Medical Cell Biology 2002
Epithelia Study guide
Classical histologists list five basic tissue types, each specialized for a different function. These include epithelium, connective tissue, blood (sometimes called "fluid connective tissue") muscle, and nervous tissue. Today's focus will be on epithelia. The lecture will go over the answers to the questions in this study guide which are found in the beginning as well as dispersed throughout the guide.
Test yourself: What do you already know about Epithelia?
1) If you were to design a generic epithelium, what
common features would you
Common features of epithelia.
Whereas epithelia may be specialized for unique functions in an organ system, they all have some features in common. First, the cells are apposed to one another and line a surface. Second, they sit on a layer of fine filaments, called a "basal lamina". Collectively these layers form a boundary between the external environment and the remainder of the organ. Thus, at the most basic level, epithelia are organized to control movement of substances into and out of that organ and protect that organ.
Classifications of epithelia
Epithelia classifications depend on both layering and shape.
Define the importance of cell shape. The shape of the cell also must be considered when studying the function of an epithelium. Flattened, scale-like cells may be seen in one layer (simple) or in multiple layers (stratified). Such cells are called squamous. If these cells are in a single layer, they provide minimal protection, but often provide more opportunity for passive transport of substances across the cell. A good example is found in the capillary wall where epithelial cells provide the surface area for transport of gases and other molecules.
If squamous cells are in a stratified epithelium, they are often designed for protection against invasion or friction. They have desmosomes (junctions) and can be sloughed off and replaced rapidly.
Epithelia that are cube shaped are called, appropriately, "cuboidal". Often these epithelia have specialized junctions and transport processes that control movement of substances from one side to the other. Sometimes they are secretory. Thus, the taller the cell, the more active it may be in terms of regulated transport.
This is particularly true of the tallest epithelial cells, the columnar cells. Shaped like a column, these cells often have very different, specialized surfaces designed to protect the barrier and transport into the cell and then out of the cell. Some epithelial cells, such as the thyroid, become taller as they become more secretory.
A final epithelium that defies classification is the transitional epithelium found in the bladder or ureter. This epithelium may have cells that are squamous and even columnar. It is definitely multilayered. It also may distend so that it looks like it is only 2-3 cellular layers.
Define the Functional significance of layering. A stratified epithelium may provide more protection to the organ against friction, etc. Also, the outer layers of the cells could be sloughed off as the epithelium encounters friction. However, it would be more difficult to design a transport system in a stratified epithelium because of the numerous cell layers. This is why the simple epithelia have become important as "regulatory barriers". A simple epithelium may regulate transport through the epithelial cells by membrane transport proteins, endocytosis and special barrier junctions.
Polarity in epithelial cells.
The barrier function supported by epithelial cells is maintained by a well organized polarity that is expressed along all surfaces as well as inside the cell. The apical surface is that which faces the lumen or outside of the organ. The basal surfaces faces the basal lamina and the blood vessels.
The above cell is a polarized secretory cell and you see the products of the cell in secretory granules accumulating near the apical surface. Identify the following organelle layers:
If you injected radioactive amino acids into the blood stream, what would be the sequence of labeling as you followed incorporation into the proteins?
Specializations at the apical surface.
Projections from the apical surface that vary in number and density. They increase surface area for important transport and enzymatic activity needed at this surface.
Microvilli are filled with actin filaments that bind to a web of thin filaments below the microvilli. This web is called the "terminal web". The microvilli can contract, like fingers flexing. From your understanding of thin filaments, speculate how this contraction can occur. The micrograph below shows the actin filaments in the core of the microvilli. The cartoon shows actin (green rods) held together by actin binding proteins (finbrin, villin, or fascin) shown as white rods running horizontally between the actin filaments. Along the periphery are the myosin molecules (red). Amorphous capping proteins (blue cap) are at the tip of the microvillus to prevent depolymerization.
At the base of the microvillus, the actin filaments are linked to more actin filaments running horizontally (thin horizontal blue-green cylinders). The horizontal actin filaments form part of a motile "terminal web" that may help move vesicles in the region. They are connected by spectrin binding proteins (white connecting regions).
Below the actin filament bundle are bundles of intermediate filaments that serve purely a cytoskeletal role.
10 nm filaments: Intermediate in size between microtubules (24 nm) and thin (actin) filaments (7 nm)
Primary function is cytoskeletal: no known motor proteins associated with these filaments.
Unique types associated with different cell types, often used to identify particular cell types, or the origin of particular cancers.
Structure of Intermediate Filaments (IF) (See IF lecture for more information)
Central alpha helical core=4 long alpha helices separated by a non helical region
Flanking globular N and C terminal domains.
Above units form dimers. Then dimers form a tetramer.
Tetramers get together to form protofilaments and protofibrils
Characterization of Intermediate Filaments (IF)
Stereocilia look like thin irregularly distributed microvilli, however, because of their length, they were initially mistaken for cilia. They are non-motile and used mainly to increase surface area for fluid absorption in the epididymus. In addition, in specialized cells in the ear, the stereocilia are also involved in hearing and balance.
These structures are designed for motility. Epithelia that need to move substances across their surface (like mucous in the air passages) have cilia inserted in the cytoplasm by basal bodies. However, if you were to look at an olfactory cell, you would see cilia specialized for detection of odiforous substances. Cilia actually bear receptors for different chemicals that cause the odors.
For more information about cilia structure and function, see the cilia web page. Review the structure of cilia in the following photograph.
Looking at the cross section, what structures make up the 9 sets of doublets and the two singlets? What is the basal body (which gives rise to cilia?). How do cilia move? What proteins are used to make the cilia bend?
