Epithelial tissue

Epithelial tissue general characteristics

• Covers body surface

• Lines hollow organs, cavities and ducts

• Forms glands

Epithelial tissue key functions

• Secretion (onto free surface)

• Protection (such as from abrasions)

• Selective barrier (limit/aid transfer)

Cytoskeleton

Microfilament e.g. actin

• Bundles beneath cell membrane and cytoplasm

• Strength, alter shape, link cytoplasm to membrane, tie cells together, muscle contraction

Intermediate filaments e.g. keratin

• Strength

Tight junctions

• Individual sealing strands

• Transmembrane

• Two key proteins involved: claudins + occludins

• The more strands, tighter the junction

• Joins cytoskeletons of adjacent cells suc as ZO-1 linking to actin (microfilament)

Tight junction function

Keeps cell polarity by preventing migration of proteins between apical and basal surfaces

Lots of tight junctions are found in the:

• stomach

• intestines

• bladder

Adherens junction

• Belt desmosome/adhesion belts in some epithelial cells

• Other cells less continuous called adhesion plaques

• More basally located compared to tight junctions

• Has a plaque layer of proteins inside of cell linking actin to cadherins

  • Cadherins span gap

  • Catering link actin to cadherins

Adherens junction function

Keep cells from separating from tension forces like contractions

Desmosome junctions

• Located on lateral wall

• Have plaque like adherens junction

  • Cadherin spams gap and links to desmoplakin which is linked to keratin (intermediate filament)

  • Keratin spans from one desmosome to another on other side of cell for structural integrity

Desmosome junction function

Bind cells together, such as prevent cardiac muscles from tearing apart

Lots of desmosome junctions found in:

• Skin epithelium

• Cardiac cells

Gap junctions

• 6 connexIN protein molecules from one connexON/hemichannel

• 2 hemichannels make up one gap junctions

Gap junction function

Provides direct connection between cells allowing up to ~1kDa small molecules through

Lots of gap junctions found in:

• Epithelial tissue

• Neurons

• Smooth muscle cells

• cardiac cells

Hemidesmosome

• Integrin linker protein instead of cadherins, linking BM to keratin

• Binds to laminin in the BM and keratin in the cytoplasm

Hemidesmosome function

Connects epithelia to basement membrane

Junctional complex

• Tight junction

• adherens junction

• desmosome junction

Basal lamina

• secreted by epithelial cells

• contains collagen, laminin, other proteoglycans, glycoproteins

Reticular lamina

• Secreted by fibroblasts, cells of underlying connective tissue

• Contains fibrous proteins such as collagen and fibronectin

Epithelia contains:

• Nerves

• No blood vessels

Basement membrane functions

• Support overlying epithelium

• Provide surface along where epithelial cells migrate during growth and wound healing

• Physical barrier

• Filtration of substances in kidney

Simple epithelia

single layer (secretion; absorption; filtration)

Stratified epithelium

two or more layers (protective)

Psuedostratified epithelium

• appears to have multiple layers as judged by positions of nuclei.

• Not all cells reach the apical surface.

• All cells are in contact with the basement membrane i.e. actually a simple epithelium (secretion).

Squamous

Flat and thin (helps allow passage by diffusion)

Cuboidal

tall as they are wide (secretion + absorption)

Columnar

more tall than wide (secretion + absorption)

Transitional

a stratified epithelium in which the cells can change shape from cuboidal to flat shape depending on organ shape (allow stretch e.g. urinary bladder)

Simple squamous features

• Most delicate epithelium

• Where there is filtration (kidney); diffusion (lung); secretion where slippery surface needed (e.g. outer layer of serous membranes);

• In Bowman’s capsule of kidney, lines cardiovascular and lymphatic systems, inside eye, alveoli of lungs, visceral cavity linings, inside blood vessels and inside heart

Simple cuboidal epithelium features

• where there is absorption + secretion

• located in Pancreas ducts; parts of kidney tubules, smaller ducts of many glands; secretory chambers of thyroid; anterior surface of lens; pigmented epithelium at posterior of retina; secretory part of some glands like thyroid

Simple columnar epithelium features

More cytoplasm so more organelles

Reflects being more metabolically active than squamous cells

Non-Ciliated Simple Columnar Epithelium features

• Single layer

• Microvilli on apical surface

• Have goblet cells

• Can have microvilli—increase surface area for absorption

• Locations: lines gut mucosa from stomach to anus, ducts of several glands, gall bladder

• Function:

Ciliated Simple Columnar Epithelium (move materials)

• Single layer

• Also has goblet cells

• Locations: some bronchioles, uterine fallopian tubes, sinuses; central canal of spinal cord, ventricles of brain

• Function

Pseudo-stratified ciliated Columnar Epithelium features

• Cilia on some cells

• secrete mucus from goblet cells

• Locations: most upper air ways

• Functions: secrete mucus, move it

Pseudo-stratified non-ciliated Columnar Epithelium features

• No cilia

• Lacks goblet cells

• Locations: larger ducts of glands, epididymis, part of male urethra

• Function: absorption, protection

Simple squamous specialised subtypes

• Mesothelium lines pericardial, pleural, peritoneal cavities

• Endothelium lines inside of heart and the blood and lymphatic vessels

Endocrine gland

Secretion example: secrete directly into blood traversing interstitial fluid

Examples: pituaitary, pineal, thyroid, parathyroid

Generally strong distant effects

Exocrine gland

Secretion pathway: Secrete into ducts that empty onto the surface of a covering or lining epithelium

Examples: sweat + salivary glands, oils glands, wax glands, pancreas

Generally local effects