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EPITHELIAL TISSUE

The word epithelium means growing upon other. The word epithelium was coined by Dutch anatomist Ruysch.

Epithelium is simplest and non¬specialized tissue. It is formed of one or more layers of cells upon all external and internal exposed surfaces of the body and organs. It forms the internal coverings of organs.

Features:

1. Cells are placed very close to each other.

2. Intercellular space is absent or very little and so the intercellular matrix.

3. Cells are connected by various connections with each other (they will be described later in detail).

4. Matrix is made up of mainly glycoproteins which are secreted by the cells themselves.

5. Because intercellular space is very little so epithelia is generally not pervaded by nerve fibres and blood or lymph vessels.

6. Cells may bear microvilli, cilia stereocilia or flagella etc. generally in large cavities.

7. Main feature of this tissue is that cells rest upon a membrane caffed basement membrane. This membrane is composed of glycoprotein, mucopolysaccharide and reticular fibres. All these components are secreted by the cells themselves. This membrane acts as plasma membrane.

8. Epithelium along with underlying connective tissue layer in various organs form mucous and serous membranes.

Origin: Epithelia arises from all three embryonic layers viz. Ectoderm¬Epidermis of skin, Mesoderm-Coelomic epithelium, Endoderm-Gut epithelium.

Connections: Various types of connections are found between the adjacent cells plasma membrane.

1. Interdigitation: Finger like processes, interwoven like the threads ina cloth.

2. Tight junction: Plasma membrane of cells are tightly packed or fused together (Zonula occludens).

3. Gap Junction (Macula occludens or Nexus): Hexagonal gap of 70-75 Å. e.g. myocardial tissue. When the size of gap is about 40-70Å, it is called close junction.

4. Desmosomes or Macula adhaerens: At certain places plasma membrane becomes thickened. These thickened areas are called desmosomes. From these areas fine filaments called tono filaments arise and joins the cells together.

5. Terminal bars or Zonula adhaerens: These are desmosomes lacking tonofibrils.

Functions of epithelia:

1. Protection from dehydration, infection, mechanical and chemical injuries.

2. Acts as absorptive surface in intestine, kidney etc.

3. Acts as selective barrier across surface.

4. Conduction of mucus and other fluids.

5. Exclusively receptive in receptor organs.

6. Regeneration.

Classification: Basically classified into two types:

1. Simple epithelium

2. Compound epithelium

(1) Simple epithelium:

1. Simple squamous epithelium

2. Simple cuboidal epithelium

3. Simple columnar epithelium

4. Ciliated epithelium

5. Glandular epithelium

6. Sensory epithelium

7. Germinal epithelium

(2) Pseudo stratified epithelium

(3) Stratified epithelium

1. Stratified squamous epithelium

2. Stratified cuboidal epithelium

3. Stratified columnar epithelium

4. Transitional epithelium

Simple epithelium

(1) Simple squamous epithelium:

(1) It is plate like or scale like.

(2) Cells are flattened and fitted together by means of junctions so that it looks like the tile of a mosaic.

(3) It is also called as pavement epithelium.

(4) Nucleus is oval or spherical and lies in the centre of cell.

Pleural, pericardial and peritoneal cavities, lining of heart, lungs, blood and lymph vessels, lining of internal ear and uriniferous tubule.

Coelomic epithelium is also called mesothelium and of vessels is called as endothelium. Endothelium appears to be wavy hence called tessellated.

(2) Simple cuboidal epithelium:

(1) It occurs in sweat glands, uriniferous tubules, ciliary body, choroid of eye and thin bronchioles.

(2) Germinal epithelium is also cuboidal.

(3) This epithelia acts as secretive, absorptive and excretionary.

(3) Simple columnar epithelium:

(i) Cells are tall and prismatic.

(ii) Nucleus is elongated and placed basally. Some cells e.g. goblet cells, are also interspersed between the epithelial cells.

(iii) These cells secrete mucus and have a very short life span of 2 to 3 days and are replaced by the transformed adjacent epithelial cells.

Inner lining of stomach, intestine and urinogenital organs.

(4) Ciliated epithelium

(i) Cells may be cuboidal or columnar.

(ii) Their free ends bear cilia.

(iii) Cilia exhibit incesant movement.

Some cells may bear pigments e.g. retina.

It occurs in neck of uriniferous tubules and retina.

(5) Glandular epithelium

(i) The cells are columnar.

(ii) These constitute most important components of glands

(6) Sensory epithelium

(i) The cells of this epithelium are generally of columnar type.

(ii) Outer ends of the cells bear neurosensory hairs and inner ends are attached with the nerve fibres.

(iii) These cells may be found Singly or in groups or interspersed between epithelial (supporting cells) cells.

It occurs in retina of eye, epithelial covering of tongue, lining of internal ear, ampullae and lining of buccal cavity etc.

(7) Germinal epithelium:

(i) The cells are cuboidal.

(ii) They possess immense power of division e.g. seminiferous tubule, ovary and stratum germinativum of epidermis.

Pseudostratified epithelium:

(i) The cells are either ciliated or columnar.

(ii) It is always single layered but all cells are not of same size.

(iii) The inequality of size gives it the appearance of being multilayered (stratified).

(iv) It occurs in lining of olfactory chamber and lining of trachea, vasa differentia, epididymis etc.

Stratified epithelium

(1) Stratified squamous epithelium

(i) It is the main protective epithelium of the body and found in the regions of friction.

(ii) It is two layered to multilayered.

(iii) Cells of different layers are not similar but cells towards outer surface are more flattened.

(iv) Two types of stratified squamous epithelia are found.

(a) Non-keratinized :

(i) Cells do not loose nucleus and keratin is either absent or much reduced.

(ii) It is generally found at wet surfaces. e.g. cornea of eye, lining of buccal cavity, vagina etc.

(b) Keratinized:

(i) Cells loose nucleus and a hard insoluble fibrous protein keratin is found in the cells.

(ii) Completely keratinized layer is water proof. e.g. epidermis of skin.

(2) Stratified cuboidal epithelium

(i) The cells of the outermost layer are cuboidal.

(ii) It occurs in conjunctiva of eye, linings of the ducts of sweat glands, some part of anal canal etc.

(3) Stratified columnar epithelium:

(i) The cells of the superficial layer are columnar and rest upon the layer of cuboidal or columnar type.

(ii) It occurs in lining of the ducts of sweat and sebaceous glands and mammary glands.

(iii) This epithelium is more common in embryo.

(4) Transitional epithelium

(i) It consists of partly flattened, non-keratinized, plastic cells.

(ii) The superficial cells are in the form of large cuboidal plates and lowers ones small and irregularly cuboidal.

(iii) Cells are in 2 to 6 layers.

(iv) The foremost feature of epithelium "the basement membrane" is absent.

(v) It occurs in urinary system.