This document provides an overview of histology and tissue preparation techniques. It discusses the following key points: - Histology is the study of tissues and how cells are organized into tissues and organs. The four main types of tissues are epithelial, connective, nervous, and muscular tissue. - Tissue samples are obtained through biopsies and prepared through a process including fixation, dehydration, clearing, embedding in paraffin wax or resin, sectioning with a microtome, and staining. Special staining techniques can identify structures like glycogen or calcium. - Histochemistry uses chemical reactions to identify certain molecules in tissues, while immunohistochemistry uses labeled antibodies to identify antigens within tissues under a microscope.

1. Introduction to Histology
I. Introduction
II. Methods
I. Tissue
preparation
II. Observation
III. Histochemistry

2. Histology
 The study of the organization of cells and
extra-cellular material into tissues and
organs.

3. Tissues
 A group of similar cells that usually has a
common embryonic origin and is
specialized for a particular function.

4. Types of Tissues
 Epithelial Tissue
 Covers body surfaces, lines cavities; glands
 Connective Tissue
 Supporting tissue
 Nervous Tissue
 Excitability
 Conductivity
 Muscular Tissue
 Contractility

5. Biopsy
 Removal of a sample of living tissue for
microscopic examination.
Used to diagnose cancers, infections, etc

6.  How to get tissues for study
 Steps in tissue preparation
 Fresh tissues from the body
 1. fixation
 Formalin ( 10% formaldehyde)
 Osmium tetroxide for EM
 Mechanism - Forms cross links with proteins (Lysine)
 2. Embedding – gives support for tissue slicing
 Paraffin or plastic resin
 3. Washing & dehydration (dehydration by graded alcohols in ascending
order)
 4. clearing – to remove paraffin & alcohol
 By xylol or tulol
 5. block making
How to generate histology slides?

7.  6. section cutting – 5-10μ thick sections with microtome
 7. mounting – on glass slide ( adhesive – albumin)
 8. clearing – xylol / tulol
 9. rehydrate – alcohols in descending order
 Staining
 nuclear stain – Hematoxylin ( basic stain & water soluble)
 counter stain – Eosin ( less water soluble but soluble in alcohol)
– dehydrate in ascending order
 10. Clearing – xylol / tulol
 11.Mounting medium – cover glass
How to generate histology slides?

8. TISSUE FIXATION
 Fixation is a complex series of chemical events that differ for the
different groups of substance found in tissues.
 The 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 lost.
 Fixation is coming by reaction between the fixative and protein
which form a gel, so keeping every thing as their in vivo relation to
each other.

9. Factors affect fixation:
- PH.
- Temperature.
- Penetration of fixative.
- Volume of tissue.
According to previous factors we can determine the concentration of
fixative and fixation time.
Types of fixative:
Acetic acid, Formaldehyde, Ethanol, Glutaraldehyde, Methanol and
Picric acid.

10. TISSUE PROCESSING
the aim of tissue processing 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:
1- Dehydration.
2- Clearing.
3- Embedding.

11. Dehydration
to remove fixative and water from the tissue and replace
them with dehydrating fluid.
There are a variety of compounds many of which are
alcohols. several are hydrophilic so attract water from
tissue.
 To minimize tissue distortion from diffusion currents,
delicate specimens are dehydrated in a graded ethanol
series from water through 10%-20%-50%-95%-100%
ethanol.
 In the paraffin wax method, following any necessary post
fixation treatment, dehydration from aqueous fixatives is
usually initiated in 60%-70% ethanol, progressing
through 90%-95% ethanol, then two or three changes of
absolute ethanol before proceeding to the clearing stage.

