Epithelium
Learning Objectives:
o Classify morphological types of epithelia based on the number of cell layers, shape of apical cells, and presence of surface specializations
o Describe the structure of microvilli, cilia, and other apical specializations of epithelial cells
o Explain the composition and function of the basement membrane
o Correlate the structure of different types of epithelial cell junctions with their functions and locations
o Describe the structural and functional characteristics of exocrine glands.
o Distinguish between the types of exocrine glands based on their classification by secreted product, mechanism of secretion, and morphology
Epithelium
SLU Slide 11: Esophagus and SLU Slide 74: Esophagus
Iowa Virtual Slidebox: Esophagus (111) and Esophagus (198)
Compare these two sections of the same organ (the esophagus). Which layer is the epithelial layer? What type of epithelium does this organ have? How did you make that designation? Draw a quick sketch of this type of epithelium.
Stratified squamous nonkeratinized epithelium. Stratified = many layers. Squamous = top layer is flat. Non-keratinized = nuclei in the cells on the surface level.
No slide
Clinical correlate: Many common epithelial malignancies are the result of exposure to toxic substances. What type of epithelium do we typically have in locations that are exposed to the “outside world”? Insults to which layer of the epithelium would be most consequential?
We typically have a stratified squamous epithelium in locations exposed to the outside world. The “cheap” squamous cells in the apical layers absorb insults and are sloughed off. The layer of mitotic basal cells are more important, as mutations in their DNA are passed on during subsequent division and proliferation.
SLU Slide 9: Submandibular gland
Examine this glandular epithelium, noting that you are observing the secretory portion and the ducts of this gland. What type of epithelium does this organ have? How did you make that designation? Draw a quick sketch below:
Simple Cuboidal Epithelium which transitions to stratified columnar epithelium.
SLU Slide 13: Duodenum and Slide 14: Ileum
Iowa Virtual Slidebox: Duodenum (183) and Ileum (141)
Compare and contrast an epithelial cell from these two organs. What type of epithelium does each organ have? How did you make that designation? What apical specializations does it have, and how do those help this cell perform its function?
Both the duodenum and the ileum have a simple columnar epithelium. Epithelial cells in the duodenum have microvilli, villi, and circular folds that all act to increase the surface area available for absorption of material. Goblet cells that produce mucus are also visible. The ileum has these same structures, though the villi are shorter and goblet cells may be more numerous.
SLU Slide 35: Trachea, Slide 79: Bronchus
Iowa Virtual Slidebox: Trachea (212)
Compare and contrast an epithelial cell from the lumen of these two organs. What type of epithelium does each organ have? How did you make that designation? What apical specializations does it have, and how do those help this cell perform its function?
Trachea has pseudostratified columnar epithelium- nuclei are seen at various levels. All cells rest on the basal lamina, but only the apices of some cells reach the lumen. The trachea is ciliated, which generate movements and help to keep the lungs free from debris. Mucus-secreting goblet cells are also visible.
The bronchus is also lined with pseudostratified ciliated columnar epithelium. However, it is thinner.
At the base of the row of cilia, you will see an eosinophilic line. What does that line represent?
Basal bodies, which are only present in cilia (not stereocilia). These are extensions of microtubule triplets, but with no central pair. You will also want to examine these structures in EMs.
SLU Slide 26: Epididymis
Iowa Virtual Slidebox: Epididymis (186); Epididymis (260)
Compare the epithelial cells in this organ to those that you’ve seen previously. What type of epithelium does this organ have? How did you make that designation? What apical specializations exist in this cell that are unique to its function?
The Epididymus has a pseudostratified columnar epithelium lined by stereocilia. The two layers of nuclei that will be most apparent are a layer of basal cells, and a layer of columnar cells. Although not always evident, those columnar cells are in contact with the basement membrane. The epididymis also possesses sterocilia, which tend to be longer and more “clumpy” than cilia. Stereocilia also lack the eosinophilic line of basal bodies present in ciliated epithelium.
