University of Texas Medical Branch

Cell Biology Graduate Program

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. During the next 4 weeks, we will learn about each of these tissues. 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.  . It is assumed that the students will have read the material before they come to class.

Test yourself: What do you already know about Epithelia?

1) If you were to design a generic epithelium, what common features would you add?

2) Epithelia can be classified by the number of layers of cells. Why would a particular organ need a multilayered epithelium ? Name the single vs multilayered subtypes.

3) Epithelia are also classified by the shape of the cells. These shapes help the cells perform their functions. What might be the significance of a flattened shape vs a cube or columnar shape?

4) Define a polarized cell. Why would polarity be important to an epithelium?

5) What specializations would you find at the apical surface? Define the function of each.

6) Name the specializations along the lateral surface? Why are they critical to the barrier function of epithelia?

7) How is the basal surface of some epithelia specialized?

8) What factors are important in regeneration and repair of epithelia?

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.

Classifications of epithelia

Epithelia classifications depend on both layering and shape. Consult your text for a diagram.

• Epithelia that are one cellular layer thick are called____________?.
• Stratified epithelia are ___________?
• If an epithelium looks multilayered (has different rows of nuclei), but is not (as seen by electron microscopy), it is called ________________?

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.

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.

Polarity in epithelial cells.

The barrier function supported by epithelial cells is maintained by a well organized polarity that is epressed 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:

The Golgi complex

The Nucleus

The rough endoplasmic reticulum

The basal lamina.

Where are the blood vessels that supply blood to this group of cells?

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.

Microvilli

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.

Stereocilia

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.

Cilia

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.

Tight Junctions.

The most apical is the tight junction. Recall from Cell Biology how this junction is structured. 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.

• How "tight" are tight junctions?
• Name the proteins found in the tight junctions. How could you tell morphologically if a tight junction was leaky or tight?
• Tight junctions have another function in maintaining the domains of membrane proteins? Explain how this is accomplished?

Zonula Adherans.

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

• What important contractile proteins are found in this complex?
• What is the significance of these proteins?
• What is the adhesion protein at this junction.

Macula Adherans (Desmosome)

The next junction is like a "button" or snap that connects the cells. Thick fibers loop into a junctional region and the cytoplasm.

• What types of filaments are in this junction?
• What is the name of the glue-like proteins?
• What happens to a desmosome if the extracellular calcium is bound by a chelating agent?
• What are hemi-desmosomes?

For a diagram, see the above polarized cell

Gap Junctions

The next junction in the series is the Gap junction. Recall from Cell Biology that the Gap junction allow cells to communicate with one another. Small molecules can pass through pore-like transmembrane proteins called connexons. 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.

Specialization of the basal surface.

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 octapus. 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?

Basal Lamina

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.

• How might integrins help in repair and wound healing?
• How is calcium involved in this process?
• Early in wound healing, what is the first response by the injured epithelium?
• When and where does cell division occur?

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