Membrane Modifications and Cellular Connections

Looking at the figure of simple columnar epithelium in your textbook, let's examine some cellular structures.

Cilia are organelles involved in moving fluid over the surface of an epithelial layer. A ciliated epithelium lines most of the respiratory tract (forming a mucociliary elevator in the lower tract). The mucous layer actually floats atop a serous (watery) fluid through which the cilia beat.

In kids with cystic fibrosis, don't forget, the cells lose the ability to form watery secretions. The mucus in the respiratory tract settles onto the cilia and clogs the 'elevator' so that bacteria, instead of being coughed out, can thrive in the lungs! The bacterial presence prompts the cells to secrete more mucus, which just gives the bacteria more room to live in - definitely a pathological positive feedback mechanism at work here.

Microvilli (translation: small folds) are found in epithelia where absorption is an important function. The tiny folds in the apical membrane - finger-like projections, really - increase the amount of membrane packed into the apical area occupied by a single cell. More membrane means more room to stick in some carrier and channel proteins. This is the classical example of the following general principles:

FOLDING INCREASES SURFACE AREA.

STRUCTURE AND FUNCTION ARE COMPLEMENTARY (FORM FOLLOWS FUNCTION).

If you remember from our study of carrier proteins that they can become saturated, then you will understand how folding the membrane can increase the rate of transport of carrier-dependent solutes.

As an analogy, let's consider a trip to a busy store - the checkout lines are long, and the manager is concerned, because he sees people leaving just at the sight of the long lines, without shopping first! We could say that the cash registers are saturated - that is, the checkers are moving as fast as they can, and they still can't keep up with the number of shoppers - so shoppers leave without going through the lines. Now, if the manager can just add more registers (and cashiers) he can get more people through the lines in the same amount of time. BUT, what if there's no more room? He has cash registers set up all the way across the front of the store already? Visit your local super-wonder-mart, and you'll see the solution - stack the checkout lanes 2 and 3 deep, and you can add more checkers in the same amount of store front - not wider, but deeper.

A typical store……………………….……………… A store with twice as many checkers.

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This is what cells do - they fold the membrane so that they can load up more channels and carriers - deeper, not wider. Take a look:

A typical cell…………………………………….…………. A cell with many more carriers!

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Let's look closer to see where the carriers are - then you'll understand why there are more of them:

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So, imagine how many carriers and channels can be packed into the membrane folds of a cell with microvilli!

Now, let's reconsider what we learned about the 3 kinds of simple epithelia - the kinds thin enough to be involved in transport:

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We said that a thinner membrane is better at transport - the shorter the distance, the faster substances can move…

Also, don't forget which end is which in each of these tissues:

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And, don’t forget where these tissues are located – all are required to allow the passage of some material IN (absorption) and/or some material OUT (secretion). While the thicker tissue types are used in areas with greater stress, they still must have a way to allow rapid transport.

This is not a problem for the amazingly thin simple squamous epithelium – rapid diffusion of oxygen and carbon dioxide through the air sacs of the lungs is easy.

Let’s look at the other extreme – simple columnar epithelium, which is found lining the small intestine, where absorption of dietary nutrients is a major function. You absorb through your 2 meter length of small intestine, daily, about 8 liters of fluid volume. This is not a continuous process – it occurs over the course of only a few hours, several times a day. Imagine the rate of absorption of nutrients, minerals, and water that occurs when the intestine is well stocked with food and digestive juices! It’s easy to see why there are microvilli on the surface of intestinal absorptive cells.

