Cell Junctions

1. Cell Junctions

3. Connections between the cells of tissue . • Hold the cells together • Types • Tight junctions or zonulae occludens. • Desmosomes • Gap junction

4. Cell-to-Cell Interactions Cells within a tissue are connected to each other by cell junctions 1. Tight junctions – create sheets of cells 2. Anchoring junctions – connect the cytoskeletons of adjacent cells 3. Communicating junctions – permit small molecules to pass between cells a. Gap junctions – in animal cells b. Plasmodesmata– in plant cells

5. Tight junctions • Ridge like structures between adjacent cells. • Made up of proteins • Cells are bound together . • Found in epithelial cells • intestinal mucosa • renal tubules • Form selective barrier

6. They are important in preventing molecules leaking between the cells. • They block the movement of integral membrane proteins between the apical and basolateral surfaces of the cell • It is made up of proteins called claudins and occludins.

7. Desmosome or maculae adherens Fasten cells together strengthening the tissue. • Desmo (‘bound’), soma (‘body’). • Desmosomes are localized patches that hold two cells tightly together. They are common in epithelia (e.g., the skin). • Fibrils arise from this portion to the interior of the membrane. • Hemi desmosome – half desomoses on the basal sides of the cell .

9. Gap junctions(communicating junctions) • Gap junctions form channels between cells. • Allow ions and small molecules to pass directly from one cell to another. These channels are formed by membrane proteins called connexin. • Six connexins in the cell membrane form a channel called a connexon

10. Functions • Gap junctions permit electrical signals to pass directly from one cell to another. • Gap junctions are found in heart and smooth muscle cells and between some neurons. • To regulate the membrane potential between cells and allow electrical impulses to pass between cells. • Electrical impulses passing between cardiac muscle cells creating the heart beat. • Also gap junctions provide the contractions of the smooth muscle cells of the uterus during birth

13. Cell communication

14. Cell Communication There are four basic mechanisms for cellular communication: 1. Direct contact 2. Paracrine signaling 3. Endocrine signaling 4. Synaptic signaling

15. Direct contact

16. Paracrine signaling – signal released from a cell has an effect on neighboring cells

17. Long-distance signaling Blood vessel Endocrine cell Hormone travels in bloodstream to target cells Target cell (c) Hormonal signaling. Specialized endocrine cells secrete hormones into body fluids, often the blood. .

18. Communication between cells requires: ligand: the signaling molecule receptor protein: the molecule to which the receptor binds -may be on the plasma membrane or within the cell

19. How do Cells Communicate? • .

20. Signal pathways • Signal molecule (ligand) • Receptor • Intracellular signal • Target protein • Response Figure 6-3: Signal pathways

21. Second Messengers • Many signaling pathways involve small, nonprotein, water-soluble molecules or ions, called second messengers. • These molecules rapidly diffuse throughout the cell. • Second messengers participate in pathways initiated by both G-protein-linked receptors and tyrosine-kinase receptors. • Two of the most important are cyclic AMP and Ca2+.

22. Hormones act on specific receptors. • The receptors are proteins. • When the Ligand binds the receptor, the receptors sends a signal within the cell to modify the activity of the cell. • Different types of receptors • Intra cellular and cell surface receptors • Ion channel receptors ,enzymatic and G-protein receptors.

23. GENERAL PRINCIPLES OF CELL COMMUNICATION Extracellular signal molecules bind to specific receptors An intracellular signaling molecule whose concentration increases (or decreases) in response to binding of an extracellular ligand to a cell-surface receptor.

24. Gate closed Signalmolecule(ligand) Ions Ligand-gated ion channel receptor Plasma Membrane Gate open Cellularresponse Gate close Figure 11.7 • Ion channel receptors