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EpitheliaEpithelia are tissues consisting of closely
apposed cells with very little intercellular substances.
They Epithelia are avascular but all epithelia grow on vascular connective tissue.
Epithelia and underlying connective tissue are seperated by basement membrane
FunctionsCovering (eg: skin)Lining of all the body cavities (eg :
intestines)Secretory(eg: glands)Epithelia covers all free surface of the
body, where it is called mesothelium and The internal surfaces of blood and lymph
vessels are lined by epithelium, here they are called endothelium
The cells on the free surface are called apical cells
Classification of covering epithelium
Based on number of cells:
1. Simple epithelium: there is one layer of cells
2. Stratified epithelium:two or more layers of cells, it is termed stratified
Based on the shape of cells in the surface layer:
1. Squamous : flattened scale or plate like cells
2. Cuboidal : Cube shaped cells
3. Columnar : cells are taller than wide
Epithelial tissue is classified based on the number of cell layers and the shape of cells in apical layer
Nucleus of the cells corresponds to the shape of the cells and also the long axis of the nucleus is always parallel to the main axis of the cell.
Simple Squamous EpitheliumOne layer of flattened, scale or plate like
cells. It lines all serous lining of body cavities;
pericardium, pleura, peritonium ; blood vessels and lymph vessels(endothelium)
The nuclei are often flattened or ovoid, ie egg shaped, and they are located close to the centre of cells
Diffusion and filtration takes place across the single layer of cells
Simple Cuboidal Cells
One layer of cube shaped cellsIt occurs in,
small excretory ducts of many glands(eg salivary glands),
follicles of thyroid gland, tubules of kidney and on the surface of ovaries.
Nuclei are spherical and are at the centreHelps in secretion and absorption
Simple Columnar EpitheliumOne layer of tall,rectangular shaped
cells;cells are taller than they are wideLines the internal surface of GI tract from
cardia of the stomach to the rectumLining of gall bladderThe nucleus is located at the same height
within the cells,often close to the base of the cells
Helps in secretion and absorption
Pseudo-stratified Columnar epithelium
One layer of columnar or irregularly shaped cells. Some kinds have cilia(pseudo stratified ciliated columnar epithelium) and goblet cells.
Lines trachea,bronchi and nasal cavityHelps in protection and secretion
Stratified Squamous EpitheliaBasal cells are present in the basal layer
which are in contact with basement membrane
Mitotically active and renew the cellsThese basal cells are either columnar or
cuboidal in shape.The apical layer cells of the epithelium
become more flattened.Present in the places subjected to wear
and tear like skin, mouth, esophagus and vagina
Classification of stratified squamous epithelium
Keratinized: dry surfaces such as the epidermis of the skin
Contains keratin, most superficial layer consisting of dead cells
Functions in protection
Non-Keratinized: most superficial cells are alive kept moist
Linings of nasal cavity, oral cavity,pharynx, vagina and the lower anal canal.
Functions in protection
Stratified Cuboidal and Columnar
Stratified cuboidal epithelium, for example is seen in the ducts of the sweat glands.
For example found in the excretory ducts of the mammary glands and the main excretory ducts of the large salivary glands
Transitional epithelium
Dome shaped cells which are neither squamous nor columnar.
Some cells are binucleatedFound in ureter,urinary bladder and upper
part of urethraWithstands distention and relaxing of
bladder.
Glandular Epithelium
Exocrine glands retain their connection with the surface epithelium from which they originated.
Their connection is transformed into tubular ducts lined with epithelial cells through which the glandular secretions pass to reach the surface
Endocrine glands are the ones which lose their connection during development process
Releases their secretory products (typically hormones) into spaces between secretory cells from which it enters the bloodstream
All glands of the body arise from covering epithelium by means of proliferation and invasion of the epithelial cells. Glands further differentiate into exocrine and endocrine glands
Exocrine glandsAccording to the # of cells the glands are
classified into unicellular and multicellular glands
Multicellular glands are further classified into merocrine, apocrine, holocrine glands based on the secretory mechanisms
Unicellular GlandsGoblet cells are the only unicellular glands
secreting mucusRespiratory and Digestive system and
other mucus membransesGoblet cells secretes glycoprotin mucin
which after combination with water is secreted as mucus
Merocrine glandsMerocrine secretion: corresponds to the
process of exocytosis. Vesicles open onto the surface of the cell
and the secretory product is discharged from the cell without any further loss of cell substance.
Secretes a watery fluid through cell membrane
Eg:salivary glands
Apocrine glandsMechanism in which part of apical
cytoplasm of the cells is lost together with the secretory product.
Apocrine sweat glands, the mammary glands, and the prostate.
Halocrine glandsDesignates the break down and discharge
of the entire secretory cellEntire cell is released and it ruptures and
dies Eg:sebaceous glands
A. Simple columnarB. Simple columnar
epithelium with ciliaC. Stratified squamousD. Simple squamousE. TransitionalF. PseudostratifiedG. Cuboidal epitheliumH. ChoanocytesI. Stratified columnar
epithelium with cilia
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