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Types of epithelial tissue: Definition, Characteristics and Functions

In this article we will discuss about the 2 types of epithelial tissue: simple epithelial tissue and compound epithelial tissue

Epithelial Tissue Definition

Epithelial tissue is a type of animal tissue it forms the outer covering of the skin and also forms the covering on all internal and external surfaces of animal body. Epithelial tissue performs various functions such as absorption, protection, sensation and secretion.

Characteristics of Epithelial Tissue

Covers and lines organs and body cavity walls
Closely packed cells with little extracellular material
Many cell junctions often provide secure attachment
Cells sit on basement membrane
Exhibits polarity apical (superior) and basal (inferior) surfaces
Avascular, that means without blood vessels and nutrients and waste must move by diffusion
Rapid cell division (high mitotic rate)

Types of Epithelial Tissue

The epithelial tissue is classified according to number of layers and the shape of the outermost layer

Number of cell layers

Simple epithelium – single layer of cells
Stratified epithelium – several layers of cells
Pseudostratified epithelium – single layer of cells of variable size and shape, with nuclei at a different layer

Shape of cells

Squamous – the width of the cell is greater than its height
Cuboidal – the width, depth, and height are approximately the same
Columnar – the height of the cell exceeds the width

Based on above criteria the epithelial tissue classified into two types:

Simple epithelium
Compound epithelium

1. Simple Epithelium

Simple epithelium consists of a single layer of epithelial cells
These are direct contact with the basement membrane
On the basis of shape and structural modifications of cells the simple epithelium divided into following types:

1. Simple squamous epithelial tissue
2. Simple cuboidal epithelial tissue
3. Simple columnar epithelial tissue
4. Pseudostratified epithelial tissue

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a. Simple squamous epithelial tissue

Simple = one layer & squamous = flat
It’s consist of a single layer of thin and flattened cells with nuclei
On surface view, like floor tiles so also known as pavement epithelium
Found in lung alveoli, parietal layer of bowman’s capsule of kidney, inner aspect of tympanic membrane, mesothelium and endothelium
Function is rapid transport of substances, secretion of fluid, diffusion of gases and osmosis

b. Simple cuboidal epithelial tissue

Simple = one layer & cuboidal = cube
Made up of single layer of cuboidal shaped cells
It has round nuclei
On surface view, cells look like mosaic (hexagonal)
Found in thyroid follicles, tubules of nephrons, pigmented layer of retina, germinal layer of ovary, inner layer of lens and choroid plexuses of brain
Function is secretion and absorption

c. Simple columnar epithelial tissue

Simple = one layer & columnar = rectangular (column)
These cells having more height than width
Nuclei are elongated, located in the lower half of cells
Cells may show some surface modifications
Common surface modifications of columnar epithelial tissue are microvilli, stereocilia and cilia
Found in lining of stomach to anus, gall bladder, uterine tube & cavity, central canal of spinal cord
Function is secretion, absorption and ciliary action

d. Pseudostratified epithelial tissue

Pseudostratified = false layered (appears to be more than one
These cells appear layered, but not multi layered
Nuclei stacked at different levels
Found in trachea, bronchi, olfactory epithelium, eustachian tube
Function is secretion and movement of mucus by ciliary action

2. Compound Epithelial Tissue

It’s consists of many layers, relatively thick
Cells nearer the surface are flat whereas the deeper are cuboidal and columnar
On the basis of stretching ability, it is of two types
1. Stratified epithelium (non-stretchable)
2. Transitional epithelium (stretchable)

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a) Stratified epithelium

On the basis of shape of the cells of outermost layer it is of three types

i) Stratified squamous epithelium

Innermost layer of cells is cuboidal
Cells of outermost layer are scale like flat cells
On the basis of presence of keratin protein in the outer most cells this epithelium is of two types

Keratinized stratified squamous epithelium

Keratin protein is present in scaly cells
Cells become non nucleated dead cells
Epidermis of skin, scale, horn, nail, feathers etc

Non keratinized stratified squamous epithelium

Keratin protein is absent
Cells are nucleated and living
Buccal cavity, inner lining of cheeks, lips, hard plate, pharynx, oesophagus & anal canal

ii) Stratified cuboidal epithelium

Inner most layer is cuboidal
Outermost layer of cells is cube like, cells are nucleated & living.
Secretory duct of sweat glands, mammary glands and sebaceous gland, pancreas, salivary glands

iii) Stratified columnar epithelium

Outermost layer is composed of pillar shaped cells, cells are nucleated. On the basis of presence of cilia this epithelium is of 2 types

(1) Ciliated stratified columnar epithelium. E.g., Buccopharyngeal cavity of frog. Larynx

(2) Non ciliated stratified columnar epithelium. Cilia absent on free end. E.g., Epiglottis

b) Transitional epithelium

In a relaxed state it’s appears as stratified cuboidal epithelium but tissue is stretched, cells become flattened, giving the appearance of stratified squamous epithelium
In this epithelium 4-6 layers of cells are present
Inner most layer of cell is composed of cube like cells
Middle 2-4 layers are composed of pear shaped
Outermost 1 or 2 layers are of oval shaped cells
This different shape of cells appears only in resting stage. When this tissue is stretched, all the cells become flattened
Found in renal pelvis, ureter, urinary bladder, proximal part of male urethra
Function the transitional epithelium cells stretch readily in order to accommodate fluctuation of volume of the liquid in an organ

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2 Types of Epithelial Tissue: Definition, Characteristics and Functions | Free Biology Notes