Epithelial Tissue

A. Simple columnar epithelium

Slide 29 (small intestine) View Virtual Slide

Slide 176 40x (colon, H&E) View Virtual Slide

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 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 (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, 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. 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 #213).

B. Simple cuboidal epithelium

Slide 9N-1 kidney, H&E View Virtual Slide

Slide 210 kidney, H&E View Virtual Slide

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 9N-1 View Image or slide 210 View Image 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 9N-1 View Image or slide 210 View Image where the lumen will be surrounded by a circle of cells.

C. Simple squamous epithelium

Slide 30 (mesentery, H&E) View Virtual Slide

Slide 29 (small intestine) View Virtual Slide

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. Observe the endothelial lining of blood and lymph vessels in the mesentery in slide 30 View Image. 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. Another excellent place to look for endothelial cells is in the many small vessels in the wall of the intestine shown in slide 29 --look for the vessels in the submucosal layer (the lightest staining area in the wall of the intestine) View Image.

Going back to slide 30, move to the periphery of the tissue section and observe a simple squamous epithelium (flattened cells) covering the surface of the mesentery View Image. This epithelium --also called mesothelium-- and the irregular connective tissue immediately underneath together make up the peritoneum (a term that you will encounter in your anatomy studies) that lines body cavities and most of the viscera contained therein. Specifically, the portion of the peritoneum that is 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."

While understanding the light microscopic structure of tissues and organs is important for interpreting pathological change, much of the really interesting biological side of medicine now involves understanding cell structure in more detail. Therefore, we feel you should be comfortable with interpreting electron micrographs by the time you complete your histology course, as they are becoming more important in diagnosis and many of the micrographs in your professional literature will be of this sort. We assume that you already have a general knowledge of cell ultrastructure and can recognize the nucleus, mitochondria, cell membranes, endoplasmic reticulum and ribosomes. The Wheater's Atlas contains an excellent review of cell ultrastructure, as does the Pawlina text. Please review these structures in these sources and the electron micrographs listed below and in the other chapters.

156 Cilia - Cross-sectioned in human trachea View Virtual EM Slide
Cross sections of cilia. The typical 9+2 arrangement of the microtubules is especially evident. The basal bodies are centrioles and have 9 triplets of microtubules with no central pair.

213 Small intestine - microvilli View Virtual EM Slide
Study the typical appearance of microvilli on the apical surface of this cell forming part of the striated or brush border in small intestine.

16 Epithelium - Desmosome and Intermediate Filaments View Virtual EM Slide
A desmosome can be seen in the upper right corner of this transmission electron micrograph. The cytoplasm is full of intermediate filaments (tonofilaments), some of which are attached to the desmosomal plaque.

120 Left Atrioventrical Valve - Bicuspid or mitral valve Simple Squamous Epithelium View Virtual EM Slide
The endocardium, the simple squamous epithelial lining of the heart, can be seen at the top of this section. Under it, you can see connective tissue, which we will study next time. Note how thin the epithelium is.

170 Kidney - Cortex, proximal tuble Brush Border View Virtual EM Slide
This EM micrograph depicts the typical appearance of microvilli on the apical surface of two types of cells with a striated or brush border. Shown is the epithelial lining cell of a proximal tubule in the kidney.

65 Epidermis Stratified Squamous Keratinizing Epithelium. VIew VIrtual EM Slide
You can appreciate that this epithelium (skin) is stratified (has multiple layers of cells) and that the layers near the surface (at the top of the micrograph) have keratinized (lost their nuclei and become a layer of keratin). The spiny appearance of cells deeper in the epithelium can also often be seen in the light microscope. They are points of cell-cell attachment, made more obvious by shrinkage during preparation. What junctions are found here?

190 Esophagus - Stratified Squamous non-keratinized epithelium View Virtual EM Slide
Compare this micrograph to EM#65. This is the lining of the esophagus, where it is no longer necessary to have an outer keratinized layer to protect against desiccation, as it was for skin. Thus, the outermost layer is still cellular and contains a nucleus. Note again the spiny appearance of the cells, due to the desmosomal attachments.

153 Trachea - ciliated pseudostratified epithelium Respiratory View Virtual EM Slide
Compare the morphology of the goblet cells with that of the other cells of the epithelium. Note that most of them are filled with secretory product (mucus) and that they do not have cilia. The line indicates the plane of section on EM#154.

154 Trachea - Respiratory Epithelium Respiratory Epithelium (tangential section) View Virtual EM Slide
The yellow colored area indicates the outline of one cell (see previous wall chart #153 for plane of section). Note the tops of the goblet cells protruding between the cilia. What are the differences among cilia, microvilli, stereocilia and kinocilia (which you will come across later)?

183 Urinary Bladder – Section of Wall Transitional Epithelium View Virtual EM Slide
Transitional Epithelium. This micrograph displays the transitional epithelial lining of the bladder. The wall of the urinary bladder contains 3 layers of somewhat irregularly arranged smooth muscle.