Types of Epithelium:
Mainly based on the location and functions of tissue it is of following types:-
It is simple in structure and basically formed by single layer cells.
(i)Simple squamous epithelium:
It consists of only one layer of flat, scale like cells,usually polygonal cells which are closely fitted together like the tiles of a mosaic.It is also known as pavement epithelium. e.g.,It forms lining of blood vessels,lymph vessels,heart ,peritoneum,pleura,Bowman's capsule,thin segment of loop of Henle and lung alveoli.
(ii)Simple cuboidal epithelium:
The simple cuboidal epithelium is composed of one layer of cuboidal shaped cells resting on a basement membrane.The nuclei are situated centrally.e.g.,the cuboidal epithelium is present in the small salivary and pancreatic ducts,thyroid vesicles,parts of membranous labyrinth, PCT,DCT,ovaries, seminiferous tubules of testes,ciliary bodies, choroid,iris of eye,thin bronchioles and sweat gland of mammalian skin.
(iii) Simple columnar epithelium
It consists of a single layer cells,many of which have modified structure.Three common modifications are goblet,cilia and microvilli.Simple columnar epithelium is present in the stomach and intestine e.g.,inner lining of gall bladder and bile duct.It also occurs in the gastric gland,intestinal glands,pancreatic lobules.
(iv)Simple ciliated epithelium:
It bears numerous delicate hair like outgrowths called cilia arising from basal granules which helps to create a current to transport the materials.The ciliated epithelium is of two types:-
(a) Ciliated columnar epithelium:
It lines respiratory tract(Lower end of bronchi),fallopian tubes(Oviducts),ventricles of brain (ependyma),central canal of spinal cord,tympanic cavity.
(b) Ciliated cuboidal epithelium:
It occurs in certain parts of nephrons of the kidneys
It always consist of single layer of irregularly shaped columnar cells,touches the basement membrane.The long cells have oval nuclei however,short cells have rounded nuclei although epithelium is one cell thick,but it gives the appearance of a stratified epithelium,hence it is called pseudostratified epithelium.Mucus secreting goblet cells are numerous and cilia are present.It is of two types :-
(a) Pseudostratified columnar ciliated epithelium:
It is found in the lining of trachea and bronchi(Upper).
(b)Pseudostratified columnar epithelium:
It is found in certain segments of human male urethra and parotid salivary gland, vasa deferentia and epididymis.
(c) Stratified squamous epithelium:
The cells in the deepest layer are columnar or cuboidal with oval nuclei.It is called germinative layer.The cells of this layer divide by mitosis to form new cells.
Character of Epithelial cell
1. All epithelia have an apical surface and a lower attached basal surface that differ in structure and function. For this reason, epithelia is described as exhibiting apical basal polarity. Most apical surfaces have microvilli(small extensions of the plasma membrane) that increase surface area. For instance, in epithelia that absorb or secrete substances, the microvilli are extremely dense giving the cells a fuzEpithelial tissue typeszy appearance called a brush border. Examples of this would include epithelia lining the intestine and kidney tubules. Other epithelia have motile cilia (hairlike projections) that push substances along their free surface. Next to the basal surface is the basal lamina (thin supporting sheet). The basal lamina acts as a filter allowing and inhibiting certain molecules from passing into the epithelium.
2. epithelial cells fit close together and form continuous sheets (except in the case of glandular epithelia). They do this with tight junctions and desmosomes. Tight junctions form the closest contact between cells and help keep proteins in the apical region of the plasma membrane. Desmosomes connect the plasma membrane to intermediate filaments in the
3. all epithelia are supported by connective tissue. For instance, deep to the basal lamina is reticular lamina (extracellular material containing collagen protein fiber) which forms the basement membrane. The basement membrane reinforces the epithelium and helps it resist stretching and tearing.