Examining epithelial tissue under the microscope

Information

Epithelial tissue serves two main functions in the body.

  1. It provides linings for external and internal surfaces that face harsh environments. The outer layer of the skin is epithelial tissue, as are the innermost layers of the digestive tract, the respiratory tract, and blood vessels.
  1. It forms glands that secrete materials onto epithelial surfaces or into the blood. Sweat glands, salivary glands, mammary glands, adrenal glands, and pituitary glands are examples of glands made of epithelial tissue.

Epithelial tissue is often classified according to numbers of layers of cells present, and by the shape of the cells. See Figure 5-1.

A simple epithelium is only one layer of cells thick. A stratified epithelium is more than one layer of cells thick. A pseudostratified epithelium is really a specialized form of a simple epithelium in which there appears at first glance to be more than one layer of epithelial cells, but a closer inspection reveals that each cell in the layer actually extends to the basolateral surface of the epithelium.

There are three basic shapes used to classify epithelial cells. A squamous epithelial cell looks flat under a microscope. A cuboidal epithelial cell looks close to a square. A columnar epithelial cell looks like a column or a tall rectangle. A few epithelial layers are constructed from cells that are said to have a transitional shape. Transitional epithelial cells are epithelial cells specialized to change shape if they are stretched laterally. They can transition from columnar- and cuboidal-looking shapes in their unstretched state to more squamous-looking shapes in their stretched state.

When classifying a stratified epithelial sheet, the sheet is named for the shape of the cells in its most superficial layers. So a stratified squamous epithelium only necessarily has squamous-shaped cells in its highest layers and might have a different-shaped cell in its lower layers.

Under a microscope, epithelial cells are readily distinguished by the following features:

  • The cells will usually be one of the three basic cell shapes – squamous, cuboidal, or columnar.
  • The cells will be closely attached to one another, in either a single layer or in multiple layers, and usually will not have room for extracellular material between the attached cells.
  • The epithelial layer on one side will face an empty space (or, in some organs, it will face a secreted substance like mucus) and on the other side will usually be attached to connective tissue proper.

Usually, a slide will have a section of tissue cut out of a larger organ. Slides with epithelial tissues usually have some of the underlying tissue found beneath the epithelial tissue with them.

423_table_04_02_summary_of_epithelial_tissue_cellsn

Figure 5-1. The different ways sheets of epithelial cells are categorized.

 

A layer of epithelial layer always serves as an outer layer for some structure, but, when looking at a tissue preparation on a slide, do not assume that just because you have found one end of the tissue sample you are automatically looking at epithelial tissue. Look for the cell characteristics listed above to be sure you are on the epithelial side of a tissue slice.

In Figure 5-2, only one edge of the tissue slice has epithelial cells. In Figure 5-2A that edge is indicated with an arrow, but when looking at a specimen under a microscope, you have to figure out for yourself where the edge with the epithelial cells is.

trachea_epithelia

Figure 5-2. A slice of a trachea. A. Magnified 1.8x. The arrow indicates which edge in this slice contains the epithelial cells B. Magnified 20x. The arrow indicates an individual columnar epithelial cell.

In the tissue slice in Figure 5-2, there are three edges that are not epithelial cells. If you just mindlessly started viewing the first edge you find, you have a good chance of looking as something other than the epithelial cells in the preparation. Be sure what you are looking at has the three visual characteristics of epithelial tissue:

  • The cells will usually be one of the three basic cell shapes – squamous, cuboidal, or columnar.
  • The cells will be closely attached to one another, in either a single layer or in multiple layers, and usually will not have room for extracellular material between the attached cells.
  • The epithelial layer on one side will face an empty space (or, in some organs, it will face a secreted substance like mucus) and on the other side will usually be attached to connective tissue proper.

In the following figures are some more epithelial layers. Note that whether the epithelial layer of the specimen is on the top, bottom, right, or left of the slice varies with how the specimen slice was positioned on the slide. In every case, you have to find which edge has the epithelial layer.

colon_simple_epithlelia

Figure 5-3. A slice of the colon, 20x.

 

esophagus_epithelia

Figure 5-4. A slice of the esophagus, 10x.

 

stomach_epithelia

Figure 5-5. A slice of the stomach, 20x.

 

urinary_bladder_epithelia

Figure 5-6 A slice of the urinary bladder, 10x.

Lab 5 Exercises 5.2

  1. Obtain a slide of epithelial tissue from the instructor.
  1. Follow the checklist in Lab exercise 5.1 to set up your slide for viewing.
  1. View the slide on the second-highest objective. Find the epithelial layer.
  1. In the circle below, draw a representative sample of the epithelial cells, taking care to correctly and clearly draw their true shape in the slide and an accurate number of layers if it is a stratified epithelium. Also draw some of the tissue (probably connective tissue) below the epithelial layer. Draw your structures proportionately to their size in your microscope’s field of view.
draw_side_exercise
  1. Fill in the blanks next to your drawing.