Epithelial Tissue

Last Updated on Thursday, 16 December 2010 01:12 Written by Sandesh Sunday, 15 August 2010 04:45

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General characteristics:

Epithelial tissue is derived from all the three primary germ layers. An epithelial tissue is the tissue that covers an external or internal surface. It is the simplest type of tissue found in the body of animals. It is a lining tissue, which covers the surface of the body, body cavities and the internal organs. It lines heart, blood vessels and lymph vessels and forms the walls of capillaries. Such an epithelial is usually referred to as endothelium. The cells of an epithelial tissue are firmly attached to one another with very little or no inter-cellular substance (matrix) between the adjacent cells. On surface or the end of these cells is always free, while the other surface or the end rests on thin non-cellular basement membrane consisting of special type of collagen and glycoprotein. Epithelial cells are arranged in continuous sheets that mat be either single or multilayered. These cells are devoid of blood vessels. The vessels that supply nutrient and remove wastes are present in the underlying connective tissue.

Functions of Epithelium:

Epithelial tissue performs the following important functions:

1. Protection (lining of skin)
2. Sensation (lining of skin and other sensory organs)
3. Absorption (lining of intestine)
4. Secretion (lining of glands)
5. Excretion (lining of sweat glands)
6. Transportation (ciliated ducts)
7. Production of gametes (germinal epithelium)

Classification of Epithelial Tissue:

The epithelium is arranged in different ways in relation to its location and function.

Based on the arrangement of layers, epithelium is classified into the following three types:

1. Simple epithelium
2. Stratified Epithelium
3. Pseudostratified epithelium

1. Simple epithelium:

The cells of epithelium are arranged in a single layer, it is called simple epithelium. Such an epithelium is generally specialized for absorption or filtration and occurs in an area where wear and tear is minimal.

Location- lining of digestive tract, alveoli of lungs, smaller ducts of many glands, kidney tubules, blood vessels etc.

2. Stratified epithelium:

If the cells of epithelium are arranged in many layers, it is called stratified epithelium. Such an epithelium is not specialized for absorption or filtration and occurs in an area where high degree of wear and tear occurs.

Location- lining the surface of tongue, skin, mouth, oesophagus, vagina etc.

3. Pseudostratified epithelium:

Like simple epithelium it has only one layer of cells. However, all the cells do not reach the surface and thus gives the false appearance of being multilayered. Hence it is known as pseudostratified epithelium.

Location- lining the larger ducts of the larger glands, upper respiratory tract, some ducts of male reproductive system etc.

Based on the cells shapes, structures and functions, the epithelium is classified into the following types:

1. Cuboidal epithelium
2. Squamous epithelium
3. Columnar epithelium
4. Ciliated epithelium
5. Glandular epithelium
6. Sensory epithelium
7. Germinal epithelium

1. Cuboidal epithelium: It is the simplest and the least specialized epithelium. It is composed of cells which are cubic or box-like. The cells are uninucleate and the nucleus is central in position. The cells are compactly arranged on a structure less basement membrane.

Location—ovaries, nephrons, vesicles of thyroid glands etc.

Stratified cuboidal epithelium which is relatively rare type of epithelium if found lining the ducts of sweat glands, pharynx and epiglottis.

2. Squamous epithelium: The squamous epithelium is of following types:

1. simple
2. stratified and
3. transitional epithelium.

i. Simple squamous epithelium:

This type of epithelium consists of a single layer of thin flat, broad scale-like cells. The cells are compactly arranged without intercellular spaces. They resemble to the tiles paved on the floor thus they are often known as “pavement epithelium”. Each cell has a center bulging portion and thinner edges. The cells are uninucleated with centrally located nucleus.

Function: Simple squamous epithelium is highly adapted for osmosis, filtration and diffusion.

Location: It is found lining the alveoli of lungs, Bowman’s capsule of nephrons, tympanic membrane of ear and blood vessels etc.

ii. Stratified squamous epithelium: This type of epithelium consists of several layers of cells. The cells are thicker than those of simple squamous epithelium and form relatively tough, impervious barrier. Each cell is formed by the mitotic divisions of germinal layer which rests on the basement membrane. The first formed cells are cubic in shape but as they are pushed upwards they become flattened (squames). The cells while moving away from deep layers, loose blood supply, become dehydrated, shrink and finally become dry. The cells at the surface are rubbed off and new cells continuously emerge to replace the flaked off cells.

The stratified squamous epitheliums are of two types: Keratinized and Non- keratinized.

1. Keratinized: These cells are found on the dry surfaces, the cells are modified into tough layer due to the deposition of keratin. Keratin is a protein which is water proof and resistant to friction.

Location: skin, hair, nails claws etc.

1. Non-kertainized: This tissue is found on wet surfaces which are subjected to considerable wear and tear. They do not perform the function of absorption.

Location: lining of mouth, tongue, oesophagus, female reproductive organs etc.

iii. Transitional epithelium:

It is a modified form of stratified epithelium and consists of three to four layers of cells. The cells are able to modify their shapes when placed under differing conditions.

Location: This tissue occurs in those places where considerable stretching occurs for example, bladder, ureter and the pelvic region of the kidney.

3. Columnar Epithelium: The cells of the columnar epithelium are tall, pillar like or column like. Basal ends of these cells are generally narrower than their free ends. The cells are uninucleated and the nucleus is situated towards the narrow basal end.

Function: The cells of columnar epithelium are usually specialized for absorption and secretion. In absorptive cells the free ends may be produced into microvili to increase the surface of absorption. Specialized flask shaped glandular cells called goblet cells are found scattered among the columnar cells. They secret mucus onto the free surfaces of the tissue.

Location: lining of uterus, stomach, intestine, gallbladder and ducts of many glands.

4.  Ciliated epithelium: Ciliated epithelium is a specialized form of epithelial tissue. It is a modified columnar epithelium. It consists of tall columnar cells with narrow basal ends and broad free ends. The free ends of each cell bears numerous, short, hair-like protoplasmic cilia arising from underlying basal granules. The cilia are capable of beating rapidly and rhythmically to sweep materials.

Location: Ciliated epithelium is found lining passages of the respiratory tract and various tubes of reproductive systems.

Function: In respiratory passage the mucous covered linings are sticky and tend to trap particles of dust and microorganisms that enter along the air. The cilia move the mucus and its captured particles upward and out of the air passages. In genital ducts, cilia aid in moving sex cells from one region to another.

5. Glandular epithelium: It is also a specialized form of epithelial tissue. Glandular epithelium is a modification of columnar or cuboidal epithelium which is specialized for the purpose of secretion. The cells of glandular epithelium secrete substances into ducts or directly into the blood. Such specialized secretory cells are known as gland cells.

The gland cells form unicellular or multicellular glands.

Unicellular: Large number of columnar cells in the lining of mammalian intestine is secretory in nature. They are known as goblet cells referred to as unicellular glands.

Multicellular: When gland cells occur in groups to form a definite gland in some places of body then they are known as multicellular glands. They consist of glandular epithelium folded in various ways to increase the surface of secretion.

The gland cells release their secretion into a single space or cavity from where they pass out of gland by means of duct. Such glands are known as exocrine gland. For eg. Liver, salivary glands etc.

Some glands are devoid of ducts and they release their secretion directly into the blood. These glands are known as endocrine glands. For eg. Pituitary, thyroid, adrenals etc.

The gland is said to be tubular gland if it has tubular secretory portion. Similarly it is called alveolar gland if the secretory portion is saccular.

Multicellular glands can be either simple or compound depending upon their complexity.