EPITHELIUM

Dr. Maryam Fatima

Anatomy Department

Objectives

• Cell junctions

• Epithelium: Definition, functions, classification

• Simple epithelium

Cell Junctions

• Adhesion is due in part to the binding action of a family of transmembrane glycoproteins called cadherins. Cadherins lose their adhesiveness in the absence of calcium.

• While describing these cell junctions, two factors are taken into account:

• 1. extent of contact area: macula( spot), zonula (belt) • 2. nature of cell contact: occludens, adherens

(intercellular space =20 to 25 nm wide), gap junctions (intercellular space= 3nm)

So keeping in view the mentioned factors Cell junctions are classified as:

• 1. Macula Adherens (desmosome or spot desmosome)• Small discoid structures• Located at various levels on lateral cell surface• Intercellular gap is 25 nm n contains an adhesive

glycoprotein called desmocollin.• Intracytoplasmic densities- attachment plaques are seen

beneath the plasma membranes of both cells making the junction.

• Hemidesmosome:

• The main structures that participate in cohesion among epithelial cells. The drawing shows 3 cells from the intestinal epithelium. The cell in the middle was emptied of its contents to show the inner surface of its membrane. The zonula occludens and zonula adherens form a continuous ribbon around the cell apex, whereas the desmosomes and gap junctions make spotlike plaques. Multiple ridges form the zonula occludens, where the outer laminae of apposed membranes fuse.

• 2. Zonula Adherens (belt desmosome)• Intercellular gap is 25nm• This gap is not bridged by filaments.• Submembrane cytoplasmic densities and filaments are

present.• Example: this type of cell junction is found among the

epithelial cells, fibroblasts, smooth muscle cells etc.

• 3. Zonula Occludens (tight junctions):• Plasma membranes of adjacent cells fuse leaving no

intercellular gap.• E.g; intestinal mucosa, urinary bladder mucosa. It

provides the sealing effect, preventing the substances from diffusing into the intercellular space.

• 4. Gap junctions (nexus):• Intercellular gap is of 2-3nm• These are permeable to molecules <1500 Da e.g; ions,

proteins, hormones, AMP etc.• Connexins (gap junction protein units-hexamers,

individual unit is called connexon) pore size 1.5nm• Examples: cardiac muscles, smooth muscles, liver,

kidney, nervous tissue.

• 5. Junctional complex:• ZO+ ZA+ MA= Junctional complex.

EPITHELIUM

• Definition: collection of closely packed cells with very small amount of intercellular substance.

• Avascular• Rests on basement membrane.• Nourishment??• Functions: protection (st. sq-oral

cavity,esophagus, skin), secretion (goblet cells, glands), absorption (intestinal cells), excretion (sweat, CO2), transport (cilia), reproduction (germinal epithelium of gonads), sensory reception (neuro-epithelial cells)

Classification:

Epithelium

Covering epithelium Glandular epithelium

Covering epithelium

• Definition: “The component cells of these epithelia are organized into one or more layers which coat the body surface.”

• Covering epithelium is divided into two main types:

1. simple epithelium

2. Stratified epithelium

Simple epithelium• Squamous (endothelium,

mesothelium,alveoli)

• Cuboidal (thyroid gland, distal convoluted tubules of kidney, small excretory ducts of glands, on the surface of ovary)

• Columnar (internal surface of stomach, intestines, gall bladder and uterus),

• Pseudostratified (conducting part of repiratory tract, male genital system)

• Section of a vein. All blood vessels are lined with a simple squamous epithelium called endothelium (arrowheads). Smooth muscle cells in the vein wall are indicated by arrows. Pararosaniline—toluidine blue (PT) stain. Medium magnification

• Simple cuboidal epithelium from kidney collecting tubules. Cells of these tubules are responsive to the antidiuretic hormone and control the resorption of water from the glomerular filtrate, thus affecting urine density and helping retain the water content of the body. PT stain. Low

magnification.

• Simple columnar epithelium that covers the inner cavity of the uterus. Note that the epithelium rests on the loose connective tissue of the lamina propria. The epithelium and the lamina propria constitute the mucosa.

Stratified Epithelium

• Stratified means: more than one layer of cells• Classified acc.to the shape of the cells in the

superficial layer:• Stratified squamous epithelium: keratinized and

non-keratinized• Stratified cuboidal epithelium• Stratified columnar epithelium• Special type of epithelium: Transitional

epithelium.

Stratified squamous epithelium

• The deepest or basal layer which rests on a basement membrane is formed by cuboidal or low columnar cells.

• Next to basal layer are present a few layers of larger polygonal cells.

• As the free surface is approached the cells gradually become flattened and at the surface they are squamous.

