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Lecture 6

Cellular Physiology 3140A Lecture 6.docx

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Physiology 3140A
Anita Woods

Cellular Physiology 3140A Lecture 6 September 20, 2013 Lecture 5 Cont’d Occluding Junctions: Ex. Kidney Tubule Tight Junctions - Blood is filtered in the kidney and it is filtered in the filtering apparatus of the nephrons. Anything filtered into the nephrons is called the filtrate. The passage of the filtrate travels down the tubule of the nephron. The important parts of the tubule are the proximal tubule, ascending limb of the loop of Henle, and the collecting duct. - The tube is not like a garden hose or not just matrix; it is cellular, specifically epithelial. If you take a section of the tubule, the part of the section that has the villi is on the inside of the tubule and the cells are facing the body of the kidney. - Components of the filtrate can continue to pass or absorbed/ reabsorbed through the cells. In case of tight junctions, it is movement of molecules in between cells, passed the tight junctions. This movement is called paracellular motion. The occluding junctions are between the cells. If the occluding junctions are tight, then there would be no paracellular motion and vice versa. - The proximal tubule is also known as the “leaky” portion of the nephron. There are a lot of reabsorption going on in the proximal tubule; both through the cells and in between the cells. - In the ascending limb of the loop of Henle, there are some paracellular motions in between cells. - In the collecting duct, there are no paracellular transports. - Recall: occluding junctions are made with proteins called claudins and occludins. Claudins are cell-cell adhesion molecules that may not/may interact directly. In the diagram, since they are not touching, it allows for paracellular motion. There are 2 occludins and they are interacting to each other. - In the tubule portion of the nephron, there are variable expressions of claudins. In the proximal tubule, Caludin 2 is expressed, it is known as the leaky claudin and it allows for passage of a number of macromolecules. Ex. Potassium and water. Claudin 2 would look like it has a gap that would allow for molecules to pass through. It also allows water to come with it. The paracellular reabsorption that occurs are both K+ and H2O. In the ascending LOH is less leaky and it expresses Claudin 16. Claudin 16 allows for some paracellular motion and not as much as claudin 2. Claudin 16 allows for divalent cations (positively charged ion: +2). For example: Mg2+ and Ca2+. These can be passed through paracellularly. In the collecting duct, it is a non-leaky epithelum. It expressed Claudin 4. Claudin 4 is the gatekeeper that allows nothing to pass through. - Recall: There are adapter proteins that keep the occluding junctions stable and there is an anchor to the cytoskeleton in particular the actin cytoskeleton. Lecture 6: Transport Across Epithelium Epithelial Cell Properties - Epithelial cells are tightly bound together into sheets of cells called epithelia. You can make epithelium from all different types of the germ layers (ectoderm- >epidermis (skin), mesoderm->inner lining of body cavity, endoderm->GI). The epithelial often functions as a barrier. Types of Epithelial Cells: - Types 1-3 are considered simple and 4-6 is considered stratified (more complex). - Simple epithelial cell layers are single cell layers with different shapes. Stratified layers are hung layers. Simple squamous epitheliums are very thin cells. Ex. Endothelium is found in the capillaries. Endothelial cells are the inner lining of our blood vasculature. It is a very thin fragile cell type. Alveoli are the smallest structure in our lungs that allows for gas exchange. They are not really able to withstand a lot of mechanical stress and they do not adhered very well together side by side because there is a very small junction. They are a non-mechanical type epithelial layer. Ex. Alveoli, thin cells are good for diffusion for gas exchange. Simple Cuboidal epithelial cells look like cubes. They are taller than the squamous. They are better at standing mechanical stress because of more cell-cell interaction along cells. The nuclei are directly in the middle and they are evenly spaced. They are still fairly thin for passage of molecules. They are good for secretion/filtration so they can make products that can secreted into the external environment or internal. Ex. Occurs mostly in tubule cells. They experience some mechanical stress but not a lot. Simple columnar epitheliums are good for absorption/secretion. It maximizes cell/cell interaction to withstand mechanical stress. Ex. Stomach. These cells are good at absorption/secretion as well as protection of t
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