Epithelial Tissue

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 epitheliaCovering epitheliaGlandular epithelia(with secretory

function)

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

Multicellular exocrine glands

A. Simple columnarB. Simple columnar

epithelium with ciliaC. Stratified squamousD. Simple squamousE. TransitionalF. PseudostratifiedG. Cuboidal epitheliumH. ChoanocytesI. Stratified columnar

epithelium with cilia

Simple columnar epitheliumSimple columnar epithelium with ciliaStratified squamous epitheliumSimple squamous epitheliumTransitional epitheliumPseudostratified epitheliumCuboidal epitheliumChoanocytesStratified columnar epithelium with cilia