Find A PhysicianHome  |  Library  |  myDownstate  |  Newsroom  |  A-Z Guide  |  E-mail  |  Contact Us  |  Directions
curve gif

Epithelia

Epithelium is one of the 4 primary tissues of the body. It consists of cells usually arranged in sheets or tubules that are attached to the underlying basement membrane. The basement membrane, a structure seen with the light microscope, has been subdivided into a basal lamina (thought to be produced by the epithelium) and a reticular lamina (produced by connective tissue cells). You will usually see the basal lamina only with the electron microscope.

Epithelium is found covering the numerous internal and external surfaces of the body and may also be modified to form glandular structures. As a physician, it is very important to be able to recognize and identify the various types of epithelia since they are important in organ identification and essential in the detection of pathological processes.

In general, the various types of epithelia are classified according to the shape of the cells and whether or not they are layered. In a simple epithelium the cells form a single layer and are all attached to the basement membrane. A stratified epithelium consists of multiple layers of cells in which only the basal layer is attached to the basement membrane. A simple epithelium which appears to be stratified is classified as "pseudostratified" (the nuclei are in different layers, but all the cells are in contact with basement membrane). A stratified epithelium in which the number of cell layers varies upon physiological stretching is called "transitional". Epithelial cells are classified by shape into squamous (flat), cuboidal and columnar varieties. The name of a stratified epithelium is determined from the shape of the apical layer of cells.

Objectives

You should aim to:

  1. Recognize the presence of an epithelium and understand its defining characteristics.
  2. Distinguish the different morphological types of epithelium.
  3. Relate morphology to function where possible.
  4. Identify ultrastructural features of epithelial cells and their functional significance.

1. Simple Squamous Epithelium

Squamous cells are flattened cells which are wider than they are tall. The coelomic cavities within the body are covered by a simple squamous epithelium which is called mesothelium. Slide #2 (Mesentery) is a piece of mesentery which has been spread flat on the glass and stained with silver nitrate. The mesentery is a sandwich composed of two sheets of simple squamous epithelium separated by a relatively thicker layer of connective tissue, which may contain blood vessels and fat cells. The edges of the squamous cells can be made out as uneven corrugated black lines due to precipitation of silver in the intercellular space. The nuclei are stained very lightly, if at all. By focusing the microscope up and down, both epithelial layers may be brought into view separately.

Blood and lymphatic vessels are lined with a special type of simple squamous epithelium called endothelium. Examine slide #70 (nasal mucosa) for blood vessels and locate their endothelial lining. Blood vessels are usually recognized by their content of red blood cells (small red staining spheres). In these preparations nuclei of the squamous endothelial cells are prominent, but cell boundaries cannot be made out because the cells are so flattened

2. Simple Cuboidal Epithelium

This type of epithelium consists of a single layer of cells which is thicker than the squamous endothelial cells. Cuboidal epithelial cells are approximately as wide as they are tall and can be found in the kidney tubules, glands and ducts. A good example is the thyroid gland, slide #67 which is composed of follicles that are discrete areas of colloidal material surrounded by a simple cuboidal epithelium. Note, that there are some variations in cell thickness.

3. Simple Columnar Epithelium

This epithelium is composed of a single layer of cells which are taller than they are wide and can be found lining the stomach, intestine, uterus and gall bladder. Locate the columnar epithelium on slide #3 (rat intestine) noting the presence of mucous-secreting goblet (these appear as clear, non-stained rounded areas due to extraction of the mucous) and the brush border (see below: non-motile processes). The cells are polarized with the nuclei at the basal end (closest to basement membrane). How could you avoid mistaking simple columnar epithelium for stratified columnar epithelium?

4. Stratified Cuboidal and Columnar Epithelia

These epithelia can be found in the tubules of the testis, ducts of some glands, and in regions of the urinary and respiratory tracts. They will be examined later when you study organ systems.

5. Pseudostratified Columnar Ciliated Epithelium

Pseudostratified ciliated epithelium is typically found lining the respiratory system and is also found in other tissues such as the epididymis. Three types of cells make up this epithelium - basal, columnar and fusiform cells. The first two are easy to see, but fusiform cells are difficult to demonstrate due to their shape. All cells rest on the basement membrane, but vary in height, and, as a result, the nuclei are at different levels giving the appearance of stratification.

