Epithelial tissue and its variations

141

Definition Avascular tissue Cells are closely apposed and adhere to

each other by cell junctions They exhibit functional and

morphological polarity They are attached to basal lamina

Classification according to function

Covering and lining epithelial tissue– Protection– Absorption– Transportation

Secretory – glandsReceptor – sensory organs Contractile (myoepithelial cells)

Cell polarityApical domainLateral domainBasal domain

Functionally different = specialized surfaces

Cell junctions divide cell membrane into apical and basolateral region

The apical domain modifications

Microvilli – frequent in all cell types – sign of absorption – brush border (striated border)

Stereocilia – – Male genital system – extremely long

and branching microvilli– Sensory hair cells – derived from

microvilli - mechanoreceptorsCilia – motile cilia, primary cilia (sensoric),

nodal cilia

MicrovilliActin filaments 20-30 –

anchored to membrane by villin

Crosslink by actin bundling proteins

Terminal web – actin, spectrin, myosin II

Brush border

Enlargement of apical domain

Enzymes, carrier proteins, ion channels

Glycocalyx

Occurence : enterocyte, cells of proximal tubule in kidney

Stereocilia

Extremely long microvilli – resorption – actin,

α actinin instead of villin

Stereocilia in sensory hair cells of inner ear

Mechanoreceptors – actin and espin crosslink

Structure is similar to microvilli

Cilia

Kinocilia and flagella – axonema – 9+2 microtubules + dynein

Primary cilia – immobile – lack of motor-proteins.

• Sensoric function – chemoreceptor, osmoreceptor, mechanoreceptor

• Primary cilia of developing cells are essential for normal morphogenesis

Nodal cilia – bilaminar embryonic disc – primitive node - similar to primary cilia but wit ability of rotational movement

KinociliaLength 5-10μm Axonema: 9 duplets A13,

B10 + 2 central microtubules

Basal body = centriol – 9 triplets

Basal foot and striated rootlet – coordination of ciliary movement and anchorage to cytoplasm

Primary cilia9+0Lack of motor-

proteinand central pair of

microtubulesBasal body Primary cilia

formation is synchronized with cell cycle progression and centrosome duplication events

Receptor:Rods and conesHair cells of inner earOlphactory

epitheliumlMechanoreceptor – in

ducts of glands – control of Ca++ in cell – polycystic kidney disease, retinitis pigmentosa

Nodal cilia

9+0, but motile!!!Rotational

movementMechanim

generating the right-left asymetry

Lateral domain modification

• Junctional complex:• zonulae occludentes

– tight junctions• zonulae adherentes• maculae adherentes

(desmosomes)• gap junctions• Lateral infoldings

(plicae), interdigitations

Intercellular cohesiveness is established by cell-to-cell adhesive molecules

• CAM – cell adhesion molecule• Cadherins - transmembrous proteins

(intracellular domain is bound to the cytoskeleton, actinu/intermediate filaments, extracellular domain fills up the intercellular space)

• Disorder: lost of cohesiveness – invasiveness of tumor cells

Zonula occludens – tight junction

Apical region of the junclional complex

Diffusion barrier Separation of apical

and lateral domains

Occludins and claudins

-anastomosing belts - actin

Anchoring (adhesive) junctions - zonula adherens

E-cadherin, Catenin, vinculin +

α-actininActin

Ca++ dependent

Anchorin junctions- desmosome

Desmocollins and desmogleins

Intermediate filaments (cytokeratins)

Ca++ dependent

Communicationg junctions – gap junction (nexus)

Accumulation of transmembrane channels or pores – connexons – inchange of molecules between cells - signaling

Basal domain – basal labyrint

Enlargement of basal surface + mitochondria

= Absorption of ions – Na/K ATPase and sodium pump

Occurence – proximal and distal tubules of kidney

Basal domainAttachment to the

extracellar matrix (focal adhesions – FAK and hemidesmosomes) - integrins

Focal adhesion – actin

Hemidesmosome - cytokeratin

Basement membrane (LM) is the layer of extracellular material separating epithelia from the

connective tissue• Basal lamina and lamina reticularis • Basal lamina (EM): lamina densa and lamina

rara • Lamina rara: receptors for laminin and

fibronectin • Lamina densa : laminin + collagen IV,

proteoglycans and glycoproteins• Lamina reticularis: reticular fibres (collagen III),

anchoring fibrils (collagen VII), fibrilin

Epithelial tissueCovering – cells are

polarized with free surface

Forming cords- no free surface – endocrine glands

Reticular – epitheloreticular cells - network - thymus

Secretory portion = parenchyma

Classification of epithelial tissue (covering)

SimpleSquamousCuboidalColumnarPseudostratified

columnar

Classification of epithelial tissue (covering)

StratifiedSquamous

• Keratinized• Nonkeratinized

CuboidalColumnarTransitional (Urothelium) -

Glands

Exocrine glands – secrete their product onto a surface

Secretory portion and duct – cells have a free surface

Endocrine – secrete their product (hormones) into the blood Cells with free

surface – thyroid gland

Cords of cells