BCH 443BCH 443Biochemistry ofBiochemistry of

Specialized TissuesSpecialized Tissues 4. Epithelial Tissues4. Epithelial Tissues

EpitheliaEpithelia• Epithelial tissues arise from any of the 3 primary

germ layers of the embryo.

• EpitheliumEpithelium– Skin (ectoderm)– Digestive tract (endoderm)

• MesotheliumMesothelium– Peritoneal cavity (mesoderm)

• EndotheliumEndothelium– Blood vessels, lymphatic vessels, heart

(mesoderm)

Functions of EpitheliaFunctions of Epithelia• ProtectionProtection

– Against wear and tear– Keratin and mucus prevent drying

• AbsorptionAbsorption– Microvilli in kidney and intestine

• Surface transportSurface transport– Via cilia on cell surface

• SecretionSecretion (all glands are epithelia)– Hormones, digestive enzymes, mucus

• Sensory receptionSensory reception

How to classify epitheliaHow to classify epithelia

(1) Shape(1) Shape

(2) Number of layers(2) Number of layers

(3) Specializations(3) Specializations

(4) Covering and lining vs. (4) Covering and lining vs. glandularglandular

All Epithelia are All Epithelia are AvascularAvascular

• Blood vessels supplying epithelia are found in the underlying connective tissue.

• Beneath the basement membrane

Basement MembraneBasement Membrane

• Lies between epithelium and underlying connective tissue

• It is the external lamina (coat) for muscle and nervous tissue

• Three layersThree layers– Top two from

epithelia, also called basal laminabasal lamina

• Lamina rara (lucida)Lamina rara (lucida)

• Lamina densaLamina densa

– Lowest from connective tissue

• Reticular lamina Reticular lamina (lamina (lamina fibroreticularis)fibroreticularis)

Basement membrane components

Basement membraneCan be seen with light microscopy

lumen

Reticular laminaType II collagen(reticular fibers)

Basal laminaLamina raraLamina densa

Parts of the basement membraneParts of the basement membrane

• Basal laminaBasal lamina– type IV collagentype IV collagen

– Heparan sulfateHeparan sulfate

– Fibronectin and Fibronectin and lamininlaminin

• Reticular laminaReticular lamina– Type II collagen

– Also called reticular reticular fibersfibers

Functions of the basement Functions of the basement membranemembrane

• Structural support via cell-matrix adhesions

• Allow nutrients and waste to diffuse

• Filter for macromolecules (kidneys)

• Zone for differentiation and polarization of cells

• Plays a role in regeneration by acting as a “highway” for cell migration

Simple Vs. Stratified EpitheliaSimple Vs. Stratified Epithelia

• SimpleSimple– One layer of cells– All cell touch basement membrane

• StratifiedStratified– Two or more layers– Only bottom layer of cells touch basement

membrane

Simple Squamous EpitheliaSimple Squamous Epithelia

• Flattened, scale-like, disc shaped nucleus

• Exchange simple gases, protection

• Kidney tubules, blood vessels, alveoli, lining major body cavities

Simple Cuboidal EpitheliaSimple Cuboidal Epithelia

• Cuboidal shape, spherical nucleus

• Secretion and absorption

• Ovary, renal medulla, ducts

Simple Columnar EpitheliaSimple Columnar Epithelia• Column shaped

• Organelles near lumenal surface

• Nucleus near basement membrane,

• Height varies by functional activity

• Absorption in small intestine

• Secretion in stomach

Cell surface specializationsCell surface specializations

• MicrovilliMicrovilli– Contain actin filament

core

– Atop most absorptive columnar epithelia

– Called striated or brush border

• Kidney, small intestine

• CiliaCilia– Contain 9x2

arrangement of microtubules

– On pseudostratified epithelia

• trachea

– Some columnar epithelia

• fallopian tube

One more specialization

• StereociliaStereocilia– Modified microvilli,

longer

– do NOT contain microtubules

– May help facilitate absorption

– Epididymis, hair cells of ear

Pseudostratified Ciliated Columnar Pseudostratified Ciliated Columnar EpitheliaEpithelia

• All touch basement membrane• Columnar cells, ciliated, some secrete mucus,

nuclei at varying levels• Traps and moves dirt out• Trachea and bronchi

Stratified Stratified Squamous Non-keratinizingSquamous Non-keratinizing EpitheliaEpithelia

