Introduction to Histology

Epithelial tissue

Dr. Hersh Abdul Ham-KarimBVM&S, PG Dip, MSc and PhD

▪ The human body consists of two basic components: cells and products of

cells (extracellular materials). The discipline of histology is concerned

primarily with the microscopic examination of these two components and

how they are organized into the various tissues of the body.

▪ The aim of this course is to provide students with the skills and knowledge

needed to identify and describe the microscopic anatomy of various

organs, with attention to the arrangement of the four primary tissues

(epithelium, connective tissue, muscle, nerve) within each organ.

Course Description and Objectives

▪ Histology (microscopic anatomy) is the study of the microscopic structures of

cells and tissues of plants and animals.

▪ The Greek root histo can be translated as either "tissue" or "web" and both

translations are appropriate because most tissues are webs of interwoven

filaments and fibers, both cellular and noncellular, with membranous linings.

▪ It is often carried out by examining a thin slice (called a "section") of tissue

under a light microscope or an electron microscope.

▪ In order to distinguish different biological structures more easily and accurately

histological stains are often used to distinguish between the different types of

biological material within the tiny sample on the slide

▪ Education: Histology slides are often used in teaching laboratories to

help students learn about the microstructures of human (and animal)

biological tissues.

▪ Diagnosis for treatment : Biological tissue samples taken from a patient

to understand its causes and make recommendations for treatment or

management of the condition.

▪ Forensic Investigations: to clarify the cause of sudden unexpected deaths

and other issues in forensic science.

▪ Archaeology: Study of biological cells and tissues recovered from

archaeological sites can provide information about history.

Here is some stuff you should know about histology before we get started.

Tissue comes to the lab fresh or in formalin

Fresh lung specimen Formalin container for specimen

▪ The most common procedure used in the study of tissues is the

preparation of histological sections or tissue slices that can be studied

with the aid of the light microscope.

▪ Under the light microscope, tissues are examined via a light beam that is

transmitted through the tissue.

▪ Because tissues and organs are usually too thick for light to pass through

them, they must be sectioned to obtain thin, translucent sections and then

attached to glass slides before they can be examined.

▪ The ideal microscope tissue preparation should be preserved so that the

tissue on the slide has the same structure and molecular composition as

it had in the body.

▪ However, as a practical matter this is seldom feasible and artefacts,

distortions, and loss of components due to the preparation process are

almost always present.

▪ Fixation: Samples of biological tissues are "fixed" to preserve the

cells/tissue and prevent postmortem decay.

• It can be done by chemical method: the tissues are usually immersed

in solution of stabilizing agents called fixatives (e.g. Formaldehyde and

glutaraldehyde). The tissues are usually cut into small fragments before

fixation to facilitate the penetration of the fixative and to gurantee

preservation of the tissue.

• Or, less frequently, physical method

o Heating and Freezing

▪ Processing (dehydration): Tissue processing is done to remove water from the

biological tissues, replacing such water with a medium (alcohol) that solidifies,

setting very hard and so allowing extremely thin sections to be sliced.

▪ The tissues are passed through a series of progressively more concentrated

alcohol baths. Concentration of first alcohol bath depends on the fixative and

size and type of the tissue, e.g. delicate tissue needs lower concentration of

alcohol and smaller interval between two strengths of alcohol.

▪ The tissue must not be dehydrated rapidly because this will cause distortion of

the tissue.

▪ Usually 70% alcohol is employed as the first solution and100% as the last

solution.

▪ Infiltration and Embedding: Tissues are placed

in paraffin waxes. The tissue and paraffin attached

to the cassette, which is ready for sectioning.

▪ Sectioning: Sectioning an embedded tissue

sample is the step necessary to produce

sufficiently thin slices of sample that the detail of

the microstructure of the cells/tissue can be

clearly observed.

• In the case of using light microscopy, a

steel knife mounted in a microtome

may be used to cut 10μm tissue

sections.

• In the case of using electron

microscopy, a diamond knife mounted

in an ultramicrotome may be used to

cut 50 nm tissue sections.

Sectioning

▪ Mounting: In this procedure, the sections are permanently attached to

microscope slides.