12. Types of dehydrating agents:
Ethanol, Methanol, Acetone.
 Duration of dehydration should be kept to the minimum consistent
with the tissues being processed. Tissue blocks 1 mm thick should
receive up to 30 minutes in each alcohol, blocks 5 mm thick require
up to 90 minutes or longer in each change. Tissues may be held and
stored indefinitely in 70% ethanol without harm

13. Clearing
 replacing the dehydrating fluid with a fluid that is totally miscible with
both the dehydrating fluid and the embedding medium.
 Choice of a clearing agent depends upon the
following:
- The type of tissues to be processed, and the type of processing to
be undertaken.
- The processor system to be used.
- Intended processing conditions such as temperature, vacuum and
pressure.
- Safety factors.
- Cost and convenience.
- Speedy removal of dehydrating agent .
- Ease of removal by molten paraffin wax .
- Minimal tissue damage .

14.  Some clearing agents:
- Xylene.
- Toluene.
- Chloroform.
- Benzene.
- Petrol.

15. Embedding
 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 sectioning.
 Paraffin wax
properties :
 Paraffin wax is a polycrystalline mixture of solid hydrocarbons
produced during the refining of coal and mineral oils. It is about two
thirds the density and slightly more elastic than dried protein.
Paraffin wax is traditionally marketed by its melting points which
range from 39°C to 68°C.
 The properties of paraffin wax are improved for histological
purposes by the inclusion of substances added alone or in
combination to the wax:
- improve ribboning.
- increase hardness.
- decrease melting point
- improve adhesion between specimen and wax

16. Precaution while embedding in wax
 The wax is clear of clearing agent.
 No dust particles must be present.
 Immediately after tissue embedding, the wax must be rapidly cooled
to reduce the wax crystal size.

17. Tissue processing
Embedding moulds:
(A) paper boat;
(B) metal bot mould;
(C) Dimmock embedding mould;
(D) Peel-a-way disposable mould;
(E) base mould used with embedding
ring ( F) or cassette bases (G)

18.  General Embedding Procedure
1- Open the tissue cassette, check against worksheet entry to ensure the
correct number of tissue pieces are present.
2- Select the mould, there should be sufficient room for the tissue with
allowance for at least a 2 mm surrounding margin of wax.
3- Fill the mould with paraffin wax.
4 Using warm forceps select the tissue, taking care that it does not cool in
the air; at the same time.
5- Chill the mould on the cold plate, orienting the tissue and firming it into
the wax with warmed forceps. This ensures that the correct orientation is
maintained and the tissue surface to be sectioned is kept flat.
6- Insert the identifying label or place the labeled embedding ring or
cassette base onto the mould.
7- Cool the block on the cold plate, or carefully submerge it under water
when a thin skin has formed over the wax surface.
8- Remove the block from the mould.
9- Cross check block, label and worksheet.

20.  ORIENTATION OF TISSUE IN THE BLOCK
Correct orientation of tissue in a mould is the most important step in
embedding. Incorrect placement of tissues may result in
diagnostically important tissue elements being missed or damaged
during microtomy.
 elongate tissues are placed diagonally across the block
 tubular and walled specimens such as vas deferens, cysts and
gastrointestinal tissues are embedded so as to provide transverse
sections showing all tissue layers
 tissues with an epithelial surface such as skin, are embedded to
provide sections in a plane at right angles to the surface (hairy or
keratinised epithelia are oriented to face the knife diagonally)
 multiple tissue pieces are aligned across the long axis of the mould,
and not placed at random

21. Processing methods and routine
schedules
 Machine processing
 Manual processing

22. CUTTING
 using the microtome

23.  A microtome is a mechanical instrument
used to cut biological specimens into very
thin segments for microscopic
examination. Most microtomes use a steel
blade and are used to prepare sections of
animal or plant tissues for histology. The
most common applications of microtomes
are

24. Microtome knives
 STEEL KNIVES
 NON-CORROSIVE KNIVES FOR
CRYOSTATS
 DISPOSABLE BLADES
 GLASS KNIVES
 DIAMOND KNIVES

25. 1- Traditional histological technique:
tissues are hardened by replacing water with paraffin. The tissue is
then cut in the microtome at thicknesses varying from 2 to 25
micrometers thick. From there the tissue can be mounted on a
microscope slide, stained and examined using a light microscope

26.  2- Cryosection:
 water-rich tissues are hardened by freezing and cut frozen;
sections are stained and examined with a light microscope.
This technique is much faster than traditional histology (5
minutes vs. 16 hours) and are used in operations to achieve a
quick diagnosis. Cryosections can also be used in
immunohistochemistry as freezing tissue does not alter or
mask its chemical composition as much as preserving it with a
fixative.