• Several cell layers deep

• Cells toward basement membrane cuboidal, near lumen, flattened

• Protects against abrasion in moist areas

• Mouth, esophagus, vagina and anus

Esophageal-gastric junctionEsophageal-gastric junction

Stratified squamous non-keratinizing epitheliaStratified squamous non-keratinizing epithelia

stomach

esophagus

lumenlumen

lumenlumen

Stratified Squamous KeratinizingStratified Squamous Keratinizing• Same as non-

keratinizing but with layers of keratin and dead cells on surface

• Protects against abrasion in non-moist areas

• Skin– thicker in areas

prone to more abrasion

Thick skin

Epithelial cells

Connectivetissue

keratin

Stratified Cuboidal EpitheliaStratified Cuboidal Epithelia

• Rarely found

• Usually only two or three layers thick

• Stronger than simple epithelium

• Found lining larger ducts – salivary, pancreas, sweat ducts

Transitional EpitheliaTransitional Epithelia

• Able to stretch and relax to accommodate urine in bladder,

• Morphology changes depending on distention.– Rounded, scalloped

edges when bladder is empty

– Stretched-out, stratified squamous appearance when bladder is full

Types of JunctionsTypes of Junctions

• Tight junctions Tight junctions ==occluding junctions

• DesmosomesDesmosomes– belt desmosomes belt desmosomes

==zonula adherens– spot desmosome spot desmosome

==macula adherens

• HemidesmosomesHemidesmosomes

• Gap junctionsGap junctions

Tight junctionsTight junctions

• Extracellular surfaces of two adjacent plasma membranes are joined together so there is no extracellular space between them

• Occurs in a band around the entire cell

Sertoli cellsSertoli cells

• In the seminiferous tubule

• Sit on basal lamina• form zonula

occludentes• protect sperm cells

from immune system• Artificial pancreas?

Tight junctions (Con’t)Tight junctions (Con’t)

• Restrict the movement of most organic molecules between cells, but may leak small ions and water

• Not associated with any cytoskeletal components

Belt desmosomeBelt desmosome

• Zonula adherens• Another belt around

the cell• Below the tight

junctions• An anchorage

junction• Associated with

actinactin filaments• Space between

membranes can be seen

Spot DesmosomesSpot Desmosomes• A region between two cells where

membranes are separated by 20nm

• Dense accumulation of protein at the cytoplasmic surface of the membrane

Desmosomes (Con’t)

• Keratin fibers extend from the cytoplasmic surface to other side of cell to next desmosome

• Holds adjacent cells together in areas of stretching– skin, cardiac muscle, bladder

Gap junctionsGap junctions

• Protein channels link the cytosols of cells– Passage of small molecules and ions (Na+, K+) – Excludes large molecules– Transmits electrical activity between cardiac

and smooth muscle cells– Allows chemical messengers to cross from one

cell to another– Coordinates activities between cells

Gap junctionsGap junctions

Freeze fracture freeze etchFreeze fracture freeze etchTEMTEM

Gap junction connexonsGap junction connexons

• A connexon is a cylinder with a central open pore

• One gap junction connexon is made up of six connexins

• The pore is a hydrophilic channel between two cytoplasms

• Plasma membranes come within 2-4nm of each other

Cellular JunctionsCellular Junctions

Occluding jxns

zonula adherens

macula adherens

HemidesmosomeHemidesmosome

• Assymetrical structures• A plate anchors the basal part of cell to the

basal lamina• This plate contains IFs called keratins or

tonofilaments• Membrane plaque linking hd to bl via

anchoring filaments• Contributes to overall stability of epithelia

Hemidesmosomes

Melanin Formation

CH2CHCO2-

NH3

O

O

+

CH2CHCO2-

NH3+

HO

HO

Highly colored polymeric

intermediates

Melanin(Black polymer)

Tyrosinase

DOPA

Dopaquinone

CH2CHCO2-

NH3+

HO

Tyrosine

Tyrosinase

Melanin formed in skin (melanocytes), eyes, and hairIn skin, protects against sunlightAlbinism: genetic deficiency of tyrosinase www.albinisim.org