Epithelial Sheets & Cell Junctions

epithelial sheets

cover the external surface of the body and line cavities

absorptive cells

one way movement of nutrients. Take up nutrients from intestine and put into gut epithelium with goblet cells which secrete mucus into gut lumen.

Goblet cells

Secrete mucus at the apical end

tight junction function

seals neighboring cells together in a epithelial sheet to prevent leakage of molecules between them

adherens junctions function

joins an actin bundle in one cell to a similar bundle in a neighboring cell

desmosome junction function

“spot weld” that anchors the touch intermediate filaments in one cell to those in a neighbor

gap junction function

cell-cell junction allowing passage of small water-solubule ions and molecules

hemidesmosomes junction function 

anchors intermediate filaments in a cell to the basil lamina

Occluding junctions

tight

anchoring junctions

actin based: cell-cell (2), cell-matrix

intermediate filament based: cell-cell (3), cell-matrix (5)

communicating junctions

gap junctions (4), synapses, plasmodesmata

structure of tight junction

cells are sealed together by branching strands of transmembrane proteins called claudins anc occludins

tight junctions allow epithelial sheets…

to serve as a barrier to solute diffusion

anchoring junction components

Molecules in the plasma membrane mediate the adhesion, membrane associated intracellular proteins, and cytoskeletal elements 

what filament is adherens junction based? 

actin

what filament is hemi adherens based? 

actin

what filament is desmosome based? 

intermediate

what filament is hemidesmosome based? 

intermediate

structure of cell-cell adherens

MP: cadherins

IP: catenins, vinculin, actinin

Cytoskeleton: actin

structure of cell-matrix adherens

MP: integrins

IP: talin, vinculin, actinin

cytoskeleton: actin

desmosome structure

• cytoplasmic surface of each plasma membrane has dense plaque made of intercellular linker proteins

• bundle of keratin filaments is attached to the surface of each plaque

• cytoplasmic tails of transmem cadherins bind to plaque

• their extracellular domains hold cells together

intercellular proteins

plakoglobin, demoplakin

hemidesmosomes structure

TP: Integrin

IP at plaques; desmoplakin-like

Cytoskeletal : Intermediate

Hemidesmosomes anchor the…

keratin filaments in an epithelial cell to the basal lamina

gap junctions structure

connexon - entire structure

connexin - each subunit

gap junctions provide…

neighboring cells with direct communication

how many proteins make up connexon?

6 protein subunits (connexins)

how many connexons make a channel

2

gap junctions allow…

allow molecules to pass through cytoplasm

electrically couples as well as metabolically

gap junctions

plasmodesmata

how adjacent plant cells connect/communicate

plasmodesmata structure

• lined with plasma membrane

• usually also contains a fine tubular structure the desmotubule

• derived from smooth endoplasmic reticulum