Khaleel Alyahya
Monday March 22, 2009

There are only 4 basic types of tissues
1. Epithelial tissue
2. Connective tissue
3. Muscular tissue
4. Nervous tissue

is a tissue composed of cells that line the cavities and surfaces of structures throughout the body

 Epithelial cells are closely packed cells, and form continuous sheets.
 Adjacent cells are bound together by cell junctions.
 They have one free surface (apical).

 The lower surface of all types of epithelia rest on
Basement membrane /Basal lamina , a structureless material secreted by the cells.
 Epithelial tissues are avascular ( no direct blood supply ). Nutrition depends on diffusion from underlying connective tissue.
 Epithelial cells can regenerate easily.

 Protection
 Absorption
 Secretion
 Exchange of gases, nutrients, & waste products

The epithelium is classified according to:
1) the number of layers.
2) the shape of the outermost layer.

 The first & second names are combined to describe the epithelium fully.
 Stratified epithelia are described according to the shape of their superficial cells.
 Thus, depending on the number of cell layers, the epithelia is divided into:
I.
Simple Epithelium
Consisting of a single layer of cells
II.
Stratified Epithelium
Consisting of two or more layers of cells

Simple Epithelia :
1) Simple Squamous Epithelium
 Composed of a single layer of flattened cells.
 The term ‘Squamous’ is derived from the comparison of the cells to the scales of a fish.
 Examples:
Blood capillaries
Pulmonary alveoli

Simple squamous epithelium

Simple Epithelia :
2) Simple Cuboidal Epithelium
 Consists of a single layer of square shaped cells
 Centrally placed nucleus
 Examples:
Kidney tubules
Thyroid gland
Surface of ovaries

Kidney tubule
Thyroid gland

Simple Epithelia :
3) Simple Columnar Epithelium
 Single layer of tall column-like cells
 Goblet cells (mucus producing cells) may be seen in this type of epithelium
 Its has ovoid nuclei located in basal half of the cells
 Examples:
Stomach and intestines

Simple Epithelia :
4) Pseudostratified Columnar Epithelium
 Single layer of cells but gives false impression of more than one layer of cells. (stratified)
 There are tall cells which reach the surface and shorter ones that don’t
 All cells rest on the same basement membrane
 The nuclei show different levels of height
 The higher cells may be ciliated.
 Goblet cells are seen in respiratory epithelium
 Example:
Trachea

Pseudostratified Columnar Epithelium

Stratified Epithelia:
 Consists of two or more cell layers
 Their main function is protection.

Stratified Epithelia:
1) Stratified Squamous Epithelium
 Stacked epithelial cells with the surface cells being flat
 The cells at the base are cuboidal or columnar
 The basal and intermediate cells maintain the ability to divide
 Cells at the surface are dead and lack nuclei

Stratified squamous ( Non-keratinized) epithelium
 Is formed of several layers of cells
 The most basal layer rests on basal lamina and it is cuboidal cells
 The most superficial layer is formed of flat squamous cells not covered with keratin
 Example: lining of mouth, oral pharynx, esophagus, true vocal cords and vagina)

Stratified Squamous ( Non-keratinized) Epithelium

Stratified Squamous Epithelium (Non-keratinized)

Stratified Squamous Epithelium (Non-keratinized)

Stratified squamous (Keratinized) epithelium
 similar to non keratinized type but the superficial layer is covered with keratin
 Example: epidermis of skin

Stratified Epithelia:
2) Stratified Cuboidal & Columnar Epithelium
 Usually have two cell layers with surface cells cuboidal or columnar
 The basal cells vary in size & shape
 Examples:
Ducts of the large exocrine glands such as salivary glands
(columnar), sweat glands (cuboidal).

Stratified Cuboidal

Stratified Epithelia:

3)Transitional Epithelium:
Is a modified type of ‘Stratified Squamous epithelium
 It is present in the urinary system organs


In the relaxed state, it shows 5-8 layers
Basal cells are cuboidal or columnar and surface cells are rounded


When the epithelium is stretched, it appears 2-3 layers thick and the surface cells appear flattened
Example:
Urinary bladder
Ureters

Transitional Epithelium (relaxed)

Transitional Epithelium ( full)

Simple Epithelium
Simple Squamous Epithelium
Simple Cuboidal Epithelium
Simple Columnar Epithelium
Pseudostratified Columnar Epithelium
Stratified Epithelium
Stratified Squamous Epithelium
Non-keratinized
Keratinized
Stratified Cuboidal Epithelium
Stratified Columnar Epithelium
Transitional Epithelium

Glandular Epithelium:
 The gland consists of one cell (unicellular gland like
Goblet cells), or more than one cell (multicellular gland like pancreas & salivary glands)
 Classification of glands:
1) Exocrine Glands:
They retain their ducts and secrete their product through them, e.g. pancreas & salivary glands.
2) Endocrine Glands:
They lose their ducts and secrete their products directly into the blood, e.g. pituitary & thyroid glands.