EPITHELIAL TISSUE_1_

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					EPITHELIAL TISSUE


       ATLAS:        W., pgs. 82-100, Epithelial tissues
       TEXT:         R., Chapters 1, 2 and 3 Cells and methods of study (Review)
                         Chapter 4 Tissues: Concept and Classification pgs. 92-96
                         Chapter 5 Epithelial Tissue pgs 98-145

OBJECTIVES:

              1.     Be able to classify epithelia.
              2.     Know the structure and function of junctions.
              3.     Know the structure of apical specializations and their functions.
              4.     Be able to correlate different types of epithelia to their functions.

       In epithelia, cells are organized in sheets, either a single layer thick (simple
epithelia) or made up of multiple layers (stratified epithelia). Be able to identify the
classes of epithelia underlined in the text below, and give some thought to why these
different classes of epithelia have such different morphologies.

         The glass slide sets occasionally contain different stains or even different
 slides of the same tissue in place of the slide that you have in your set. Certain stains
 are much more instructive than others and different (alternate) tissues often help to
 explain functional changes. When these differences occur, the lab guide will usually
 refer to the particular slides as being in either "even" or "odd"-numbered slide
 collections. If you are studying from the glass slide material try to look at the
 alternates by borrowing from your lab mates.


I. SIMPLE EPITHELIA

A. Simple columnar epithelium (W pg. 84, 5.3; pgs. 277-8, 14.20, 14.22; pg. 283, 14.29)
slide 29 (small intestine) [WinLab] [Mac] [WinHome]
slide 176 (colon) [WinLab] [Mac] [WinHome]

Remember that epithelia line or cover surfaces. In slide 29 and slide 176, this type of
epithelium lines the luminal (mucosal) surface of the small and large intestines,
respectively. Refer to the diagram at the end of this chapter for the tissue orientation
and consult the atlas (W pg 277 14.20; pg 278, 14.22) for the cell types that make up
the epithelium. First, examine slide 29 at low power and note finger-like projections of
intestinal villi. Also, note that, in some areas, these villi are transversely sectioned and
appear instead as circular profiles. The villi are lined by a simple epithelium, and,
therefore, you should ideally see a single row of dark nuclei toward the base of the cells
as, in W pg. 84, 5.3 (you may see more if the epithelium is cut tangentially). Also, it is a
columnar epithelium, so the cells should be taller than they are wide. Look around on
the slide until you find a region of epithelium having this appearance.
Note the presence of goblet cells (W pg 278, 14.22), which look like balloons
suspended in the epithelium. These cells secrete mucus; the clear "balloon" in the
apical region of each goblet cell is where the stored mucus is located (mucus does not
react with H&E stain). Look at the columnar epithelium in slide 176 and note that it
contains a very large population of goblet cells relative to that found in slide 29. In the
colon, villi are absent and, instead, a simple columnar epithelium forms the intestinal
"glands," which are invaginations made up of mostly goblet cells (W pg 283, 14.29).

Identify the microvillous border at the apex of the epithelial layer at 40x magnification in
slide 29. This region appears as a darker staining line at the top of the cells. In a very
good section, it may appear striped, or "striated", because it is made up of finger-like
projections called microvilli (seen at the electron microscopic level in EM #128, also in
W pg 280, 14.25 a&b).


B. Simple cuboidal epithelium (W pg. 84, 5.2; pg. 321, 16.21a)
slide 9 (digital slide 9N-1, kidney, H&E) [WinLab] [Mac] [WinHome]
slide 210 (kidney, H&E) [WinLab] [Mac] [WinHome]

These slides show simple cuboidal epithelium, lining tubules in the kidney. The
tubules are cut in all different orientations; look for a region toward the middle of the
slide where the tubules are cut more or less in longitudinal section in slide 9 or slide
210, and appear as parallel wavy rows (at 4x magnification). Look for a favorable area
where you can see a space (the tubule lumen) lined on either side with simple cuboidal
epithelium. Note also that there is very little other tissue between tubules, so that you
often see two rows of cuboidal epithelia from adjacent tubules back to back. In other
parts of the section, look for tubules in cross-section in slide 9 or slide 210 where the
lumen will be surrounded by a circle of cells.


C. Simple squamous epithelium (W pg 83, 5.1)
slide 30 (mesentery, H&E) [WinLab] [Mac] [WinHome]

Simple squamous epithelial cells are flattened, i.e., wider than they are tall. A simple
squamous epithelium, called "endothelium," lines blood vessels, lymphatic vessels, and
the chambers of the heart. When sections through endothelial cells are viewed with the
light microscope, the cytoplasm cannot be seen, because the flattened cell is so thin.
Thus, endothelium is generally identified on the basis of the structure and position of
nuclei alone; that is, the nuclei are also often flattened and elongated, and are found
lining the lumen of the vessel (W pg 83, 5.1). Observe the endothelial lining of blood
vessels in the mesentery. Sometimes the blood vessels contain red blood cells and can
be identified that way. Otherwise, look for tubular or circular profiles at low power and
examine the endothelial lining of these vessels at high power. Note that the endothelium
may be damaged during processing such that it separates from the vessel wall or it may
slough off entirely and not be visible at all. In areas where you can find an endothelium,
note that the nuclei do not always look flattened in vessels that have contracted. Now,
move to the periphery of the tissue section and observe a simple squamous epithelium
(flattened cells) covering the surface of the mesentery. This epithelium --also called
mesothelium-- and the irregular connective tissue immediately underneath together
make up the peritoneum that lines body cavities and most of the viscera contained
therein. The portion applied against the walls of the cavities is known as parietal
peritoneum whereas the portion covering the organs (and their associated mesenteries)
is the visceral peritoneum……