Epithelium
Definition
A type of tissue covering the body and lining cavities and tubes.
Also found in glands
Structure
Cells are closely packed
Intercellular substance is minimal
Cells lie on a basement membrane
The tissue may be a single layer of cells or multi-layered (stratified)
Functions
Protection of the structures covered
Secretion
Absorption
Simple epithelium
Single layer of identical cells
Found on absorptive or secretory surfaces
Not on areas of stress
Named according to the shape
Shape depends on the function
The more active the tissue the taller the cells
4 types
Squamous (pavement) epithelium
Single layer of flattened cells
Cells fit closely
Forms a thin and smooth and inactive membrane
May be called endothelium
It lines : -
heart
blood vessels
lymph vessels
alveoli of the lungs
Cuboidal (cubical) epithelium
Cube shaped
Lying on basement membrane
Forms tubules of the kidney
Found in some glands
Function : secretion, absorption and excretion
Columnar epithelium
Single layer
rectangular
basement membrane +
Found in organs of GIT
A mixture of cell found : some absorb products
of digestion some secrete mucus (mucus : goblet cells)
Ciliated columnar epithelium
Have fine hair like processes called cilia
Wave like movement of the cilia propels the
contents of the tubes which they line in one direction
found in
uterine tubes - ova propelled
respiratory passages - mucus propelled
Stratified epithelium
Stratified epithelia
Many layers
Cells of different shapes
Superficial cells grow up from below
Basement membrane usually absent
The main function of compound epithelium is protection
2 main types : stratified and transitional
Stratified squamous epithelium
Many layers of cells
Different shapes
Deepest layers are columnar
Grow towards the surface and become flattened
Non-keratinised stratified epithelium
Found on wer surfaces e.g. conjunctiva of the eye,
lining of the mouth, the pharynx, the oesophagus, the vagina
Keratinised stratified epithelium
Found on dry surfaces
e.g. skin, hair and nails
Surface layer consists of dead cells to which the protein keratin has been added. This forms a tough, relatively waterproof protective layer that prevents drying of the underlying live cells. The surface layer of skin is rubbed off and is replaced from below.
Transitional epithelium
Several layers of pear-shaped cells and is found
lining the urinary bladder.
It allows for stretching as the bladder fills.
