Epithelium Flashcards
Terminal Bars
Terminal bars represent cell-to-cell attachment sites in apical region
Characteristics of Epithelium
Avascular; dependent on diffusion of nutrients thus, limits its thickness.
Cytokeratin intermediate filaments
Continuouslly renewed by mitosis; in layer closest to the basal lamina (stem cells)
Variable mitotic rate:
Intestinal epithelium half-life = 4-6 days
Skin epidermis half-life = 2-4 weeks
Can form various glands
Cilia
5-10 mm long; can see individually
motile structures
function to move fluid over cells
found in trachea, bronchi, oviducts, flagella of sperm
Cilia/Flagella Internal Structure
Motile; 7 – 10 mm long
Microtubular core: 9+2
Dynein motor proteins: ATPase
Basal body: attachment of a cilium to the cell’s apex; 9 triplets of microtubules; no central pair
Microvilli
Can’t see a single microvillus
Striated border (gut)
Brush border (kidney)
Function: involved in absorption of excess fluid, nutrients, etc.
Microvilli Internal Structure
Non-motile; 1-2 mm long
Core of actin filaments
Anchored in terminal web (contractile)
Intermediate filaments = cytokeratin
Other cross-linking proteins:
villin: anchors actin to the tip
myosin I anchors actin to cell membrane
Has fuzzy coat of glycoproteins with enzymatic activity = Glycocalyx
Stereocilia/Stereovilli
Long microvilli; 120 um
No villin in the tip
Ezrin anchors actin to cell membrane (no myosin I)
Limited in distribution: in epididymis, inner hair cells, trachea
Function is absorption and are non-motile
Types of Cell Junctions
Occluding (tight) junctions (seals) join cells together to form an impermeable barrier.
Adhering junctions (cables) provide mechanical stability by linking cytoskeletons of adjacent cells.
a: zonula adherens
b macula adherens or desmosome
Communicating (gap) junctions (channels) permit movement of ions or molecules between cells.
Zonula Occludens
A belt of membrane fusion
Relatively impermeable diffusion barrier between adjacent cells
A physical & chemical separation b/n lumen and intercellular space
Prevents movement of fluid, so must go through cytoplasm, basement membrane, and go into blood vessels underneath
Excess fluid and tight junction destroyed = diarrhea and fluid leaving into lumen of intestines
Zonula Occludens Molecular Structure
3 transmembrane proteins, JAM, occluding, and claudin function like a zipper between cells
3 cytoplasmic proteins: ZO-1, 2, 3, connect to actin microfilaments
Zonula Occludens Pathologies
Can have mutations of proteins or bacterial toxins act on proteins
Cholera toxin work on ZO 1 and 2; causes dehydration and death
Claudin: clostridium bacterium destroys this transmembrane protein to increase permeability and causes diarrhea
Zonula Adherens
Belt Desmosome
Cell membranes not fused but velcro-like
Calcium-dependent cell adhesion molecules link cytoskeletons of adjacent cells
Actin filaments facilitate attachment to adjacent cells
alpha-actinin – an actin-binding protein also found in the Z-line of skeletal muscle and the dense bodies of smooth muscle.
Proteins that are Ca2+ dependent adhesion molecules
Macula Adherens
Desmosome/Spot Weld
Intermediate filaments help dissipate force pulling on attachment site = tonofilaments
Desmocollin and desmoglein are types of calcium-adhesion molecules.
Intracellular space between cells: desmocolin and desmoglein attach to attachment plaques
Hemidesmosome
Cells on basement membrane will be attached via a hemidesmosome
Plaque of different proteins with keratin filaments attached
Gap Junction
Allow diffusion of small (
Basement Membrane
Underlies all epithelia
A barrier for tumor cell invasion b/n the epithelium and the adjacent CT
Basement Membrane Components
Basal Lamina : laminin & type IV collagen (epithelial cells)
fibronectin (fibroblasts)
heparan sulfate proteoglycans (perlecan)
nidogen/entactin (glycoprotein)
Reticular fibers: type III made by fibroblasts from CT
Major Proteins in Basal Lamina
The interwoven network of proteins allows for: structural attachment, filtration, compartmentalization, tissue scaffolding, regulation & signaling
Base of cells to attach to basement membranes: integrins and integrin receptors
Sheet like layer called laminin = cross shape, and self assembles into the sheet and below that sheet is type IV collagen and proteins act as bridges between these two sheets called nitogen and entactin and perlecan (heparin sulfate)
Perlecan is the most common proteoglycan in BL
All made by epithelial cells
Cell-Basal Lamina Interactions
Basal epithelial cells have cell-surface molecules (integrins) that bind to collagen, laminin, and fibronectin in the BL
