Epithelium - Histology

Sections

Full-Length Text

Squamous

Simple

• Single layer of flattened cells
• Bulging nuclei
• Basal surface rests on basement membrane
• Apical surface faces the environment

Stratified

• Comprises several layers of cells
• Can be keratinized, with surface layers of dead, anucleic cells
• Non-keratinized has surface layers with nuclei

Functions and Locations:

• Provide an ideal surface for diffusion and filtration; hence, simple squamous epithelia is located in the alveolar sacs of the lungs and in the endothelium of cardiovascular linings.
• Because of its multiple layers, stratified squamous epithelium provides protection from abrasion, dehydration, or potentially harmful substances.
  • For example, the outer layers of the epidermis protect the underlying layers from constant abrasion with the external environment, and the squamous epithelium of GI tract faces the lumen, so it protects underlying tissues from gastric contents.

Cuboidal Epithelium

Simple

• Comprises a single layer of cubed, block-like cells resting on a basement membrane
• Its nuclei are centrally located, and line up in rows
• May have cilia, which increase surface area for absorption and secretion
• If arranged around a lumen, apical surface faces it

Stratified

• Comprises multiple (usually two) layers of cuboidal cells
• Round nuclei

Functions and Locations:

• Facilitates absorption and secretion
  • For example, the kidney tubules, which are responsible for fine-tuning the urine contents, are lined with simple cuboidal epithelium
  • Cilia, which can be motile or non-motile, and/or microvilli may be present. Recall that microvilli are cytoplasmic extensions that increase the surface area of the plasma membrane.
• Protection
  • For example, stratified cuboidal epithelia of the large salivary gland ducts protects surrounding tissues from salivary secretions.

Columnar

Simple

• Comprises a basement membrane supporting cells that are taller than they are wide
• Goblet cells secrete mucus to trap foreign particles
• Cilia are often present; these hair-like extensions "sweep" materials along passageways; for example, the cilia of the bronchioles continuously remove foreign matter from the lower respiratory tract.

Stratified

• Comprises more than one layer of cells; notice that more columnar-like cells reside in the apical layer, while the basal layer is more cuboidal.

Pseudostratified

• Appears (at first) as if it comprises multiple layers, but, upon closer inspection, does not.
• Goblet cells and cilia; microvilli may also be present.
• Cells of pseudostratified epithelium fit together like puzzle pieces, with nuclei at various distances from the basal membrane; in the sample, it almost looks as if there were two rows of nuclei.

Functions & Locations

• Absorption and secretion
  • For example, the simple columnar epithelium of the GI tract has microvilli, which provide a large surface area for absorption, while the mucus-secreting goblet cells moisten and protect the GI lining.
  • And, as we discussed earlier, the ciliated columnar epithelia of the respiratory tract sweep foreign materials away from the lungs.
• Protection
  • Stratified columnar epithelium provides protection in the large ducts of salivary glands

Full-Length Text

• Here we will learn key histological features of epithelia, which is one of the four major tissue types in the body.

• To begin, indicate that we'll learn squamous, cuboidal, and columnar epithelia; because transitional epithelia is only present in the urinary system, we address it in that context, elsewhere.

First, let's draw simple squamous epithelium; "simple" means that there is only one layer of cells.

• Show a single row of flattened cells on a basement membrane, and indicate their nuclei.

• Look at respiratory alveoli in the histology sample, and note that the flat squamous cells have a scaly, irregular appearance, with bulging nuclei.

• Then, indicate that, like all epithelial tissue, simple squamous epithelium is polar: its basal surface rests on the basement membrane, and, and its apical, aka, free surface, faces the environment.

Next, let's show stratified squamous epithelium; "stratified" means it comprises several layers of cells.

• Indicate that stratified squamous epithelium can be keratinized, with surface layers of dead, anucleic cells, or, non-keratinized.

• Write that the thin cells of squamous epithelia provide an ideal surface for diffusion and filtration; hence, simple squamous epithelia is located in the alveolar sacs of the lungs and in the endothelium of cardiovascular linings.

• Because of its multiple layers, stratified squamous epithelium provides protection from abrasion, dehydration, or potentially harmful substances;

• For example, the outer layers of the epidermis protect the underlying layers from constant abrasion with the external environment, and the squamous epithelium of GI tract faces the lumen, so it protects underlying tissues from gastric contents.

Next, let's learn about simple and stratified cuboidal epithelium.

• Show that simple cuboidal epithelium comprises a single layer of cubed, block-like cells resting on a basement membrane;

• Indicate that its nuclei are centrally located, and line up in rows;

• Show that it may have cilia, which increase surface area for absorption and secretion.

• In the histological sample, indicate the basal and apical surfaces; show that the apical surface opens to a lumen.

• Next, show that stratified cuboidal epithelium comprises multiple (usually two) layers of cuboidal cells;
  • For context, indicate the round nuclei and the lumen.

• Write that cuboidal epithelia facilitates absorption and secretion.
  • For example, the kidney tubules, which are responsible for fine-tuning the urine contents, are lined with simple cuboidal epithelium, and, secretory glands are often lined with cuboidal epithelium;
    -For example, stratified cuboidal epithelia of the large salivary gland ducts protects surrounding tissues from salivary secretions.

• Cilia, which can be motile or non-motile, and/or microvilli may be present. Recall that microvilli are cytoplasmic extensions that increase the surface area of the plasma membrane.

Next, we'll draw simple, stratified, and pseudostratified columnar epithelia.

• First, draw a section of simple columnar epithelium, which comprises a basement membrane supporting cells that are taller than they are wide;

• In our drawing and in the histological sample, indicate the goblet cells, which secrete mucus to trap foreign particles, and, show that cilia are often present.
  • These hair-like extensions "sweep" materials along passageways; for example, the cilia of the bronchioles continuously remove foreign matter from the lower respiratory tract.

• Next, indicate that stratified columnar epithelium comprises more than one layer of cells; notice that more columnar-like cells reside in the apical layer, while the basal layer is more cuboidal.

Lastly, we'll look at pseudostratified columnar epithelium; as its name implies, it appears (at first) as if it comprises multiple layers, but, upon closer inspection, does not.

• To show this, draw a basement membrane and a few cells with wider basal surfaces than apical surfaces;

• Then, between these cells, draw cells that are wider at their apical surfaces.

• Indicate goblet cells and cilia in our drawing and in the histological sample; microvilli may also be present.

• Notice that the cells of pseudostratified epithelium fit together like puzzle pieces, with nuclei at various distances from the basal membrane; in the sample, it almost looks as if there were two rows of nuclei.

• Write that the function of columnar epithelium is to facilitate absorption and secretion; for example, the simple columnar epithelium of the GI tract has microvilli, which provide a large surface area for absorption, while the mucus-secreting goblet cells moisten and protect the GI lining.

• And, as we discussed earlier, the ciliated columnar epithelia of the respiratory tract sweep foreign materials away from the lungs.

• Stratified columnar epithelium also provides protection in the large ducts of salivary glands.

Images:

Histology (Mark Braun, MD, & Indiana University: http://medsci.indiana.edu/c602web/602/c602web/toc.htm; http://www.indiana.edu/~anat215/virtualscope2/start.htm)