▪ Clearing: Before a section can be stained the

paraffin must be removed, a process called clearing.

After clearing, only the tissue remains adhering to

the slide. Clearing is accomplished by passing the

mounted sections through the solvent Clearene

(Surgipath/Leica) that dissolves the paraffin

▪ Staining: Finally, tissue sections treated with an appropriate

histology stain.

- Variable stains are used for specific tissues

• Heamatoxylin and Eosin are most common used.

• Trypan blue stain (for living tissues)

• Geimsa stain (for blood smear)

• Histo-chemical stain (Staining the different chemical

components of the cell)

• Immuno-histo-chemical stain (Localization and staining

specific proteins by the antigen antibody reaction)

Hematoxylin and Eosin (H&E)

- standard histology stain and the most frequently used combination of stains

used in the histology lab for routine tissue preparation.

- Basophilic components stained by basic dye (H); purple-blue [Nucleic

acid]

- Acidophilic components stained by acidic dye (E); pink [cytoplasm &

collagenous fiber].

Levels of structural organization

▪ In complex organisms, cells group together with one another based on

similar structure and function to form tissues.

Three major germ layers that form the embryonic disc (source of stem cells)

▪ Endoderm

• Forms lining of digestive tract, forms the lining of the urinary and

respiratory tracts, as well as other organs and glands (for example, the

pancreas, liver, thyroid gland, and parathyroid glands)

▪ Mesoderm

• Form muscle; bone and other connective tissue; and various organs,

including the heart, kidneys, ovaries, and testes.

▪ Ectoderm

• Forms skin and its derivatives such as hair, nails, oil glands, sweat

glands, and mammary glands.

• Nervous system

▪ The human body is composed of four basic types of tissues;

• Epithelium

• Connective

• Muscular

• Nervous tissues.

▪ These tissues vary on;

• structure of cells

• composition of extracellular matrix

• cell function.

▪ So how are these tissue types different? Let's zoom in on each one to

better understand.

▪ The epithelial tissue, or epithelium, consists of sheets of cells that cover the

external surfaces of the body, line the internal cavities, form various organs

and glands, and line their ducts.

They are typically classified into three categories;

1. Covering or membranous epithelia:

Are simply defined as those epithelia that cover and line all body surfaces. They are

also the varieties that are given specific names based largely on their layers and cell

shape.

2. Glandular epithelia:

Represent all the varieties of epithelia that are specialized for secretion,

which is the process by which cells release specific substances onto their

apical surfaces.

3. Special epithelia:

May also be classified as surface or glandular, but have specialized functions.

Examples of these include neuroepithelium (e.g., olfactory epithelium),

reproductive epithelium (e.g., seminiferous epithelium), and myoepithelium

(epithelial cells specialized for contraction).

▪ Cilia: Hair-like appendages attached to the

apical surface of cells that act as sensory

structures or to produce movement.

▪ Villi: Finger-like projections that arise from

the epithelial layer in some organs. They

help to increase surface area allowing for

faster and more efficient adsorption.

▪ Microvilli: are small, nonmotile projections

that cover all absorptive cells in the small

intestine and proximal convoluted tubules

in the kidney.

1. Epithelial tissues are composed only of cells.

2. No extracellular matrix.

3. Cells are connected to each other by intercellular junctions and form layer.

4. One side of the epithelial cell is oriented towards the surface of the tissue,

body cavity, or external environment and the other surface is joined to a

basement membrane.

5. Epithelia is avascular but innervated in nature.

6. The epithelial cells are nourished by substances diffusing from blood vessels

in the underlying connective tissue.

7. Epithelia have high ability to regeneration because of stem cells.

➢ Protection; Epithelial cells from the skin protect underlying tissue from

mechanical injury, harmful chemicals, invading bacteria and from excessive

loss of water.

➢ Sensation; Sensory stimuli penetrate specialized epithelial cells. Specialized

epithelial tissue containing sensory nerve endings is found in the skin, eyes,

ears, nose and on the tongue.

➢ Secretion; In glands, epithelial tissue is specialized to secrete specific chemical

substances such as enzymes, hormones and lubricating fluids.

➢ Absorption; Certain epithelial cells lining the small intestine absorb nutrients

from the digestion of food.

