Epithelium

Learning Objectives

After reading this post you will be able to:

• Define epithelial tissue and describe its characteristic features .
• Classify epithelial tissue.
• State the function of each type of epithelium.
• Identify surface modifications of epithelia and state the function of each modification.
• Identify all types of epithelia.

Q. Define Epithelium.

A. Epithelium is a sheet of cells arranged in single or multiple layers that covers the external surface of solid structures and lines the internal surface of hollow viscera and body cavities.

Q. Where will you find epithelium?

A. Epithelial tissue

• covers the surface of body
• lines body cavities, digestive tract , urinary tract, respiratory tract and blood vessels.
• Secretory/Glandular epithelium: Epithelial tissue cells also make up glands of the body (formed by invagination of surface epithelium).

Q. List the characteristic features of epithelial tissue.

A. Characteristic features of epithelial tissue

• Are composed almost of only cells with very little intercellular tissue.
• The Basal surface of cells rests on a basement membrane.
• The lateral surface of cells are adherent to each other via junctional complexes.
• The apical surface of cells may show surface modification ( microvilli, cilia, sterocilia).
• There are no blood vessels and lymphatics in epithelial tissue.
• They have good regenerative capacity.

Q. What are the functions of epithelia?

A. Functions of epithelia are:

• Protection and barrier: protect underlying tissue from mechanical injury, harmful chemicals and pathogens and excessive water loss.
• Diffusion: allows diffusion of gases, liquids and nutrients.
• Absorption: epithelial cells lining the small intestine absorb nutrients.
• Secretion: glandular epithelium is responsible for secretion of enzymes, hormones and lubricating fluids.
• Sensory perception: Sensory stimuli are detected by specialized epithelial cells ( taste buds, retina, olfactory epithelium).
• Contracitilty: Myoepithelial cells .

Q. Classify epithelium.

A. Classification of Epithelium is :

a. Based on number of layers of cells

b. Based on shape of cells

Q. Write the identifying features, function of simple epithelia and list some sites where they are located.

A.

Q. Describe briefly pseudostratified  columnar epithelium.

A. Pseudostrtified columnar epithelium  is an epithelium that gives an appearance of being stratified because the cell nuclei are at different levels, but in which all cells reach the basement membrane, hence it is a type of  simple epithelium.

• Ciliated pseudostratified columnar epithelium is  found in the linings of the  nasal cavity, larynx, trachea and bronchi. Cilia are motile and help  keeping repiratory  passage free of mucus or foreign particles by propelling them towards the exterior openings.
• Pseudostratified columnar epithelia with stereocilia are located in the epididymis of male reproductive tract. Stereocilia are non motile .They are structurally similar to microvilli  and help in absorption.

Q.. Write the function of stratified epithelia and list some sites where they are located.

A

Q. What is metaplasis? Explain with examples.

A. Metaplasia is a  change that occurs in the epithelial lining in response to chronic physical or chemical irritation (pathological metaplasia) so as to adopt to the physiological or pathological stresses.

Examples:  

1. Pathological irritation by cigarette smoke  causes the pseudostratified ciliated columnar epithelium of the airways to be replaced by stratified squamous epithelium.
2. A stone in the bile duct may  cause the replacement of the secretory columnar epithelium with stratified squamous epithelium.
3. Barrett’s oesophagus: In the  lower part of the  esophagus the normal  stratified squamous epithelium is replaced by simple columnar epithelium with  goblet cell ( found in intestine).The main cause of Barrett’s esophagus is an adaptation to chronic acid exposure from reflux from stomach.