Stratified Epithelium

Here, the cells are arranged in more than one layer. Stratified epithelium is classified into the following types based on the shape of the constituent cells.

Stratified squamous epithelium in which, more than one layer of flat, polygonal cells are found arranged on a basement membrane. It is a characteristic feature of the skin. It also occurs in the lining of the tongue and the oesophagus.

The outer most layer of cells in the stratified squamous epithelium of the skin, undergoes a process called keratinisation in which the cytoplasm of each cell gets replaced by a rigid protein called keratin.

 

Stratified Squamous

Stratified Cuboidal

Stratified Columnar

 

    Transitional

fig 5.55 Types of Stratified Epithelium

 

Note:

The keratinised layer of skin is involved in the production of derivatives like hairs and nails.

There is also a regular replacement of this keratinised layer by a process known as moulting.

Stratified cuboidal epithelium

In which, more than one layer of cuboidal cells are found arranged on a basement membrane. It occurs in some parts of the reproductive system, ducts of sweat glands and in the lining of the pharynx.

Stratified columnar epithelium

In this type, the epithelium has several layers of polyhedral cells with columnar cells found only in the superficial layer. It occurs in the large ducts of some glands and in the male urethra.

Transitional epithelium in which, the cells in the superficial layers are not truly squamous, cuboidal or columnar. These cells are large and rounded or conical. The epithelium allows distention. It occurs in the urinary bladder.

Pseudo-stratified epithelium

fig 5.56 Pseudostratified Epithelium

In this type, only a single layer of cells are found on a basement membrane. However, all the cells do not reach the surface and the nuclei of the cells are found at different levels. Hence, there is a false appearance of more than one layer.

Pseudo-stratified epithelium is found in parts of the respiratory system such as nasal cavity and trachea, some ducts of male reproductive system and also in the auditory canal.

Functions of epithelial tissue

The epithelial tissue is involved in a variety of functions in the body. Following are the most important functions.

Protection is the main function of the epithelium. As an important component of the skin, the epithelium provides maximum protection to the body.

Absorption in the internal organs like the small intestine and kidney. The epithelium is mainly involved in the absorption of nutrients and other useful substances.

Secretion

The epithelium becomes a part of all the glands in the body which are involved in producing a variety of secretions like enzymes and hormones.

Transport

In different regions of the body, the epithelium, particularly the ciliated epithelium, is involved in the transport of substances.

Excretion

The epithelial cells play an important role in the removal of unwanted and harmful substances from the body.

Sensory-reception in different regions of the body

The epithelial cells become specialized to form receptors that are capable of receiving the various external and internal stimuli, and transferring them to the nerve cells. E.g. Photoreceptors, thermoreceptors, osmoreceptors, pressure receptors.