Epithelia Lab

Learning Objectives

  • Explain the ways in which epithelia are classified
  • Distinguish between, simple, stratified, and pseudostratified epithelia
  • List characteristics of squamous, cuboidal, columnar, and transitional epithelia
  • Identify some common surface specializations of epithelia
  • Discuss how the organization, thickness, surface specializations, and turnover of an epithelial layer might reflect its function
  • Identify key differences between normal and pathological epithelial specimens

Keywords

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Pre-Lab Reading

Introduction

Epithelia are tissues composed of closely aggregated cells that cover most body surfaces, cavities, and tubes. These include the outer surface of the body (skin), tracts traversing the body (gastrointestinal tract), dead-end tracts that have openings at the body surface (respiratory, urinary, and genital tracts), and ducts that open into these tracts (exocrine glands). The functions of epithelia are numerous, and a single epithelium may have several functions. The most important of these include physical protection and selective transport (diffusion, absorption, secretion).

Basic Structure of Epithelia

Surface epithelia form continuous sheets that can have one or multiple cell layers. Several different types of cell junctions mediate physical strength and cell communication within the epithelium. A basement membrane lies beneath the epithelium and separates it from underlying tissue; because blood vessels do not penetrate the basement membrane, nutrients like oxygen and metabolites reach the epithelium by diffusion. Epithelia are polarized, with an apical surface that faces the external environment and a basal surface that faces the basement membrane.

Histological Classification of Epithelia

The most common manner of classifying epithelia is based on three criteria:

  • Number of Cell Layers. Simple epithelia comprise a single layer of cells. Stratified epithelia comprise two or more layers. Pseudostratified epithelia are simple epithelia that appear to be stratified when they are viewed in section, even though they are truly simple epithelia.
  • Shape of Cells. In squamous epithelia, the outermost layer is flattened. A cuboidal epithelium is characterized by cells that are as tall as they are wide. When these cells are taller than they are wide, they make up a columnar epithelium. Stratified epithelia are named after the shape of their most superficial layer of cells. A transitional epithelium is a type of stratified epithelium that has cells with shapes that fall between cuboidal and squamous. Transitional epithelia are specialized to deal with stretch and to serve as a barrier against toxicity.
  • Surface Specializations. It is customary to take note of the surface specializations of epithelia, which allow them to perform their function as a selective permeability barrier. These specializations can be located on one of the three surfaces of an epithelial cell.

Apical Specializations

The apical surfaces of epithelial cells frequently feature three surface specializations:

  • Microvilli are small processes that project from the apical surface of most types of epithelial cells, greatly increasing the surface area. They are notably found at the intestinal brush border and in the proximal convoluted tubule of the kidney.
  • Stereocilia are unusually long, non-motile microvilli that are found on the surface epithelium in the epididymis and ductus deferens.
  • Cilia are actively motile processes with a complex internal structure found on the epithelial lining of the respiratory tract and the female reproductive tract.

Lateral Specializations

The lateral surfaces of epithelial cells that are adjacent to their neighbors are usually separated by a space of 20 nanometers and may show several specializations

  • Tight junctions, or zonula occludens, block the movement of extracellular molecules between adjacent cells and maintain the polarity of the cell membrane. For example, the tight junctions located beneath the luminal surface of the simple columnar epithelium of the intestine prevent the contents of the gut from penetrating the cell lining. Tight junctions are composed of proteins known as claudins and occludins.
  • Adhering junctions, or zonula adherens, provide strength to the epithelium by linking the actin cytoskeletons of adjacent cells. They are located just beneath the zonula occludens and form a continuous contractile band around the cell. The zonula adherens is composed of transmembrane proteins called cadherins, whose cytoplasmic tails bind to anchor proteins in an intracellular plaque.
  • Desmosomes, or macula adherens, are typically located deep to adhering junctions and are abundant in stratified epithelia exposed to stress. Cadherins are also the transmembrane proteins of desmosomes, but the intracellular segment binds to intermediate filaments instead of actin.
  • Gap junctions allow for the passage of small molecules and nutrients between adjacent cells in order to coordinate the functions of the epithelium. They are composed of transmembrane connexin proteins that together form connexons. Multiple connexons from two adjacent cells align to form the gap junction.

The combination of the zonula occludens, zonula adherens, and macula adherens is known as the junctional complex. This unit divides the plasma membrane into apical and basolateral surfaces. An electron microscope is necessary to see the individual components; with the light microscope, this region is visualized as a terminal bar.