• Taking into account the state of most superficial cells, two sub-varieties of stratified squamous epithelium are recognized:

• Keratinized stratified squamous epithelium• Non-keratinized stratified squamous epithelium

Keratinized stratified squamous epithelium

• Other name is stratified squamous cornified epithelium.

• It is present in those places where the surface cells are subjected to more attrition and drying.

• The surface cells are non-nucleated and their cytoplasm contains large numbers of filaments of a protein called “keratin”.

• Due to the presence of keratin the surface cells become dry and horny.

• Example: epidermis

Non-keratinized stratified squamous epithelium

• Other name is non-cornified stratified squamous epithelium.

• This epithelium covers the wet surfaces (e.g; esophagus).

• The surface cells become very flat but mostly remain nucleated and their cytoplasm contains little or no keratin.

• Examples: lining of most of the oral cavity, oropharynx, esophagus and vagina.

Stratified cuboidal epithelium

• This type of epithelium consists of two layers of cuboidal cells.

• this epithelium has a very restricted distribution in the human body; the best example is the epithelium of the ducts of sweat glands.

Stratified columnar epithelium

• It consists of columnar surface cells which rest on one or more layers of roughly cuboidal cells.

• It also has a restricted distribution in the body.

• Examples: it lines some parts ductus deferens and male urethra. Main ducts of salivary glands and pancreas. Conjunctival epithelium of eye.

• Note: the surface cells of st. cuboidal and cst. Columnar epithelium are not continuously replaced by basal mitosis and there is no progress from base to surface.

Transitional epithelium

• Name: earlier, histologist considered this epithelium to be intermediate between stratified squamous and stratified columnar epithelia. Later studies proved this concept to be incorrect but the term continues.

• Alternative name is urothelium because this special variety of epithelium is found exclusively in excretory urinary passages: the renal calyces, renal pelvis, ureters, urinary bladder and part of urethra.

• Appearance: in undistended state, the epithelium is seen to consist of 5 or 6 cell layers. The cells of the most basal layer are cuboidal in shape,mononucleate and their cytoplasm stains basophilic due to the presence of abundant ribosomes.

• Over the basal are present several layers of larger polygonal cells. These cells are either binucleate or uninucleate.

• The superficial layer of epithelium appears to consist of very large cells with a characteristic highly convex(dome-shaped) luminal surface.

• In fully distended state: the transitional epithelium is seen to be comprised of 2 to 3 layers: a basal layer of cuboidal cells over which are present one to two layers of large squamous (flat) cells.

• The luminal plasma membranes of the large surface cells are unusually thick.

• Occluding junctions are present between adjacent cells.

• Membrane internalization………………

Revision slides

Revision

Revision

Modifications at cell surface

• 1. Microvilli: small, finger like projections found on the free

surface of epithelial cells in variety of locations. • Size: 1-2 micrometers in height, 0.1 micrometer in diameter• Structure: contains a central bundle of 20-30 microfilaments (actin)

extend into the transverse network of the cytoplasmic filaments (myosin) forming the terminal web.

• Example: located on the luminal surface of small intestine, cells lining the proximal convoluted tubules of the kidney

• Functions: increase the surface area of the cells in correlation with the absorptive function. Myosin filaments of the terminal web results in oscillatory contractile movement of microvilli which facilitates the absorptive process.

• Electron micrograph of the apical region of an intestinal epithelial cell. Note the terminal web composed of a horizontal network that contains mainly actin microfilaments. The vertical microfilaments that constitute the core of the microvilli are clearly seen. An extracellular cell coat (glycocalyx) is bound to the plasmalemma of the microvilli.

• 2. Cilia (Kinocilia):• Hair-like processes found chiefly on the free surface of those

epithelial cells which are specialized for transport of fluid or film of mucus over the epithelial surface.

• Size: 5-10micrometers in length, 0.2 micrometer in diameter.• Sites: conducting part of respiratory system, fallopian tubes of

uterus• Structure: axoneme (microtubes)…, • basal body (structure similar to centriole)• Functions: kinocilia beat rapidly in constant direction resulting in

propulsion of a layer of mucus, fluid, particulated matter or ovum over the epithelium.

• 3. Flagellum: long motile whip like projection from the cell is called a flagellum.

• Structure: axoneme• Undulating movement helping in the locomotion of the entire cell as

a whole.• Example: spermatozoan.

• 4. Stereocilia: appear a sthin, hair-like structures which are seen to be in contact with each other to form small tufts. Size: 30 micrometers.

• They appear to resemble cilia but because of their incapability to exhibit movements in living cells they were givem the special name of stereocilia.

• Structure: long microvillus which lacks a well-developed central microfilament bundle. Hence they are fexible and wind around eachother resulting in a tuft-like appearance.

• Example: location: duct of epididymis, neuro-epithelial cells, sustentacular cells of tastebuds, hair cells of internal ear.