Epithelial Tissue

Epithelial Tissue: (Gr; Epi = on; thelial = nipple)

• An epithelium is a tissue composed of one or more layers of cells that cover the body surface and line its various cavities.

• General Function: It serves for protection, secretion and excretion.

• Types of Epithelial tissues:

Based on number of cell layers, epithelial tissues divided into two types as follows;

1. Simple Epithelial tissue and

2. Compound Epithelial tissue.

1.1 Simple Epithelial Tissue:

• It is composed of a single layer of cells.

• Major Function: It is a lining for body cavities, ducts and tubes.

• Other functions like diffusion, secretion, absorption and movement of the particles.

• Types of Simple Epithelial Tissue:

Based on shape of epithelial cell, it is further divided into four types as follows;

1. Squamous Epithelium

2. Cuboidal Epithelium

3. Columnar Epithelium and 

4. Ciliated Epithelium.

1.1.1 Squamous Epithelium:

• It is made up of flattened cells with irregular boundaries. Its nuclei are located at the center.

• Ex: Walls of Blood Vessels, air sacs of lungs.

• Function: forming a diffusion boundary.

1.1.2 Cuboidal Epithelium:

• It is composed of cube-like cells. Its nuclei are located at the center.

• Ex: ducts of glands and tubular parts of nephrons.Savinay Kumar JC

• Function: secretion and absorption.

• The epithelium of proximal convoluted tubules (PCT) of nephrons in the kidney has microvilli.

1.1.3 Columnar Epithelium:

• It is composed of tall and slender cells (column like cells).

• The nuclei of the columnar cells are located at the base and in few cases center.

• Free surfaces may have microvilli.

• Ex: lining of stomach and intestine.

• Function: secretion and absorption.

1.1.3 Ciliated Epithelium:

• It is a modified Cuboidal or columnar epithelium cell that bears cilia on their free surfaces.

• Ex: inner surface of hollow organs like bronchioles and fallopian tubes.

• Function: to move particles or mucus in a specific direction over the epithelium.

1.2 Compound Epithelial Tissue:

• It is composed of more than one layer of cells.

• Ex: surface of skin, buccal cavity, duct of salivary gland and pancreatic duct.

• Major Function: protection against chemical and mechanical stress.

• Other functions like secretion and absorption are observed in a few cases.

1.3 Glandular Epithelium:

• Some of the columnar or cuboidal cells specialized for secretion is called glandular epithelium.Savinay Kumar JC

• Types of glandular epithelium: Based on number of epithelial cells participate in secretion, glandular epithelium divided into two types as follows;

1. Unicellular Glandular Epithelium and

2. Multicellular Glandular Epithelium

1.3.1 Unicellular Glandular Epithelium:

• It is also called simple gland or goblet cells.

• It consists of isolated glandular cells (one cell) in the epithelium.

• Ex: Alimentary canal.

1.3.2 Multicellular Glandular Epithelium:

• It consists of many cells for secretion.

• Ex: Salivary Gland.

Note: According to the Greek language Exo = outside; Endo = inside; and crine = secrets.

1.3.3 Difference between Exocrine and Endocrine Glands:

Exocrine Gland	Endocrine Gland
They produce onto an epithelial surface.	They produce directly into the bloodstream
They secrete enzymes (like digestive enzymes), mucus, saliva, earwax, oil, milk and other cell products	They secrete Hormones.
They control short term activities.	They control long term activities.
They have ducts.	They don’t have ducts.
Ex: Sweat Gland, Gastric Glands	Ex: Pituitary Gland, Thyroid gland