Cell Membrane, Part 3 - Cell Junctions

Question 1

tissues

Answer 1

tight communities of cells

Question 2

fundamental social interactions btwn cells

Answer 2

• those that hold cells together

Question 3

cell junctions

Answer 3

• consist of multi protein complexes that provide contact btwn neighboring cells or btwn a cell and ECM

Question 4

cell junctions functions

Answer 4

• bond cells together
• reduce stress placed upon cells
• build up paracellular barrier of epithelia
• control paracellular transport
• enable communication btwn neighboring cells

Question 5

2 types of tissues where cell junctions are abundant

Answer 5

1. epithelial tissues

2. connective tissues

Question 6

epithelial tissues

Answer 6

• ex: lining of gut or epidermal covering of skin
• cells tightly bound together into sheets called epithelia
• direct attachments btwn cells are made by cell-cell junctions
• cytoskeletal filaments are anchored onto these junctions and also linked to basal lamina through cell-matrix junctions

Question 7

connective tissues

Answer 7

• ex: bone or tendon
• formed from an ECM produced by cells that are distributed sparsely in matrix (fibroblasts)
• matrix bears most of the mechanical stress to which tissue is subjected, not cells
• direct attachments btwn cells are rare
• cells have important attachments to matrix via cell-matrix junctions

Question 8

cell junctions are also present in other tissues

Answer 8

• cardiac muscle
• smooth muscle
• nervous tissue

Question 9

types of cell-cell junctions

Answer 9

• tight junctions
• cell-cell anchoring junctions
  
  • adherens junction
  • desmosomes
• gap junctions (communicating junctions)
  
  • channel-forming junctions

Question 10

types of cell-matrix junctions

Answer 10

• cell-matrix anchoring junctions
  
  • actin linked cell-matrix junctions
  • hemidesmosomes

Question 11

tight junctions

Answer 11

• seal gap btwn epithelial cells
  
  • seal the intercellular space from luminal environment
  • seal is not absolute
    
    • impermeable to macromolecules
    • permeability to ions and other small molecule varies
  • serve as selective permeability barriers for paracellular transport
• mostly at apical location
• found in epithelial cells
  
  • ex: lining of intestinal mucosa, bladder, etc

Question 12

cell-cell anchoring junctions

Answer 12

• depends on transmembrane adhesion proteins called CADHERINS (calcium-dependent adhesion) that span membrane, with one end linking to cytoskeleton and other end linking to structures outside it
• anchoring junctions form strong, membrane-spanning structures that are tethered inside cell to the tension-bearing filaments of the cytoskeleton
• widely distributed in tissues and most abundant in tissues that are subjected to severe mechanical stress such as heart, muscle and epidermis

Question 13

cell-cell anchoring junctions occurs in 2 functionally different forms

Answer 13

1. adherens junctions (desmosomes): hold cells together and are formed by transmembrane adhesion proteins that belong to CADHERIN family
2. focal adhesions and hemidesmosomes bind cells to the eCM and are formed by transmembrane proteins of the INTEGRIN family

Question 14

adherens junctions

Answer 14

• aka zonula adherens, intermediate junction, “belt desmosome” or adhesion belt
• connect one actin filament bundle in one cell with that in the next cell
  
  • provide anchorage site for cytoskeleton
• help modeling shape of multicellular structures
  
  • guides organization of developing tissues
    
    • ex: construction of vertebrate nervous system
  • important in tissue remodeling
• located more basal than tight junctions
• found in epithelial cells, cardiac muscle, fibroblasts

Question 15

desmosomes

Answer 15

• aka macula adherens
• connect intermediate filaments in one cell to those in the next cell
  
  • provide anchorage site for cytoskeleton
  • structurally similar to adherens junctions but contain specialized proteins that link with intermediate filaments instead of actin
• main function: provide mechanical strength
• located in tissues that suffer a lot of stress/stretch like skin, intestines, stomach, bladder

Question 16

gap junctions

Answer 16

• allow passage of small water-soluble molecules from cell to cell
  
  • without having to pass through plasma membrane
  • ex: ions, glucose
• gap is narrow
• very important in tissues containing electrically excitable cells
  
  • action potentials can spread rapidly from cell to cell
  • ex: cardiac contraction, peristaltic movements of intestines
• located in connective tissue, epithelial tissue, cardiac muscle, neurons…

Question 17

cell-matrix anchoring junctions

Answer 17

• depend on interns (transmembrane proteins (receptors)) like cadherins that bridge cell-cell and cell-matrix interactions
• activated after binding ECM ligands or intracellular activator proteins, and produce intracellular signals that can influence almost an aspects of cell behavior (proliferation, survival, polarity and migration)
• sense and respond to mechanical forces acting across junction

Question 18

actin-linked cell matrix junctions

Answer 18

• aka focal adhesion
• anchor actin filaments in cell to ECM
• can be small and transient or large and durable
  
  • ex: focal adhesions of fibroblasts to a culture dish; myotendinous junctions attaching muscle cells to their tendons

Question 19

hemidesmosomes

Answer 19

• anchor intermediate filaments in a cell to ECM
• similar in form to desmosomes
  
  • instead of desmogleins, hemidesmosomes use integrins
  • found in epithelial cells connecting basal face of cell to basal lamina