How about simple cuboidal epithelium? We find this tissue type busily absorbing nutrient-laced fluid from the filtrate produced in the kidneys. Your produce (by filtration) about 180 liters (yes, that’s one hundred and eighty liters) of fluid EVERY DAY. Considering you only have about 3 liters of fluid (the plasma) in your blood stream, it’s obvious that most of that fluid must be reabsorbed. Since urine output is only about 1 liter, the kidney tubules reabsorb 179 liters of fluid in a 24-hour period. Clearly, this is a job for microvilli! And that’s not all…the kidney cells responsible for almost all of this reabsorption also have specialized folds on their basal surfaces (called, conveniently enough, basal folds). Again, this increases the surface area – this time, of the basolateral membrane – to increase the space available for carrier molecules. Look again at the figure on page 108. Can you see the basal folds? The folds on kidney cells go much deeper than the ones in the figure.

While we’re looking at this figure, let’s turn our attention to something else. Look where all the mitochondria are…and remember what they do. There are a couple of good reasons for mitochondria to be situated at the basal pole of the cell: (1) that’s as close to the blood supply as they can get, and that gives them ready access to the oxygen that they absolutely must have to produce ATP; and (2) it puts them close to the basolateral surface, where they can conveniently supply ATP to power all the solute pumps found there. Yes – almost all (if not all) solute pumps (remember, that means active transport) are found on the basolateral membrane, actively pumping solute to the tissue fluid, so that there is a high solute gradient to move passively into the blood. The other effect of pumping solute OUT of a cell is to lower that solute’s concentration INSIDE the cell…so typically there is a good diffusion gradient between the lumen and the cell’s interior. SO, solute is pumped out at the basal pole, and diffuses in at the apical pole – you only have to pump it once to get it to move through BOTH the basal AND the apical membranes.

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Cell to Cell: Structural Connections in Epithelia

As it is the primary function of epithelium to form a barrier between an external environment and an internal environment, it makes sense that cells are tightly connected – both to each other and to the basement membrane that holds the epithelium to the connective tissue at its base. The connections between cells are called junctions, and they are built on integral proteins in the plasma membrane (review these on pages 68-69 – the anchoring proteins).

The connections called tight junctions form a continuous, leak-proof barrier around the apical margin of epithelial cells. The result is like a network of zip-lok seals that prevents ECF from leaking between cells and escaping from between cells, as well as preventing anything in the external environment from leaking in. Since nothing can leak, the only way through from external to internal (or the other way ‘round) is through the epithelial cells – not between them.

Just deep to the tight junctions is a band of desmosomes. Desmosomes are also found in a button form. Desmosomes tightly fasten cells to each other so that any shearing or twisting forces won’t separate the cells. The junction is tight, but there is a space between the cells through which ECF can flow. *Because columnar epithelium has more cell-to-cell connection, there is more room for more desmosomes. Squamous epithelium has the least – this partly explains why the tissue is so fragile.* The function of desmosomes is better understood if you have ever seen metal sheets welded together. Take a close look at a water tank sometime (the one on Glenwood Blvd, in the west of Tyler, is a good example – just don’t run off the road because you’re examining the welds!). Notice that the metal plates higher up on the tank are smaller and connected with fewer welds than the plates at the bottom- where the water pressure is greatest.

At the basal membrane, what looks like half a desmosome – a hemidesmosome – connects each cell to the basement membrane, preventing the entire epithelial layer from being sheared off its foundation.

The gap junctions are different in function from the other types, which are designed to structurally reinforce the connections within an epithelial layer. These specialized, hollow junctions allow the flow of ions from one cell’s cytoplasm directly to the next cell. This is kind of like adjoining hotel rooms – if you and your family occupy two adjacent rooms with a door between, you don’t have to go out in the hall to get to the room next door. The purpose of gap junctions is to create a form of intercellular communication. It allows coordination of the activity of the interconnected cells. An example of this is the coordinated beating of cilia within the respiratory tract.

Other tissue types have some of the above junctional types. Cardiac muscle and smooth muscle both have gap junctions, as do some neural networks. Cardiac muscle is packed with desmosomes, too – so your heart cells don’t separate with the force of each heart beat. They also have tight junctions. Only epithelia, though, have a basement membrane – so that’s the only place you’ll find hemidesmosomes.