27. STAIN and MAINTAIN Structural Integrity

28. H & E is a charge-based, general purpose stain. Hematoxylin
stains acidic molecules shades of blue. Eosin stains basic
materials shades of red, pink and orange. H & E stains are
universally used for routine histological examination of tissue
sections.
Hematoxylin and Eosin (H & E)

29. Fixation
Any well fixed tissue.
Staining Procedure
1- Deparaffinize and hydrate to water
2- If sections are Zenker-fixed, remove the mercuric chloride crystals
with iodine and clear with sodium thiosulphate (hypo)
3- Mayer's hematoxylin for 15 minutes
4- Wash in running tap water for 20 minutes
5- Counterstain with eosin from 15 seconds to 2 minutes depending
on the age of the eosin, and the depth of the counterstain desired.
For even staining results dip slides several times before allowing
them to set in the eosin for the desired time
6- Dehydrate in 95% and absolute alcohols, two changes of 2
minutes each or until excess eosin is removed. Check under
microscope
7- Clear in xylene, two changes of 2 minutes each
8- Mount in Permount or Histoclad
Results
Nuclei - blue - with some metachromasia
Cytoplasm - various shades of pink-identifying different tissue
components

30. Renal nephron

33. http://www.meridianinstitute.com/eamt/files/burns2/54burns2.jpg

34. nephrotic range proteinuria-proximal tubule
http://www.gamewood.net/rnet/renalpath/ch1.htm
Pathology

36.  Staining – routine stain – H&E
 Some structures are seen/ preserved (large molecules like
nucleoproteins, cytoskeleton proteins, ECM proteins- collagen,
membrane proteins)
 some are not seen/lost (small molecules -t-RNA, large molecules like
glycogen & Proteioglycans are dissolved, )during the fixation/staining
process
 Special fixatives to retain membrane ( phospholipids)
 Permanganate & osmium – for EM
 For Elastic fibers – Orcein/ Resorcin – Fuscin
 For reticular fibers – Silver impregnation
 Histochemistry & Cytochemistry
 Specific binding of dye with particular molecule
 Fluorescent dye labeled antibody to cell component
 Enzyme activity
 Autoradiography – radio isotopes tagged with precursors of a
molecule  molecule incorporated into cell/ tissue before fixation
Special situations

37. H&E, Hematoxylin and Eosin
•Hematoxylin stains basophilic
structures
•Eosin stains acidophilic
structures

38. Hematoxylin- nuclei; eosin- cytoplasm

40. Gomori trichrome stain
Gomori's one-step trichrome is
a staining procedure that
combines the plasma stain
(chromotrope 2R) and
connective fiber stain (fast
green FCF) in a
phosphotungstic acid solution
to which glacial acetic acid has
been added.
http://freepages.genealogy.rootsweb.ancestry.com/~gomery/gomorigeo.html

41. Trichrome stain (Generally Masson’s) –
To delineate cells from surrounding connective Tissue
www-bioc.rice.edu/bios576/immuno/Trichrome.jpg

42.  Special stain
 PAS positive substances Carbohydrate
(glycogen) or carbohydrate rich molecules,
Basement membrane, reticular fibers
 Periodic acid cleaves bond between carbon atoms 
form aldehyde group
 Aldehyde binds with Schiff to produce magenta or
pink color
PAS =Periodic Acid Schiff

43. Periodic Acid Schiff’s stain (PAS)- glycogen, mucopolysaccharides

45.  Acid hydrolyses or cleaves proteins from
deoxyribose of DNA  leads to opening of
sugar group & formation of aldehyde
 Schiff binds and gives magenta color to
aldehyde
 Can be useful to quantify amount of DNA ( by
using spectrophotmetry of Feulgen stained
tissue)
Feulgen stain for Nuclear Proteins
Why RNA cannot be stained by Feulgen?