No Slide
Arranging information in tables is a great way to consolidate information while comparing the important features of closely related structures.
Create a summary table comparing cilia, stereocilia, and microvilli with regard to structure (i.e., describe what they look like under a light microscope), function (what they do), and if you like, a drawing or screenshot.
Electron microscope image of Cilia:
Observe this image of cilia, comparing it to its light microscopic appearance. Make any notes in your lab book:
9+2 arrangement; may see basal bodies with triplets and no central pair if in cross section.
SLU Electron microscope image of Epithelium with basement membrane:
Compare the appearance of the base of an epithelial cell under electron microscopy and under light microscopy. Which layers are apparent in EM images that are not apparent under light microscopy?
The basement membrane not visible in light microscopy but is visible in EM.
SLU Electron microscope image of Junctional Complex:
Compare the appearance of the tight junction, zonula adherens, and desmosome. Pay attention to the space in between the cell membranes as well as the appearance of the cytoplasm. In what order to they appear from apical to basal?
The tight junction has a dark, electron dense appearance in the space between the two cells because at this junction, the cell membranes fuse with one another. In the zonula adherens, the intracellular space looks empty, while the cytoplasm has “fuzzy” plaques on either side. Desmosomes have an electron-dense line in the center of the intercellular space, and electron-dense plaques in the cytoplasm of both cells (which differentiates them from hemidesmosomes).
They occur from apically to basally in this order: Tight junction, zonula adherens, desmosome. Desmosomes may be found throughout the lateral membrane as well.
No Slide
Clinical Correlation: Metastasis is the spread of a primary tumor to a distant site. How might the alteration of epithelial cells to one another and to the basement membrane contribute to metastasis?
Mutations in the proteins that comprise the attachment sites might result in a cell that can detach from its neighbor cells and from its basal membrane more easily, allowing for the spread of these mutated cells.
Exocrine Glands
SLU Slide 79 (Bronchus) and 51 (large intestine)
Iowa Virtual Slidebox: Lung (82) (embedded left); Colon (142) (embedded right); Colon (393)
These organs both display unicellular glands. How can you differentiate these cells from the surrounding epithelium? What purpose do the secretions of these cells serve? Draw this unicellular gland in your notebook.
These cells stain much more lightly in normal H+E preparations because of the high concentration of mucin. In both the bronchus and large intestine, these secretions serve as lubricants.
SLU Slide 8 (parotid gland), Slide 9 (submandibular gland), & Slide 10 (Sublingual gland)
Iowa Virtual Slidebox: Parotid (350); Parotid (279); Submandibular gland (399); Submandibular gland (120); Sublingual gland (121)
Compare the multicellular glands from three different salivary glands. What is the difference between the secretions of these glands? How do these differences manifest as differences in the cytoplasm? Draw or take screenshots of each of the acini represented in these glands.
The parotid gland contains serous acini, while the sublingual gland has mainly mucus acini. Serous acini, such as those in the parotid gland, stain more strongly (with Eosin) because of the abundance of proteins/enzymes represented in saliva (zymogen granules). These acini are typically spherical.
The sublingual gland contains more mucus acini. Mucus cells have pale cytoplasm, and their nuclei are sometimes “pushed” against the basal membrane.
No Slide:
Sketch or annotate screen shots of three different glands that use three different mechanisms of secretion.
Metacognitive Tip: Sketching these structures (including the act of deciding what to include and what not to include) helps you to focus your attention on important features of each type of gland. Comparing them helps to focus your attention to what makes them distinct from one another.
The parotid gland contains serous acini, while the sublingual gland has mainly mucus acini. Serous acini, such as those in the parotid gland, stain more strongly (with Eosin) because of the abundance of proteins/enzymes represented in saliva (zymogen granules). These acini are typically spherical.
The sublingual gland contains more mucus acini. Mucus cells have pale cytoplasm, and their nuclei are sometimes “pushed” against the basal membrane.