antibodies to determine
the presence of a specific type of protein within a sample. During a
Western Blot, proteins are separated from one another based on
molecular weight using gel electrophoresis. They are then moved to
a synthetic membrane via blotting. The membrane is then probed
using antibodies that are usually labelled with peroxidase. This
catalyses a chemiluminescent reaction. If the required protein is
present within the sample, this will be shown by a stained band
present on the Western Blot.

Highlights
 Histology is the microscopic study of tissues and cells used in understanding the
pathogenesis and diagnosis of various diseases.
 Cells are the tiny living units that form the tissues, organs and structures within the
body. All cells contain cytoplasm, are surrounded by a membrane, and contain a
variety of structures and organelles.
 Body tissues are collections of cells, grouped in the body according to structure and
function. They are separated into four categories: muscular, nervous, epithelial and
connective.
 Tissues are routinely visualised using microscopy. Light microscopes (or optical
microscopes), use a combination of visible light and lenses to create a magnified
image. In contrast, electron microscopes work by emitting parallel beams of electrons
onto the sample being observed, resulting in higher resolutions.
 As cells are generally colourless, they need to be stained so that they can be easily
viewed under the microscope. The four main types of stains used in histology are
empirical, histochemical, enzyme histochemical and immunohistochemical.

Introduction to Histology - want to learn even more

about it?
Our engaging videos, interactive quizzes, in-depth articles and HD atlas are
here to get you top results faster.
Sign up for your free Kenhub account today and join over 774,000 successful
anatomy students.

CREATE YOUR FREE ACCOUNT

“I would honestly say that Kenhub cut my study time in half.” – Read more.