SN5
SHORT NOTE
5M
★ High-yield
Classify cell junctions and describe their structure and functions.
Definition
Cell junctions are specialised regions of the plasma membrane at which cells are attached to one another or to the extracellular matrix, or through which they communicate.
Classification
| Class | Junction | Function |
|---|---|---|
| Occluding | Zonula occludens (tight junction) | Seals the intercellular space |
| Anchoring — cell to cell | Zonula adherens; macula adherens (desmosome) | Mechanical attachment |
| Anchoring — cell to matrix | Hemidesmosome; focal adhesion | Attachment to the basement membrane |
| Communicating | Gap junction (nexus) | Passage of ions and small molecules |
Individual Junctions in Detail
| Junction | Proteins | Cytoskeleton | Features |
|---|---|---|---|
| Zonula occludens | Claudins and occludins | Actin | Most apical; forms a complete belt; membranes fuse at ridges |
| Zonula adherens | E-cadherin; catenins | Actin | A continuous belt just below the tight junction |
| Macula adherens (desmosome) | Desmoglein and desmocollin; plakins | Intermediate filaments (keratin) | Disc-shaped "spot welds"; the strongest junction |
| Hemidesmosome | Integrins | Intermediate filaments | Half a desmosome, binding the cell to the basal lamina |
| Gap junction | Connexins forming connexons | None | A 2 nm channel; passes molecules under about 1 kDa |
- The junctional complex, from apex downward, is zonula occludens → zonula adherens → macula adherens — a fixed order worth memorising
Functional Importance
- Tight junctions also maintain cell polarity, by preventing membrane proteins from diffusing between the apical and basolateral surfaces — without them a transporting epithelium could not work at all
- Gap junctions couple cells electrically — the basis of the syncytial behaviour of cardiac muscle, smooth muscle and the lens
Applied Aspects
- Pemphigus vulgaris — autoantibodies against desmoglein in desmosomes; intra-epidermal blisters that rupture easily, with a positive Nikolsky sign; potentially fatal
- Bullous pemphigoid — antibodies against hemidesmosomal antigens; sub-epidermal blisters that are tense and do not rupture easily; a milder disease
- Epidermolysis bullosa — inherited defects of the anchoring proteins; the skin blisters with minor trauma
- Loss of E-cadherin in the zonula adherens is a key step in tumour invasion and metastasis, since cells detach and become motile
- Cholera toxin and inflammation open tight junctions, and the resulting paracellular leak contributes to secretory diarrhoea
All questions in General Histology
- Q1Classify epithelium with sites and functions. Describe its surface specialisations, cell junctions and basement membrane. ★
- Q2Describe the cells, fibres and ground substance of connective tissue. Classify connective tissue with examples. ★
- Q3Compare the three types of muscle histologically. Describe the structure of skeletal muscle and the sarcomere. ★
- Q4Describe the structure of a neurone and classify neurones. Describe the neuroglia and add a note on nerve degeneration and regeneration. ★
- Q5Describe the histology of cartilage and bone. Add a note on the structure of blood vessels. ★
- SN1Classify glands with examples. Describe the modes and nature of secretion. ★
- SN2Describe the histology of the lymph node, spleen and thymus. ★
- SN3Describe the histology of the skin. Differentiate thick from thin skin. ★
- SN4Describe the preparation of a histological section and the common stains used. ★
- SN5Classify cell junctions and describe their structure and functions. ★
- SN6Describe the ultrastructure of a typical cell and the cytoskeleton.
- SN7Describe the histology of blood with its formed elements. ★