Basal Specializations

At the basal surface of epithelial cells are located:

  • The basement membrane, a thin non-cellular layer, intervenes between the epithelium and the connective tissue. This membrane is 30 to 60 nanometers thick and made up of collagenous and non-collagenous glycoproteins and proteoglycans. It provides structural support for the epithelium and serves as a selective barrier for the movement of materials to and from the supporting tissue.
  • Hemidesmosomes are a variant of the intercellular desmosomes and anchor the cell to the basement membrane. The transmembrane proteins are known as integrins, which bind to extracellular laminins in the basement membrane.

Pre-Lab Quiz

  1. Epithelia are often described in terms of their permeability. In what tissues would you expect to find "leaky" epithelia? Impermeable epithelia?
  2. Answer:
  3. How are the identities of the apical and basal surfaces of epithelia maintained?
  4. Answer:
  5. How would you classify the following epithelia?
    • A multi-layered epithelium with flat cells covered in keratin.
    • A single-layered epithelium with tall cells possessing cilia.
    • An epithelium with cells that change shape in response to internal pressure.
    Answer:
  6. In which specialized epithelial structures would you find the following proteins?
    • Integrin
    • Dynein
    • Connexin
    • Cadherin
    Answer:

Slides

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  1. Simple Squamous
  2. Endothelium
  3. Simple Cuboidal
  4. Simple Columnar
  5. Simple Columnar with Microvilli
  6. Pseudostratified
  7. Stratified Squamous
  8. Stratified Squamous with Keratin
  9. Transitional
  10. Stereocilia
  11. Microvilli EM
  12. Cilia EM
  13. Junctional Complexes EM
  14. Gap Junction EM
  15. Basement Membrane

Virtual Microscope Slides

  1. Blood Vessels
  2. Find and identify the epithelium that lines blood vessels.
  3. Kidney
  4. Identify two different types of epithelia in this section and describe the difference in structure and function between the two.
  5. Trachea
  6. Locate the apical and basal surfaces of the epithelium which lines the trachea. Describe the position of the nuclei. What type of epithelium is it?
  7. Epidermis
  8. Locate the position of the basement membrane.
  9. Bladder
  10. Locate the position of the basement membrane.
  11. Intestine
  12. Classify the epithelium. What microscopic surface specializations do you notice? What macroscopic structures are present? Indicate the position of the basement membrane.

Pathology

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  1. Barrett's Esophagus
  2. Cervical Intraepithelial Neoplasia

Quiz

  1. Classify the epithelium.
  2. Answer: Simple squamous. The epithelium is the layer of cells at the top. Note the flattened nuclei and thin cytoplasm.
  3. Classify the epithelium.
  4. Answer: Simple cuboidal. Note the round nuclei and square-shaped cells, indicative of cuboidal. A single layer of cells rests on the basement membrane.
  5. What does this type of epithelium line?
  6. Answer: Respiratory tract. Note the cilia on some of the cells and the presence of mucous-secreting cells. Also, the epithelia is pseudostratified.
  7. Classify the epithelium and name an organ where you would find this type of epithelium.
  8. Answer: Transitional epithelium. Found in the bladder to accommodate large increases in volume. Note the several layers of columnar and cuboidal cells which is indicative of transitional epithelia.
  9. Classify the epithelium.
  10. Answer: Simple columnar with microvilli
  11. Identify the surface specializations.
  12. Answer: Cilia with 9+2 microtubules
  13. Squamous cell carcinoma results in the formation of a disorganized, abnormal squamous epithelium. What might malignant squamous cells produce that would allow a pathologist to recognize this type of cancer?
  14. Answer: Malignant squamous cells produce large amounts of keratin, and these can be seen as nests of swirls that are diagnostic of squamous cell carcinoma.
  15. A stem cell in the gut epithelium undergoes a mutation in the gene encoding actin. Assuming its daughter cells survive and differentiate, what surface specializations might be affected? How might this affect the function of the small intestine?
  16. Answer: An actin mutation would affect microvilli, stereocilia, and adherens junctions. You would expect the intestinal lining to be less absorptive and leaky
  17. Irritation due to cigarette smoke has been shown to result in squamous metaplasia in which the normal epithelia of the respiratory tract are transformed into a stratified squamous epithelium. What is the normal epithelium of the respiratory tract? What might be the benefit of this transformation? Why would it have negative consequences?
  18. Answer: The normal epithelium is pseudostratified columnar ciliated. Squamous epithelia are better at protecting against irritation, but lack the functionality of the ciliated columnar epithelia and are unable to clear mucous and particles as effectively.