EPITHELIAL TISSUEBook Recommended:

Basic Histology Lec. By Dr. Habib Qureshi

DEFINITION

Epithelium is the collection of cells with very little intercellular substance lining internal and external surfaces of body

and performing similar function

epithelium

EPI = UPON THELE = NIPPLE DERIVED FROM ALL THREE EMBRYONIC

GERM LAYERS Basal layer of epithelium rest on

BASEMENT MEMBRANE

BASEMENT MEMBRANE

Basement membrane consists of Basal lamina proteoglycans, type 1V collagen, electron

microscopic- Forms limit of cell- Secreted by epithelial cells Reticular lamina- Consist of fine collagen fibers

CLASSIFICATION of EPITHELIUM

Simple Epithelium Simple Squamous Simple Cuboidal Simple Columnar

Stratified Epithelium Pseudostratified Epithelium Transitional Epithelim

Simple Squamous Epithelium

Single layer of flat cells. Rest on basement membrane Nucleus bulging Function: gas exchange, diffusion of fluids Endothelium = blood vessels Mesothelium = peritoneal, pleural and

pericardial cavities Sites= bowman’s capsule, alveoli of

lungs, mesothelium, endothelium, loop of Henle of kidney

Simple Squamous Epithelium

Simple cuboidal epithelium

SINGLE LAYER OF CUBOIDAL CELLS For secretion Sites=thyroid follicles, tubules of kidney,

surface of ovary All cells of same height and width and

rest on basement membrane

SIMPLE CLUMNAR

Tall cells on basement membrane Nucleus near base For absorption and secretion ciliated = uterus Non ciliated = GIT, (has brush border –

microvilli, folds of cell membrane to increase surface area

Stratified squamous epithelium

S. sq. keratinized Surface layer is squamous In between 3-7 layers are polygonal to flat Basal layer is cuboidal There is layer of keratin at the the top Sites. skin

S. Sq. Non-keratinized No keratin layer Same as other sites= esophagus, anal canal, vagina

Stratified Cuboidal epithelium Two to three layers of cells Mostly present in ducts of salivary glands.

Stratified Columnar epithelium Two to three layers of cells Present in ducts of salivary glands, Urethra.

Pseudostratified epithelium

ALL LAYERS REACH BASEMENT MEMBRANE

Nuclei at different levels Trachea Has goblet cells

Transitional epithelium

Empty urinary bladder Basal layer columnar Intermediate layers polygonal 2-3 layers Flask shaped layers Dome shaped( umbrella) at top

AٍStretched urinary bladder 2-3 layers of flat cells

Intercellular Junctions

Needed for cell aggregation and cohesion Marked in epithelial tissue Due to binding action of glycoproteins Lateral membranes have special

junctions For adhesion Act as seals for flow of materials

Present in definite order from apex to base

E/M Cellular Junctions

Cellular junctional complex

TYPES of INTERCELLULAR JUNCTIONS

Tight junctions – ZONULA OCCLUDENS Most apical Belt like , totally occluding (Closing) Have ridges and grooves forming net like structure Form tight seal (e.g Urinary bladder) to control

paracellular pathway

ZONULA ADHERENS Belt like Encircles cell For adhesion of neighbouring cells Forms terminal web on cytoplasmic surface of cell

membrane

Gap junctions –NEXUS Present anywhere Present in all tissues except skeletal muscle There is gap of 2 nm in between opposing

surfaces Protiens of junction make connexons which

control the flow Permits molecules less than 1500 Da. Eg. Heart muscle.

DESMOSOMES:MACULA ADHERENS Disc like structure Membrane > 30 nm apart Dense material in intracellular space

DESMOSIN for adhesion Attechment plaque protiens inside the cell

membrane

HEMIDESMOSOMES (Half Desmosomes)Are between basal surface of epithelial cells and

basement membrane.

E/M Cellular junctions

FUNCTIONAL CLASSIFICATION

Adhereing junctions Z. Adherens Hemidesmosomes desmosomes

Impermiable junctions Zonula occludens

Communicating Junctions Gap junctions

Specialization of cell surface

For specific activities at the cell mambrane

Microvilli Fingerlike folds of cell membrane Increase surface area Present in absorptive cells e. g. small intestine

where the make brush or striated border 1 um in height and 0.08 um wide Within mv are clusters of actin microfilaments

forming terminal web at the base

Microvilli

STEREOCILIA Long nonmotile processes of cell Long microvill Present in epididymis Increase surface area for molecular movement

CILIA AND FLAGELLA Long, motile hair like structures 5-10 um long and 0.2 in dia Surrounded by the cell membrane Contain central pair of microtubule

surrounded by9 pairs of microtubules Inserted into basal body, inside the cell Cilia eg. In trachea for movement of mucous Flagela eg in sperms. Tail like movement

Cilia

The End