Histology – the study of tissue structure
Tissue – Group of cell with similar structure and function
Epithelial tissue or epithelium (plural epithelia)
*Epi “on”
*Thele “covering or lining”
Covers the external and internal surface throughout the body
Most epithelia have:
Free Surface – not in contact with other cells
Basal Surface – adjacent to a basement membrane attaching the epithelial cells to underlying tissues
Basement Membrane – secreted partly by epithetlial cells and partly by the cells of underlying tissues
Functions of epithelia
*Protect the underlying structure
*Act as Barrier
*Permit passage of substance
*Secrete substances
*Absorb substances
Classification of Epithelia
-According to the number of cell layers and shape of the cells
According to the number of cell layers:
*Simple epithelium - Consist of single layer of cells
*Stratified epithelium – Consist of more than one layer of epithelial cells, with some cells sitting
on top of others
According to the shape of cells:
*Squamous – Flat
*Cuboidal – Cubelike
*Columnar – Tall and Thin
Simple Squamous Epithelium
Single layer, flat, often hexagonal cells
Nuclei appear as bumps because cells are so flat
Functions: diffusion, filtration, some secretion, some protection against friction
Location: lining of blood vessels of heart, lymphatic vessels, alveoli of the lungs, portions of
kidney tubules, lining of serous membranes of body cavities
Simple Cuboidal Epithelium
Single layer of cube-shaped cells
Some cells have microvilli or cilia
Function: active transport and facilitated diffusion result in secretion and absorption by cells of
kidney tubules
Function: Secretion of cells of glands and choroid plexuses
Function: Movement of particles embedded in mucus out of the terminal bronchioles by ciliated
cells
Simple Columnar Epithelium
Single layer of tall, narrow cells
Some cells have cilia or microvilli
Function: movement of particles out of the bronchioles of the lungs by ciliated cells
Function: partially responsible for the movement of oocytes through the uterine tubules of
ciliated cells
Function: secretion by cells of the glands, the stomach and intestine
Function: absorption by cells of the intestine
Location: glands and some ducts, bronchioles of the lungs, auditory tubes, uterus, uterine tubes,
stomach, intestines, gallbladder, bile ducts and ventricles of the brain
PseudoStratified Columnar epithelium
Single layer of cells
Some cells are tall and thin and reach the free surface, and others do not
Nuclei of these cells are at different levels and appear stratified
Cells are almost always ciliated and are associated with goblet cells that secrete mucus
Single layer of cells
Some cells are tall and thin and reach the free surface, and others do not
Nuclei of these cells are at different levels and appear stratified
Cells are almost always ciliated and are associated with goblet cells that secrete mucus
Location: Lining of nasal cavity, nasal sinuses, auditory tubes, pharynx, trachea and bronchi of
the lungs
Stratified Squamous Epithelium
Several layers of cells that are cuboidal in the basal layer and progressively flattened toward the
surface
Function: protects against abrasion, forms a barrier against infection and reduces water loss
from the body
Location: keratinized – outer layer of the skin
Location: non-keratinized – mouth, throat, larynx, esophagus, anus, vagina, inferior urethra and
corneas
Transitional Epithelium
Stratified cells that appear cuboidal when the organ or tube is not stretched and squamous
when the organ or tube is stretched by fluid
Function: accommodates fluctuations in the volume of fluid in an organ or a tube
Function: protects against the caustic effects or urine
Location: lining of urinary bladder, ureters and superior urethra
Stratified Cuboidal Epithelium
Consist of two or three layers of cuboidal cells
It is the epithelium that often lines the larger ducts of some glands such us major salivary
Glands.
Stratified columnar Epithelium
Consist of atleast two layers of columnar cells although sometimes the cells of the deeper layer
are cuboidal
It lines the large ducts of some glands
Surface Modification of Epithelial cells
Most epithelial cells exhibit modifications on their apical(superficial), lateral and/or basal
surface that are necessary for the discharge of their specific functions.
Modification on apical surfaces of epithelial cells
Microvilli
Has short and fine fingerlike extensions or processes of plasma membrane that protrude from
the apical surfaces of the cells
Are not individually distinguishable under the LM, but in cell where they are particularly
numerous such as those that line the small intestine
They form a fuzzy, fine certical line on the surface of the epithelium called “Striated Border”or
“Brush Border”
Coated on their outer surface by glycocalyx.
Core of microvillus is formed by a network of actin filaments
Cilia (Kinocilia)
Present in the apical surfaces of the cells that are specialized for transport of fluid or mucus over
the surface of epithelium
The core (Axoneme) of cilium consist of microtubules that run parallel to its long axis.
Flagella
Are simply long cilia.
In human, only one cell type possesses a flagellum,The Spermatozoon. Its has only one
flagellium which is otherwise called Tail
Stereocilia
Are simply microvilli that are as long as cilia.
They are non-motile and their core consist of actin filaments
Are characteristic of the epithelial cells that line the ductus (Vas) epididymis and ductus
deferens, long tubes that help convey sperm cells from the testes to the external environment
Also present on the hair cells of the inner ear where they play a role in auditory and vestibular
perception
