Items 35-44

For each numbered statement of function described in the items below, choose the most appropriate lettered component of the junctional complex or associated structure. Answers may be used once, more than once, or not at all.

(A) Tight junction (zonula occludens)
(B) Adhensive junction (zonula adhenrens)
(C) Desmosome (macula adherens)
(D) Hemidesmosome
(E) Gab junction (nexus)
(F) Microfilaments
(G) Tonofilaments
(H) Microtubules

35. This structure consists of a hexagonal array of aqueous channels between cells. These pores allow free passage of ions so that adjacent cell are electrically coupled. Small regulatory molecules such a cyclic nucleotides cam also pass through these structures.

36. This structure is thought to hold cells together. It consist of paired electron-dense plaques on the cytoplasmic faces of apposed membranes. Anchoring intermediate filaments rich in keratin insert into these plaques.

37. The outer leaflets of the plasma membranes of adjacent cells come onto direct contact. If forms a functional seal between a luminal and adluminal compartment.

38. F-actin is the most abundant and functionally important constituent of the structures.

39. These structure attach epithelial cells to their subjacent basement membranes.

40. The part of the junctional complex most directly responsible fro minimizing fluid flow between epithelial cells.

41. Regulates transfer of ions and small molecules from cell to cell in epithelia.

42. Belt-like regions where plasma membranes are in close proximity (But do not touch one another) and cytoplasmic faces of membranes have microfilaments attached.

43. These contain protein assemblies called connexons. Choose the ONE best response.

44. Which of the following statements about components of the junctional complex is CORRECT?

(A) Gap junctions, made of connexons, are found in all jucntional complexes.
(B) Occluding junctions (zonulae occuludents) are most commonly found between epithelial cells and the underlying connective tissue.
(C) At desmosomes, intermediate filaments attach to a thickening of the cytoplasmic side of the plasma membrane.
(D) Neither gap junctions nor desmosomes are found connecting cells other than epithelial cells.
(E) At hemidesmosomes epithelial cells are anchored to actin microfilaments in the underlying basement membrane.

ANSWERS AND TUTORIAL ON ITEMS 35-44

The answer are: 35-E;36-C;37-A;38-F;39-D-40-A-41-E-42-B;43-E;44-C. Epithelial tissue define boundaries and establish compartment in the human body. For example, the lumen of the lumen of the small intestine contains a complicated mixture of digestive enzymes capable of digesting the wall of the small intestine. The contents of the lumen of the GI tract are isolated from the sensitive wall of the gut guy by a membrane specialization known as the junctional complex. Forensic Comparison Microscopes, Inverted Tissue Culture Microscopes, Metallurgical Microscopes, Ore Microscopes.

At the most apical portion of the junctional complex there is a tight junction where the outer leaflets of the membranes fuse into a belt-like occluding junction called the zonula occudens (A). This structure extends around the apex of the columnar epithelial cells and makes a seal between the lumen the lateral extracellular fluid environment. In freeze-fracture-etch electron microscopy, the zonula occludens sometimes occurs as an anastomosing network of ridges (points of membrane fusion) representing multiple barriers to movement of molecule from the lumen to the lateral extracellular compartment. Below the zonula occludens there is a divergence of the plasma membrane with a clear separation of 10 to 15 nm. This structure is called the adhesive junction (zonula adherens) (B). Here there is simple membrane apposition with variable amounts of electrons dense material in the intervening 10-15 nm gap. Numerous actin-rich 6nm microfilaments (F) radiate away from the zonula adherens into the cytoplasma of apposed cells.

This structure is usually described as an adhesive junction. The desmosome (macula adherens) (C) is found below the zonula adherens, as well as at numerous other points of membrane apposition between adjacent cells. At the desmosome,the plasma membranes diverge to 25-30 nm. There is an intermediate dense line running between the cells. On the inner face of each apposed plasma membrane there is a plaque of electron dense material. Long bundles of keratin-rich 10 nm intermediate filaments (called tonofilaments ) (G) radiate away from the plaque of electron dense material. The macula adherens is thought to be a structure that holds cells together; it is sometimes likened to a “spot weal” Hemidesmosomes (D) are often found surface of epithelial cells where they anchor these cells firmly to the underlying basement membrane.

Tissue-culture microscopy studies show that starved yeast cells take up their own cytoplasm into vacuoles through autophagic bodies. Closer analysis using electron microscopy show that these bodies form a double-membraned structure called the autophagosome, which subsequently fuses with the vacuole/lysosome. The whole process is topologically the same in mammals. Screens in yeast defective in autophagy morphologically and biochemically revealed two sets of genes, the APG and AUT genes, respectively. A specific vacuolar enzyme biosynthetic pathway requires the cytosol-to-vacuole-targeting (CVT) genes. There is extensive overlap between the CVT genes and the APG/AUT genes.All apg mutants have defects at or before formation of the autophagosome.

Two ubiquitin-like systems have been discovered. The first conjugation system is unique in that Apg12 is synthesized as a mature form; it seems to have just one target, Apg5, and at steady state almost all Apg12 molecules are conjugated with Apg5. The second Apg8/Aut7 is processed at its carboxy-terminal region by Aut2/Apg4. Apg8 exists in two forms, one of which is membrane bound through a phospholipid. This lipidation is mediated by ubiqutin-like system; Apg8 is activated by Apg7 and transferred to Apg3 and finally forms a conjugate with phosphatidylethaolamine (PE). Apg4 cleaves Apg8 PE, releasing Apg8 from membrane. Morphological studies show that Apg8 localizes on the membrane of intermediate structures of the autophagosome; this transient association seems to be essential for formation of the autophagosome.

Both Apg12 and Apg8 are highly conserved, with apparent homologues in the worm, mammals and plants. In higher eukaryotes, Apg8 consists of a multigene family.

The gap junction (nexus) (E) is also commonly near the junctional complex. In the gap junction, the outer leaflets of the membranes of adjacent cells approach to within 2nm, but a small, definite gap remains. Gap junctions are composed of hexagonal arrays of barrel-shaped structures with six protein subunits (connxons) arranged around an electron lucent central core. This core is an aqueous channel between closely apposed cells, allowing the free passage of ions and other small molecules between epithelial cells.