Epithelial Tissue

EPITHELIAL TISSUE

Objectives

• By the end of this session students are expected to be able to:

Define Epithelial Tissue

• Describe the Structure of Epithelial Tissues

Classify Epithelial Tissues

Explain Functions of Epithelial Tissues

EPITHELIAL TISSUE

Epithelium is a group of closely associated cells with scanty intercellular materials that cover external and internal surfaces of the body.

They form the covering of all body surfaces, line body cavities and hollow organs, and are the major tissue in glands.

Epithelial tissues are widely spread throughout the body and are tightly packed together with no intercellular material between the cells or very little intercellular materials.

It also forms all glands including endocrine and exocrine glands.

The cells are attached to the underlying connective tissue known as the basement membrane.

The basement membrane is a connective tissue structure where the epithelial cells are attached.

Cells of epithelial tissue appear in different shapes and structure whereas the differences reflect the functions of the epithelium.

Classification of Epithelial Tissues

• Classification according to the cell shape
• Squamous cells
• These are flat cells with an irregular flattened shape
• Cuboidal cells
• They have a shape similar to a cube, meaning its width is the same size as its height
• Columnar cells

They are taller than they are wide

Classification based on number of layers (Stratification)

Simple Epithelium

Cells are organized in a single layer and therefore all cells are attached to the basement membrane.

Because the shapes of the cells include possible function, they are found in different organs depending on their function.

• There are three types of simple epithelium:
• 1.Simple squamous epithelium
• 2.Simple cuboidal epithelium

3.Simple columnar epithelium.

Simple Squamous Epithelium

• All Squamous cells are flat cells with an irregular flattened shape.

It is a one-cell layer of simple squamous epithelium.

Forms a thin membrane and therefore it is found in places where exchange of material from one compartment to the other occurs. Squamous cells can be found in the heart, blood vessels, lymph vessels and alveoli of the lung.

They allow free diffusion.

Simple Cuboidal Epithelium

This consists of cube-shaped cells fitting closely together lying on a basement membrane.

Forms secretory cells, lining of the small ducts of the gland sand it covers the outer surfaces of organs such as the ovaries and in the kidney tubules.

Are actively involved in secretion, absorption and excretion.

Simple Columnar Epithelium

This is formed by a single layer of cells, rectangular in shape lying on a basement membrane.

It is found lining the organs of the alimentary tract.

Consists of mixture of cells, some absorb the products of digestion and others secrete mucus.

Stratified squamous epithelium

• Consists of several layers of cells of different shapes

The cells are arranged into superficial, intermediate and basal (deep) layers.

In the deepest layers the cells are mainly columnar and, as they grow towards the surface, they become flattened.

It is the main protective epithelium of the body external surface for example in the skin it forms the epidermis.

General Functions of Epithelial Tissue

Protection

Epithelial cells from the skin protect underlying tissue from mechanical injury, harmful chemicals, invading bacteria and from excessive loss of water Sensation

Sensory stimuli penetrate specialized epithelial cells Specialized epithelial tissue containing sensory nerve endings is found in the skin, eyes, ears, nose and on the tongue

Secretion

In glands, epithelial tissue is specialized to secrete specific chemical substances such as enzymes, hormones and lubricating fluids.

Absorption

Certain epithelial cells lining the small intestine absorb nutrients from the digestion of food.

Excretion

Epithelial tissues in the kidney excrete waste products from the body and reabsorb needed materials from the urine.Sweat is also excreted from the body by epithelial cells in the sweat glands.

• Diffusion

Simple epithelium promotes the diffusion of gases, liquids and nutrients.

Assignment

Outline types and functions of epithelial tissues

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