Epithelial Tissue: Function, Types, & FAQs

Animal tissues fall into four categories: epithelial, connective, muscle, nervous and epithelial tissues. Epithelial tissue is the most extensively found tissue as it covers internal and external body surfaces.

The tissue lines the tracts, cavities, vessels, and organs, including the skin. The cells of tissue epithelium have a unique damage repair capacity and are resistant to abrasion. Learn every detail about epithelial tissue, its types, functions, structure, and more.  

What Is Epithelial Tissue?

The tissue that lines the body on the outer as well as the inner side contributing majorly to respiratory, digestive, reproductive, and excretory functions, is called epithelial tissue. 

Structure: The following points explain the structure of epithelial tissue:

• The adjacent cells of epithelial tissue are held together by intercellular cement.
• Epithelial tissue cells are tightly packed and held together by intercellular junctions like desmosomes, interdigitations, intercellular bridges, and fitted folds.
• Epithelial tissue stays anchored to connective tissue via the basement membrane. 
• The epithelium is generally devoid of blood vessels. It obtains nutrition from underlying blood vessels via diffusion.

Did you know: The word epithelium has a Greek origin, where “epi” is for –upon and “thelio” means –-grows. Ruysch, a Dutch scientist, coined the term for tissues that grow upon other tissues.

Epithelial tissue is also found in the form of membranes. The two types of epithelial membranes are as follows:

Mucous membrane: Also called mucosa, it contains goblet cells that secrete mucus and help in lubrication, protection, and movement. It also protects tissues from drying. 

Site of occurrence: It lines the cavities like respiratory and digestive tracts that open outside the body.

Serous membrane: The membrane secretes the fluid inside the cavity. 

Site of occurrence: Lines the body cavities such as pericardial membranes and pleural cavities that do not open outside the body.  

Epithelial Tissue Function

The major functions attributed to tissue epithelium are as follows:

1. Protection: It protects underlying tissue from injury. Dehydration, germ entry, and chemicals. 
2. Secretion: Epithelial glands secrete mucus, hormones, and gastric juices.
3. Excretion: The epithelial lining of uriniferous tubules eliminates nitrogenous waste. 
4. Absorption: Intestinal epithelial lining absorbs digested food materials. 
5. Conduction: Ciliated epithelium conducts mucus and fluids. 
6. Sensation: Sensory epithelium of sense organs sends nerve impulses to CNS.
7. Respiration: Lung alveoli tissue epithelium assists in the gaseous exchange between blood and air.
8. Reproduction: Germinal epithelium of reproductive organs produces gametes. 
9. Exoskeleton formation: Surface epithelium produces exoskeletal structures like horns, hair, nails, feathers, and more. 

Types of Epithelial Tissue

The different types of epithelial tissue based on their structure are enumerated in the table below. 

Type	Subdivision	Structure	Location	Role
Simple Epithelium (One cell thick)	Squamous Epithelium/ Pavement Epithelium	A single layer of thin, flat, scale-like cells with interdigitations and junctions to stay intact.	The peritoneal lining of coelomic cavities, including lung alveoli, kidney’s bowman’s capsule, internal ear’s membraneous labyrinth, and blood vessel lining.	Transportation by diffusion.
	Cuboidal Epithelium	Cuboidal cells with rounded nuclei. They bear microvilli.	The lining of salivary and pancreatic ducts, thyroid vesicles, choroid, and iris of the eye.  Cuboidal epithelium with microvilli is present in proximal convoluted tubules of the kidney.	Secretion, excretion, and absorption.
	Columnar Epithelium	Cells appear rectangular in VS and polygonal in surface view. Elongated cells often bear microvilli.  When microvilli are arranged regularly, they have a striated border.  When microvilli are arranged irregularly, they have a brush border.  Secretory columnar cells that secrete mucus are called goblet cells.	Without microvilli- mucus membrane of the stomach and large intestine.  With brush border- Gall bladder. With striated border – Small intestine. Goblet cells- mucosa of stomach and intestine.	Secretion and absorption.
	Ciliated Epithelium	Columnar or cuboidal cells with cilia on the free surface.	Lines respiratory tract, uterine tubes, and uterus. In different ductules of the testis, auditory tube, portions of the middle ear, and lining of neurocoel cavity of brains and spinal cord.	Movement
	Pseudostratified Epithelium	A single layer of columnar cells of varying heights. Also, some cells are broader at the base while others are at the apex.  Some are ciliated, while others are non-ciliated.	Ciliated pseudostratified columnar epithelium-trachea and large bronchi. Non-ciliated parts of the auditory tube, vasa deferentia, epididymis, and male urethra.	Transportation
Compound Epithelium (more than one cell thick)	Transitional Epithelium	Thin and stretchable.  They have 4-6 layers and extensive interdigitations.  The innermost layer has columnar or cuboidal cells.  Middle 2-3 layers have polyhedral or pear-shaped cells.   The surface layer has large, broad, rectangular, and oval cells.	Renal pelvis, urinary bladder, ureter, and parts of urethra.	Distension of organs.
	Stratified Squamous Epithelium	Several superficial layers of squamous cells. Deeper layers have interlinked polygonal cells. The non-keratinized squamous epithelium has living superficial cells and a moist surface.  The keratinized squamous epithelium has dry surfaces and dead superficial cells. They contain the protein–keratin.	Keratinized stratified squamous epithelium covers the skin of the entire body.  Non-keratinized stratified squamous epithelium-lines buccal cavity, oesophagus, pharynx, vagina, and eye cornea.	Protection
	Stratified Cubical Epithelium	The superficial layers are cuboidal, while other layers may be cuboidal or of other types.	Eye conjunctiva, the lining of the female urethra, sweat glands, and parts of the anal cavity.	Protective.

