Cellular Physiology 3140A
September 20, 2013
Lecture 5 Cont’d
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
- 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