Cell Junctions PDF

Cell junctions General Biology & Cytology Learning outcomes At the end of this lecture, the student will be able to: Define intercellular junctions and identify the different types Dr. Amira Osman Semester 1, Year 1  INTERCELLUAR JUNCTIONS Mem...

Cell junctions General Biology & Cytology Learning outcomes At the end of this lecture, the student will be able to: Define intercellular junctions and identify the different types Dr. Amira Osman Semester 1, Year 1  INTERCELLUAR JUNCTIONS Membrane associated structures join adjacent cells & provide adhesion and communication between them Types: Cell 1 Cell 2 1- Tight junction 2- Adherens junction 3- Gap junction  INTERCELLUAR JUNCTIONS 1- Occludens (tight Junction): ▪ the two cell membranes fuse with each other by their outer layers and the intercellular space between them is absent.(e.g. intestinal absorptive cells). Function: leading to severe 1- Occludens (tight) junctions Forming a complete seal that prevent passage of materials from the lumen to intercellular space (between the cells) Medical significance Defects in occludins may compromise the fetal blood-brain barrier, neurologic disorders  INTERCELLUAR JUNCTIONS 1- Occludens (tight Junction): Zonula occludens Fusion completely encircle the cell. Occluding junction Fusion isn’t complete. Fascia occludens Fusion is patchy.  INTERCELLUAR JUNCTIONS 2- Adherens junctions: ❑ Definition: Cells are joined by an intercellular binding material. ❑ Types: Zonula adherans Desmosomes  INTERCELLUAR JUNCTIONS 2- Adherens junctions: Zonula adherans ❑ Site : below Zonula occludens. ❑ Structure: normal 20 nm). filled - intercellular space is narrow 15 with nm ( an electron dense binding substance. - On the cytoplasmic surface of the opposed cell membrane there is Plaque.  Desmosomes Disc shaped structure like a spot between the adjacent cell membranes the intercellular space between the two adjacent cells is normal (20 nm). The membranes of both cells are thickened and show condensations on their cytoplasmic surface called plaques and there are Intermediate filaments (keratins) inserted into them. (i.e. 2 plaques & keratins). Functions of desmosome fix cells strongly and prevent their separation Between epithelial cells of skin & Between cardiac muscle cells  INTERCELLUAR JUNCTIONS Hemidesmosome Half desmosme between epithelial cells & basement membrane. Medical significance Autoimmunity against proteins leads to skin disorders  INTERCELLUAR JUNCTIONS 3- Gap junctions (nexus): ❑ there is narrowing of the intercellular space between the two adjacent cells to become 2 nm (normally it is 20 nm) but connected by channels (connexons) ( e.g. cardiac muscle). ❑ Structure: Cylinderical hydrophilic channels formed of integral proteins are arranged side by side in the junction.  Gap junction The 2 adjacent cell membranes are separated by 2nm gap Functions of gap junction: 1- direct passage of ions between cells 2- rapid transmission of nerve impulse between cardiac ms cells so heart contract as one unit  Epithelial polarity Cells have a top and a bottom and side modifications So different activities take place at different places Cell polarity Apical modificationsBasal modifications  Lateral modifications Apical modifications Microvilli formed of actin filaments Apical modifications Stereocilia Not true cilia but long microvilli. Non motile cilia  Apical modifications Cilia Basal modifications Basal infolding Basement membrane membrane in epithelial cells, the cell rest on basement