Epithelial Tissue:

Epithelial Tissue:

• An epithelium is a tissue composed of one or more layers of cells that cover the body surface and lines its various cavities. 
• It serves for protection, secretion and excretion. 
• The word ‘epithelium’ was introduced by Ruysch. 
• It was applied originally to thin skin covering the nipple (G. epi = upon, thele = nipple). 
• They are located on the outer surfaces of organs, including the skin. 
• They form the linings of tracts, cavities and vessels.  
• Epithelial tissue evolved first in animal kingdom. It originate from all the three primary germ layers. e.g. Epidermis arises from ectoderm, Coelomic epithelium from the mesoderm and epithelial lining of alimentary canal from the endoderm.

Structure:

• Cells are arranged in one or more layers, cells are compactly arranged and there is no inter cellular matrix between them. 
• Neighbouring cells are held together by intercellular junctional complexes like desmosomes, tight junctions, interdigitations etc. the cells of lowermost  layers always rest on a non living basement membrane or basal lamina.  
• Basement membrane is made up of no cell product of epithelial tissue. 
• It is formed of mucopolysacharides, glycoprotein and collagen or reticular fibres. 
• The epithelial cells are held together by small amounts of cementing substances is mainly composed of glycoprotein secreted by the cell themselves. 
• Blood vessels are absent in the epithelial tissues. 
• However, nerve endings may penetrate the epithelium. 
• The free surface of cells may be smooth or may have fine hair like cilia, sterocilia and microvilli. 
• Epithelium is subjected to continuos wear and tear and injury. Hence it posses very high capacity of renewal (mitotic cell division). 
• The following types of modifications and junctions are found in the plasma membrane of adjacent epithelial cells to keep the cells together.

 Modifications in epithelial tissue

• Microvilli : It is simple and minute cytoplasmic processes arising from free exposed surfaces of the cell. They absorb material.
• Stereocilia : It is non-motile cytoplasmic processes.
• Cilia : It is contractile motile fibrous processes arising from basal granules.
• Tight junctions  (Zona occludens) : At certain places the plasma membranes of adjacent cells are tightly packed or even fused together.
• Desmosomes : Desmosome is present in epithelial tissue. They consist of thickened area and several fine tonofibrils extending from  each plasma membrane into cytoplasm of respective cells. Macula adherens is a kind of desmosome.
• Gap junction : At place, the adjacent cells form ion-rich gap junctions for intercellular communication and chemical exchange. These junctions probably do not provide physical support.
• Interdigitations : These are interwoven finger-like processes of plasma membranes of adjacent cells.
• Intercellular bridges : These are minute projections that arise from adjacent cell membranes. The intercellular bridges make contact with one another.