cell-cell junctions and ECM

describe the lateral domain of cell-cell junctions

• epithelial cells are joined tightly by cell-cell junctions categorized using molecular technique:

  • tight junctions

  • desmosomes/hemidesmosomes (anchoring junctions)

  • gap junctions

• all apprear as a darkly staining structure reffered to an “terminal bar”

describe tight junctions

• form barrier that limits water and other molecules from passing

  • any molecule that pass must either diffuse/be transported through the epithelial cell

  • OR

  • diffuse through the occluding junction (varies based on tightness and number of extracellular aqueous channels present)

• located at most apical point of lateral domain

• prevent apical lipids and proteins from moving to the lateral domain

  • is limiting to the “fluid mosaic” PM

what do tight junctions do?

• seals the PM of adjacent cells by forming “focus fusion”

  • transmembrane proteins occludin and claudin

  • the extracellular portions of the transmembrane proteins form a “zipper”

  • a group of zone occludens (ZO) proteins regulate formation of the zona occludens

  • the ZO proteins (and others) localize here due to a recognizable amino acid sequence in the cytoplasmic domains

what are anchoring junctions?

• lateral adhesions between epithelial cells

• use proteins that link into the cytoskeleton of adjacent cells

main types of anchoring junctions

• zonula adherens - interact with actin filaments

• macula adherens (desmosomes) - interacts with intermediate filaments

adtl. anchoring junctions, attach the epithelial cells at the basal domain to connective tissue, 2 types

• focal adhesions and hemidesmosomes

describe desmosomes and hemidesmosomes

• cell adhesion molecules (CAMs) of neighboring cells interact:

  • homotypic binding

  • heterotypic binding

• adhesion of anchoring junctions dep on attractions of CAMs from neighboring cells

  • easier to dissociate than tight junctions

desmosomes are ___ and hemidesmosomes are ___

• cell to cell

• cell to basal lamina junctions

what do cadherins do?

• control cell-cell interactions

• participate in cell recognition

• participate in embryonic cell migration

• E-cadherin is important tumor suppressor

what do desmosomes do?

• provide very strong cell-cell attachment by anchoring to intermediate filaments

• analogous to series of “spot welds” along lateral domain

• not sufficient as a barrier alone but in conjunction with occluding junctions and zonula adherens strengthen cell-cell contact

what anchors intermediate filaments in desmosomes? members of attachment plaque?

• desmosomal attachment plaque on cytoplasmic side of PM anchors intermediate filaments

• desmoplakins and plakogolbins = members of attachment plaque

  

describe gap junctions

• structures allow passage of molecules and ions between cells

• present in many tissue types

• each communicating junction = an accumulation of transmembrane channels or pores

• connexins are a family of proteins that function to create the channels that comprise the communicating junction

  • communicating junctions can open and close via conformation changes in response to stimulus

basement membrane

• epithelial cells’ basal domain contacts basement membrane

• BM is a specialized structure that lies between the epithelial tissue and the CT

• amorphous, difficult to preserve and stain

• within the basement membrane — a thin, electron dense layer termed basal lamina

what is the basal lamina? what does it contain?

• basal lamina = fine network of sticky molecules like:

  • laminins, collagen (type IV), proteoglycans, glycoproteins

what are three major components of the ECM?

1. fibrous proteins (collagen, elastin)

2. glycosaminoglycans (GAGS) and proteoglycans

3. glycoproteins (fibronectin and laminin)

collagen fibers

• resist tensile force

• alpha-chains

• collagen molecules (triple helices)

• collagen fibrils

• collagen fibers

• 

elastic fibers involved in…

• stretching and relaxation

• single elastin molecule and crosslinks

• 

what are GAGS? what are proteoglycans?

• GAGS = chain of repeating disaccharides

  • sulfated, amino sugars are common

  • resist compression and lubricant

• proteoglycan - GAG chains linked to core protein

ECM glycoproteins

• fibronectin

• 

• laminin

• 

what is an integrin?

• it goes through the membrane — fibronectin and laminin receptor

•