6. Stratified Squamous Epithelium

Stratified squamous epithelium is found in the skin and in the passageways leading to the exterior, such as the mouth, anus and vagina.

Examine the esophagus (slide #50) and note the epithelium lining the lumen. In the odd-numbered boxes, it is cut in cross-section and the epithelium lines the lumenal surface of the tube; in the even-numbered boxes it is a longitudinal piece of tissue with the epithelium on one surface. By visual inspection, most of the tissue is pink. The epithelium will be the most basophilic material in the slide. Inspection of the epithelial layer at low-power indicates that the basal surface of the epithelium is indented by connective tissue papillae. If these papillae are cut in cross-section, they appear as circular areas within the epithelium.

Using the 10x objective, observe the many layers of cells. With this and the 40x objective, examine the changes in shape the cells undergo from the basal to the lumenal layers. New cells are formed in basal areas of the epithelium, are pushed towards the free surface eventually and are sloughed off. If the epithelium is subject to a great deal of abrasion, such as in the skin, the outer cells undergo a special type of transformation. Keratin is formed in these cells as they migrate towards the surface. The nuclei disappear and the cells become dehydrated. This process will be discussed in the lecture on skin.

Slide #43 (Thick skin), is a section through the plantar surface of a foot. The pink material is connective tissue and the epithelium is the blue layer next to it. The thick outer layer is composed of the keratinized cells mentioned above. It may be partially or completely detached in some slides. If so, examine your neighbor's slide.

7. Transitional Epithelium

This epithelium is found in urinary tract. Its appearance varies depending on whether it is in a stretched or relaxed state. Examine slide #76 (human bladder) and slide #9 (rat bladder) made from relaxed bladders. The cells in the outermost layer characteristically have a large amount of cytoplasm, which may give the cell a swollen, balloon shape appearance ("cap" cells). In slide #9, many cells in the outer layer appear to be covered by a dense-layer of material which is not part of the epithelium.

Specializations of Epithelial Cells

Epithelial cells have polarity in terms of their morphology and orientation. The plasma membrane nearest the basement membrane is the basal membrane; the apical membrane lines the free surface, and the lateral membrane is between adjacent cells. The various membranes may be modified to perform specialized functions. Projections from the cell-surface may be found on different cells depending on their function. These can be visualized in the light microscope but their structure is only apparent in electron micrographs (E.M.s). Examine the E.M.s in your textbook. Non-motile processes called microvilli may project from the surface of cells. Microvilli vary in size from extremely small processes which can only be seen with the electron microscope, to larger structures which can be seen with the light microscope. In the small intestine (slide #3), individual microvilli are not seen but the brush border they form is visible. Explain why. What is the function of microvilli?

   

Motile processes called cilia are present in the pseudostratified epithelium of the trachea (slide #72). Observe these and then study the E.M.s noting the characteristic 9 + 2 microtubule arrangement. Notice in the thin section EM of a simple columnar ciliated epithelium numerous cilia cut in both cross section and longitudinal section can be observed at the apical membrane. The cilia are attached to basal bodies in the apical cytoplasm

Cell Junctions

Thin section EM of a tight junction on the lateral membrane of adjacent epithelial cells. Using this procedure, the tight junction appears as punctate membrane fusions.

   

In freeze-fracture EM, these membrane fusions appear as intramembrane ridges. The function of tight junctions is to seal the intercellular space between epithelial cells and prevent transepithelial diffusion. Tight junctions maintain epithelial cell surface polarity.

   

Desmosomes are observed on the lateral membrane between epithelial cells. They are involved in cell-cell adhesion and are abundant in organs that undergo severe mechanical stress such as the skin. Hemidesmosomes are found on the epithelial basal membrane and anchor the epithelium to the basal lamina.

   

Gap junctions appear as plaques of tightly packed intramembrane particles in freeze-fracture electron microscopy. For comparison, the intramembrane particles of the adjacent plasma membrane (labelled M in the picture) are less densly packed. Gap junctions are involved in cell-cell communication.