The following photograph shows the difference between cilia and microvilli at the EM level. Epithelial cells with microvilli show a dense brush-like array. The cilia are long and move in a wave-like motion. In the oviduct, the cilia are important to help move the egg down the tube on its way to the uterus.
Specializations at the lateral surface.
Near the apex, there are important junctions that force molecules to enter the organ through the cells rather than between the cells. These are specializations along the lateral surface. In the above diagram of the Polarized cell, you can see the different types.
Macula Adherans (Desmosome)
One of the junctions is like a "button" or snap that connects the cells. Thick fibers loop into a junctional region and the cytoplasm.
For a diagram, see the above polarized cell
The following cartoon shows a desmosome. Two plasma membranes are connected at a site where there are plaques (blue area covering membranes) The green horizontal rods represent cadherin molecules that connect one another via homophilic binding. Binding requires calcium (shown as red patches connecting the rods). Blue plaques on plasma membrane contain desmoplakins which are intermediate filament binding proteins. Intermediate filaments loop into the plaques connecting the desmosome with the cytoskeletal system of both plaques. These are represented by the hairpin loops in the figure to the left (below). The figure to the right shows an Adherent junction that involves actin filaments.
Adherent junctions (figure to the right) are similar to desmosomes in that they have cadherin binding in the center along with calcium. However, the plaques on the plasma membranes contain alpha actinin. Review lectures on muscle and thin filaments to aid your understanding of actin and alpha actinin.
The most apical is the tight junction. It involves a fusion of the two adjacent membranes with fibrous connections, something like "strapping tape". The following photograph shows a freeze-fracture/freeze etch view of a tight junction (zonula occludens). Review the membrane web page for more information about freeze-fracture/freeze etch.
The next junction is a complex of thin filaments that form a mat that connect with the membranes. These filaments then extend in a belt like array of all along the top of the cell. They appear to be continuous with the "terminal web". See the above diagram in the Polarized cell
The next junction in the series is the Gap junction. Recall from the first Cell Biology course that the Gap junction allow cells to communicate with one another. Small molecules can pass through pore-like transmembrane proteins called connexons. Connexons are made of subunits called connexins. The following photograph shows this diagrammatically. Note that ions, molecules like fluorescein, and second messengers can pass from cell to cell. How might this be used to spread a signal?
The following photograph shows the gap junctions prepared three different ways. Recall that negative staining is the deposition of heavy metal around a structure that is floated on a water surface. The photo on the left shows the donut shape of the connexon molecules. The middle photograph is a freeze-fracture/freeze etch view and the right hand photo shows an ultrathin section through the junction.
Specializations at the basal surface.
Like a half desmosome at the base of the cells
Links cell to basal lamina
Dense plaque on plasma membrane contains desmoplakins (blue region in above cartoon). These are sites for insertion and attachment of intermediate filaments. Integrins are receptor proteins that are also concentrated at the site probably in a membrane phospholipid "raft". They are integral membrane proteins (passing through the plasma membrane). The cytoplasmic side interacts with filaments and the exoplasmic site (extracellular domain) binds a specific ligand. In this case the ligand is an extracellular membrane protein called "fibronectin".
Another area of specialization on some basal surfaces are the "focal adhesions". These are different from the hemidesmosome because they have actin molecules connected to alpha actinin in the plaque on the plasma membranes. However, they also involve integrins which are recognized by specific ligands in the extracellular matrix. In this example, the ligand is laminin.
Integrins are: extracellular matrix receptors in the cell membrane:
Composed of two glycoprotein subunits: alpha and beta. Combination and types of subunits may dictate selectivity of binding
How do Integrins function?
Epithelia that are important in regulated transport also may have specializations along their basal surface. These look as if the cell has multiple "legs" like an octopus. The "legs" of different cells interdigitate with one another and form a space into which ions and water are transported. Even though these are at the basal surface, they are called "lateral interdigitations". Also, any other substances that are moved across the cell to the blood supply are secreted into this region. The membranes have important ion pumps, like sodium/potassium ATPases. Review your Cell Biology material discussing these pumps.
Below is a micrograph that shows the basal surface of kidney epithelium.
The interdigitations of the membranes show the increased surface area available for the ion and water transport. Note the accumulation of mitochondria. What is the significance of their abundance?
This region was once called the basement membrane. However, it is not a membrane and therefore, the nomenclature has fallen out of favor. The cells sit on the innermost layer, called the lamina lucida because of its clear appearance. Underneath this layer is the lamina densa which has more interwoven filaments. The above view also shows the fine basal lamina on which the epithelia rest. In addition, the next region is actually a capillary and what we are seeing is the basal lamina of the kidney epithelial and the capillary endothelial cells. The principal components include Type IV collagen, laminins and proteoglycans.
In those basal lamina attached to epithelia sitting on connective tissue, there are additional types of anchoring fibers. These are going to be discussed in the next unit (Connective tissue).
What are the functions of basal lamina?
Regeneration, Repair and Aggregation of epithelia
Because of its location, epithelia are subject to constant injury by friction or invasive agents. Therefore, it is not surprising that there is a base of cells that divide and replace the lost cells. The rate of repair and regeneration varies with the type of epithelium. The following questions are designed to help the student think about how epithelia respond to injury and the factors that are needed for healing. For more information, see the attachment on Adhesion molecules.
Gwen V. Childs, Ph.D. Text Copyright 1999 http://www.cytochemistry.net/microanatomy/Epithelia/epith_lec.htm
visitors since 12/17/97