Epithelial Tissue

 

Structure

• Cells are closely packed together

• No blood vessels within the epithelial tissue

• Always found next to connective tissue which does have blood vessels

• Found in the lining of body organs and on the body surface

• Classified by the number of layers and shape of cells

• The lowest or bottom layer of cells is attached to a basement membrane for support and connection.

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(Image by OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons)

 

Epithelial cells arranged in one layer are known as simple epithelial tissue. More than one layer of cells is known as stratified or compound epithelial tissue.

 

Epithelial tissue is also classified by the shapes of the cells. There are three different shaped cells; squamous, cuboidal and columnar.

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The principal function of simple epithelia is to allow materials to pass through via diffusion or osmosis.

The principal function of stratified or compound epithelia is to protect deeper structures and to hamper the passage of materials. 

 

Simple Squamous Epithelium

Structure

Single-layer of flat scale-like cells

The nucleus forms a lump in the centre

Can be rolled into a tube shape

Function

Ideally designed for allowing materials to pass through e.g. diffusion and osmosis because they are so thin

Location

Alveoli of the lungs

Capillary walls

Bowman's capsules of nephrons

 

Simple Cuboidal Epithelium

Structure

Single-layer of cube-like cells

Spherical nuclei

Can be rolled into a tube

Function

They often line ducts and tubes and can allow materials to pass through similar to squamous cells. They are involved with secretion (the production and release of fluid by cells) and absorption (the intake of fluids or other substances by cells of the skin or mucous membranes)

Location

Found in the glands of the body e.g. thyroid gland

Renal tubules of the kidneys

Sweat ducts

 

Simple Columnar Epithelium

Structure and Function

• Single-layer of rectangular-shaped cells

• Slightly oval nuclei

• Can be modified to include microvilli (micro projections from cells that increase surface area for absorption), cilia (microscopic hair-like organelles that aid movement of particles e.g. dust in airways), goblet cells (cells that secrete mucous)

Location

• The lining of the stomach (modified with goblet cells)

• The lining of intestines (modified with microvilli and goblet cells)

• The lining of fallopian tubes (modified with cilia)

• The lining of the trachea and bronchi (modified with cilia and goblet cells)

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Stratified/compound Squamous Epithelium

Structure and Function

• Consists of multiple layers of squamous cells 

• More durable and provides protection from friction between the tissues and environment

• Two subtypes; non-keratinising and keratinising

• Keratinising stratified squamous epithelium consists of a dead layer of cells on the top surface, and contains a brown substance called melanin

• Melanin protects the body form harmful UV rays and the amount found in the tissue is genetically determined

• Exposure to sunlight will increase the amount of melanin in the tissue to provide further protection. This is known as tanning.

• Non-keratinised tissue has no layer of dead cells or melanin. All cells are alive because they are surrounded by fluid.

Location 

• Keratinised stratified squamous epithelium is found in the outer epidermis of the skin

• Non-keratinised stratified squamous epithelium is found on the lining of wet surfaces e.g. mouth, vagina, oesophagus

 

Stratified/Compound Cuboidal and Columnar Epithelial Tissue

• Stratified cuboidal and columnar epithelial tissue is quite rare. 

• The function of stratified cuboidal tissue is primarily the protection of larger ducts and is found the sweat glands in the skin, the mammary glands in the breast and salivary glands in the mouth

• The function of stratified columnar tissue is primarily protection and secretion and is found in the male urethra, some large excretory glands e.g. milk glands