Specialization of Epithelial Tissue 

Epithelial tissues can also be categorized as follows by the special functions they have:

Transitional epithelium

Transitional epithelium is also called urothelium. It consists of several layers of cells that flatten on a stretch. It can contract or expand when required. The cells of this tissue epithelium are cuboidal when not stretched.

However, when the organ stretches, the tissues compress, and the cells take an irregular squamous shape. They have tight junctions between the cells that prevent the reabsorption of toxic substances. It stretches readily and accommodates different liquid volumes. It also acts as a barrier. 

Examples of Transitional Epithelium: It lines the urinary tract and enables the urinary bladder to expand.

Olfactory epithelium

The olfactory epithelium has olfactory receptor cells. These cells have specialized cilia extensions. The primary function of the cilia is to trap odor molecules.

So, when you breathe, the molecules get trapped as they pass the epithelial surface. The olfactory epithelium transmits information about the molecules from the receptors to the brain’s olfactory region. Consequently, the brain interprets the smell.

Examples of Olfactory Epithelium: It is located in the nasal cavity lining. 

Glandular Epithelium 

Certain epithelial tissues are specialized to perform secretory functions. These tissues form glands. Glands can be classified into the following two types:

Exocrine Glands

These glands pour their secretions onto the epithelial surface, either directly or via ducts. They are also called externally secreting glands. They have a rich blood supply. Exocrine glands function under the influence of hormones and the nervous system. 

Examples of Exocrine glands

• Salivary glands
• Tear glands
• Gastric glands
• Intestinal glands

Types of Exocrine glands

• Holocrine: The cells disintegrate on discharging the secretion. Example: sebaceous glands. 
• Merocrine: They are also called epiccrine or eccrine glands. They throw out secretions by diffusion while the epithelial cells remain intact. Examples: salivary, intestinal, and sweat glands. 
• Apocrine: The epithelial cells shed their apical portions. Example: Mammary glands. 

Endocrine Glands

These glands lose contact with the epithelial surface from which they have developed. They secrete hormones and pour them into the bloodstream. The secretions circulate through the body and reach the target site. These are internally secreting glands or ductless glands. 

Examples of Endocrine glands

• Pituitary gland
• Thyroid gland
• Adrenal gland
• Parathyroid gland
• Gonads
• Pineal gland

Did you know: The pancreas is an example of both the endocrine and exocrine glands. The main bulk of the gland is constituted by its exocrine part.  The exocrine portions secrete enzymes that contribute to the digestion of carbohydrates, proteins, and fats.  The endocrine part includes pancreatic islets or islets of Langerhans that secrete glucagon and insulin.

Conclusion 

Epithelial tissue has various important functions to play and covers a large portion of the body. It is broadly classified into simple and compound depending on the number of layers. Also, some specialized epithelial tissues form glands and are primarily involved in the secretion process. 

Frequently Asked Questions

1. What are the major characteristics of tissue epithelium?

A.The primary characteristics of epithelial tissues are as follows:

• Attachment: They stay attached to a basement membrane. 
• Regeneration: They repair damage and grow rapidly. 
• Abrasion-resistant: They resist abrasion via rapid divisions.
• Compact Arrangement: Cells are joined by intercellular cement with connecting bridges of cytoplasm between them.

2. What does pseudo symbolize in the name pseudostratified epithelium?

A. Pseudostratified epithelium appears multilayered despite being single-layered because some cells are shorter while others are longer. Also, their nuclei are present at different levels. Some cells are broader near the base of the membrane, while others are broad at the apex. Since nuclei lie in the broader region, the epithelium appears multilayered when viewing it. Consequently, it is called pseudostratified. 

3. What is the difference between simple and compound epithelium?

A. Simple epithelium comprises a single layer of cells resting upon a basement membrane. In contrast, the compound epithelium has more than one layer, and the innermost epithelium cells rest on the basement membrane. 

4. What conditions affect the tissue epithelium?

A. Epithelial tissue has the potential for malignancy development. It is often the site of papillary thyroid carcinoma and adenocarcinoma. Adenocarcinoma is the most common cancer type responsible for most :

• breast cancers
• prostate cancers
• colorectal cancers
• pancreatic cancers
• Upto 40% of lung cancers