What is tissue?
A tissue may be defined as a group of one or more types of cells having a similar origin and specialized for a specific function or functions along with the intracellular material.
Branch of biology dealing with the study of tissue is called histology.The term 'tissue' was introduced by Bchat and also known as 'Father of histology'.Mayer coined the term 'histology' and the founder of histology is Marcello Malpighi.Following types of tissue are found in animals:
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'(G. epi=upon,thele=nipple) was introduced by Ruysch.They form the lining of tracts,cavities and vessels.Epithelium tissue evolved first in animal kingdom.It originate from all the three germ layers.
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 layer 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 mucopolysaccharides ,glycoprotein and collagen or reticular fibres.Blodd vessels are absent in the epithelial tissue.However,nerve ending may penetrate the epithelium.It possesses very high capacity of renewal(mitotic cell division).The following types of modification and junctions are found in the plasm membrane of adjacent epithelial cells to keep the cells together.