46.  For the confirmation of specific substances
 Pretreatment of sections with specific
enzymes
 Diastase/amylase  for glycogen
 DNA ase  for DNA
Enzymatic digestion

47. Figure 1—17.
Photomicrograph of a rat
kidney section treated by
the Gomori method to
demonstrate the enzyme
alkaline phosphatase. The
sites where this enzyme is
present (cell surface) stain
intensely with black
(arrows). Medium
magnification.

48. Three lenses- Compound microscope
1
3
2

50. Figure 1—16. Photomicrograph of
a bone section treated with a
histochemical technique to
demonstrate calcium ions. The
dark precipitate indicates the
presence of calcium phosphate in
calcified bone and cartilage.
Noncalcified cartilage tissue
(stained in pink) is in the upper
portion of the figure. Medium
magnification.

51.  Antibody ( Immunoglobulin) conjugated with
fluorescent dye( most common is Fluorescein)
+ Antigen ( foreign protein)
 Fluorescein  absorbs UV light and emits
green fluorescence  can be seen under
Fluorescent microscope (IF- Immuno
Fluorescence)
 Example :- actin (Antigen) of Rat  infected
to Rabbit  blood of Rabbit ( have poly -
clonal antibodies for Rat’s actin/ anti rat actin
antibodies)  bind with Fluorescent dye
Immuno Histo Chemistry (IHC)

52. Specific antigen
(actin of rat)
Monoclonal Antibodies
Multiple Myeloma pts.
Monoclonal B ells
Hybridoma cells
↓
Single specific type of antibodies (Monoclonal)
( against Actin)
B lymphocytes of
Immunized rabbit
↓

53.  Diagnosis of tumors(tumor markers) &
Infections( HIV, Infectious Mononucleosis)
 Classify sub – types (B -cell and T- cell
lymphomas)
 Treatment – Anti-TNF-α antibodies in
inflammatory disorders
Clinical Significance of Monoclonal
Antibodies

55. Direct Immunocytochemistry

56. Indirect Immunocytochemistry
2nd antibody employed

57. Figure 1—26. Photomicrograph
of a section of small intestine in
which an antibody against the
enzyme lysozyme was applied
to demonstrate lysosomes in
macrophages and Paneth cells.
The brown color results from
the reaction done to show
peroxidase, which was linked to
the secondary antibody. Nuclei
counterstained with
hematoxylin. Medium
magnification.

58. Figure 1—28. Electron micrograph showing a section of a pancreatic acinar cell
that was incubated with anti-amylase antibody and stained by protein A coupled
with gold particles. Protein A has high affinity toward antibody molecules. The
gold particles appear as very small black dots over the mature secretory granules
and the forming granules in the Golgi complex. (Courtesy of M Bendayan.)

59.  Localization of enzymatic activity in tissues
 Best fixation – mild aldehyde ( formalin)
 Basis – localized reaction production of
enzyme activity
 Used for acid & alkaline phosphatase, ATP
ases
 AB (substrate) + T (trap) AT (
reaction product) + B (Hydrolyzed
component of substrate)
Enzyme Histochemistry
enzyme

60. Other Methods
 Hybridization: for localizing
mRNA/DNA (NA)
 In Situ Hybridization: Binding (
Probe + NA) in cell/tissue
 FISH: If Fluorochrome is used
in Hybridization technique
 Autoradiography: by tagging
the precursor molecules (Amino
acids) followed by synthesis of
large molecules (NA)  localize
the particular tagged molecule

61. Orientation of cut

62. Three dimensional picture
How you get it?

63.  3- Electron microscopy:
 after embedding tissues in epoxy resin, a microtome equipped with
a glass or diamond knife is used to cut very thin sections (typically
60 to 100 nanometers). Sections are stained and examined with a
transmission electron microscope. This instrument is often called an
ultramicrotome.

64. Transmission Electron Microscope
TEM

66. TEM
Enteroendocrine cell

67. TEM-eosinophil

68. © 2002 by Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter.

69. Scanning Electron Microscope

70. Scanning electron
microscope