➢ Excretion; Epithelial tissues in the kidney excrete waste products from the

body. Sweat is also excreted from the body by epithelial cells in the sweat

glands.

➢ Diffusion; Simple epithelium promotes the diffusion of gases, liquids and

nutrients. Because they form such a thin lining, they are ideal for the diffusion

of gases (e.g. walls of capillaries and lungs).

➢ Cleaning; Ciliated epithelium assists in removing dust particles and foreign

bodies which have entered the air passages.

➢ Reduces Friction; The smooth, tightly-interlocking, epithelial cells that line

the entire circulatory system reduce friction between the blood and the walls

of the blood vessels.

Simple epithelium:

- Being composed of one layer of cells only,

they are very thin.

- Their main function is to allow passage of

substances between the lumen and the

surrounding tissues.

Stratified epithelium:

- Being composed of several layers of cells,

they are very thick.

- Their main function is to protect the tissues

that they cover.

- The shape of the cells closest to the

basement membrane is quite different from

that of the cells at the top, near the lumen.

1- The number of cell layers

2- The shape of the cells

▪ Squamous – cells wider than tall (plate or

“scale” like)

▪ Cuboidal – cells are as wide as tall, as in

cubes

▪ Columnar – cells are taller than they are

wide, like columns

1- Squamous epithelium

- Squamous cells have the appearance

of thin, with flattened nuclei.

- They form the lining of cavities such

as the kidney, blood vessels, and

lungs.

- Function: Diffusion

Simple epithelium can be subdivided according to the shape and function of

its cells.

nucleus

cytoplasm

2- Cuboidal epithelium.

- As their name implies, cuboidal cells

are roughly square or cuboidal in

shape with central rounded nuclei.

- Present in Kidney, ovary, testes,

glands and ducts of glands.

- Function: secretion, absorption,

produce egg and sperm.

nucleus

cytoplasm

3- Columnar epithelium.

- The cells are elongated and column-

shaped with basal oval nuclei.

- Present in highly absorptive surfaces

(intestine); secretory surfaces (stomach);

lining gall bladder (absorbs water).

- Some columnar cells are specialized for

sensory reception such as in the nose, ears

and the taste buds of the tongue.

- They secrete mucus or slime, a lubricating

substance which keeps the surface smooth.

nucleus

cytoplasm

4- Ciliated columnar epithelium.

- These are simple columnar epithelial cells,

but in addition, they posses fine hair-like

outgrowths, cilia on their free surfaces.

- These cilia are capable of rapid, rhythmic,

wavelike beatings in a certain direction.

- Present in the air passages like the nose and

in female reproductive tract (fallopian tube,

uterus).

nucleus

cytoplasm

cilia

5- Pseudostratified columnar epithelium

- All cells are attached to the basal lamina,

but not all cells reach the free surface

(short basal, tall columnar)

- Their nuclei located at different levels so

it appear as stratified epithelium.

- Present in several locations, but two

important examples occur in the

respiratory and urinary systems.

cytoplasm

nucleus

Goblet cellshort basal

tall columnar

- Stratified squamous keratinizing: Upper cell layer composed of squamous sells.

Present in surface of skin

- Stratified squamous non-keratinizing: Present in oral cavity, pharynx, oesophagus,

anal canal, uterine cervix, and vagina.

nucleus

cytoplasm

Keratin layer

nucleus

cytoplasm

- The stratified epithelia are further classified according to the shape of the cells at

the free surface.

- Stratified cuboidal: Upper cell layer composed of cuboidal cells. Only 2

to 3 cell layers; lining the ducts of glands (e.g. salivary gland).

nucleus

cytoplasm

nucleus

cytoplasm

Columnar cell

Cuboidal cell

- Stratified columnar epithelium: consists of two or more layers and exhibits

columnar surface cells. Examples of this variety can be found in portions of the

male urethra, and also in the larger ducts of glands.

- Transitional epithelium:

• Many layers of cells, but surface

cells change from rounded to flat.

• Line the urinary bladder, renal

pelvis, and ureters – permits

expansion and recoils after

stretching.

• Function: Protective, stretch and

relax.

Relaxed

Distended