Classically, histologists consider that there are four basic tissues. These are epithelium; connective tissue (which for our purposes includes blood); muscle; and nervous tissue. In this laboratory we will consider the first of these, epithelium.
The term "epithelium" comes from the Greek roots "epi-" = "upon" and "thele" = "breast," and originally meant only the skin on the breast, particularly around the nipple. The term as it's now used covers a good bit wider area than that, of course. An epithelium may be defined as a relatively avascular and almost wholly cellular (i.e., having little or no connective tissue associated with it) aggregation of cells which are in apposition over a large part of their surfaces, and which are specialized for absorptive, secretory, protective, or sensory activities. While this definition is useful, you must keep in mind that cells and tissues don't exist independently, and that (with the exception of a smear preparation) you will virtually never see a slide in which only one cell type and/or tissue is present. Epithelia can occur as sheets of cells, as in the lining of the intestine, or as clumps of cells, as in glandular organs. A distinction is made in many texts between covering and lining epithelium and secretory or glandular epithelium.
Covering and lining epithelium is classified histologically on the basis of two criteria: the number of cell layers present and the shape of the cells in the top layer. If an epithelial sheet is composed of one and only one layer it is a simple epithelium. If there is more than one layer, even if there are only two layers total, then it is a stratified epithelium. There is a third category, not so neatly defined, called (for want of a better name) pseudostratified epithelium which will be discussed more fully below.
There are three basic cell shapes in covering and lining epithelia: squamous, cuboidal, and columnar. These terms refer to the appearance of cells in microscopic sections. Very often the distinctions between squamous, cuboidal, and columnar shapes are not cut and dried; they may be a matter of opinion and even the aesthetics of the observer.
Simple epithelia can have cells of any shape. Remember that the definitive characteristic of a simple epithelium is that it has only one layer of cells.
To see examples of simple squamous epithelium, click here.
If you will now examine slide 8, you will see simple squamous epithelium. This slide contains an artery, a vein, and a nerve, all cut in cross section. The artery has the thickest wall of the three; the nerve has numerous smaller profiles inside it.
Examine the innermost wall of the artery at high magnification; you will be able to see that it's completely covered with simple epithelium. You are viewing these cells (as is almost always the case with this type of epithelium) along their cut edge, so the cytoplasm is virtually invisible because the cell is so thin. About all that can be clearly seen is the nuclei bulging into the lumen.
In virtually any tissue section, you will be able to find a blood vessel, and all of them have this same type of lining.
Simple cuboidal epithelium is classically demonstrated in the ductwork of exocrine glands. It's found in many other sites, too. To be "cuboidal" the cells should be about as high as they are wide, but sometimes whether a given epithelial sheet is cuboidal or columnar is a matter of judgement and even artistic sense!
To see examples of simple cuboidal epithelium, click here.
Simple columnar epithelium is very common. It comprises most of the lining of the digestive tract.
On slide 40, the tall absorptive cells comprising the bulk of the lining of the duodenum is a very good example of this shape.
To see an example of simple columnar epithelium, click here.
The lining epithelium of the intestine, as seen in slide 40, isn't entirely uniform in nature: there's more than one cell type present. Epithelial sheets of this type usually (not always) contain "goblet cells," which appear as large clear spaces here and there among the columnar absorptive cells. The name comes from the cup-like shape of these cells. The "cup" is filled with mucus droplets that are released into the lumen of the tract. Goblet cells are actually unicellular exocrine glands.
Stratified epithelia are those composed of more than one layer of cells. It is important to remember that a stratified epithelium is classified on the basis of the shape of the cells in the TOP LAYER ONLY. Cells in the lower layers may take any shape, and often do.
Stratified squamous epithelium is the most common stratified type. It lines the oral cavity, the anus, the vagina, and several other openings. The lumen of the esophagus is lined with it, too. The best example of stratified squamous epithelium, however, is the skin: the general body surface is covered with it, as slides 7 and 11 demonstrate. Slide 7 is made from thin skin; slide 11 is thicker skin. Both of them, however, are classified as stratified squamous.
Slides 7 and 11 illustrate the keratinization of stratified squamous epithelium. Keratinization is the process of making the top layer(s) of a stratified squamous sheet hardened and dead. It's an adaptation to wear and tear found on abraded surfaces. Not all stratified squamous epithelial sheets are keratinized, but most are. Very heavy keratinization can be seen on slide 24. This example is from the footpad of a dog.
To see examples of stratified squamous epithelium, click here.
Stratified cuboidal epithelium is fairly common in large gland ducts and in some of the very largest ducts of large glands, such as the mammary gland, it may grade into stratified columnar. Try slide 28 again and examine the larger ducts to find this type of epithelium.
To see an example of stratified cuboidal epithelium, click here.
In this imperfect world, nothing is as simple as we'd like it to be, and it's time to consider pseudostratified epithelium. This is usually treated as a separate category, although in fact it is a subset of the simple form. In pseudostratified epithelium, the general appearance is that of several layers, but in reality all of the cells reach the basement membrane, and the epithelium is truly a simple one. Since all of the cells do rest on the basement membrane there is only one layer, although the location of nuclei at different levels gives the specious appearance that more than one exists. This is easy to demonstrate in an electron micrograph, or with some special stains, but very hard to verify with only H&E staining.
The best examples of pseudostratified epithelium are found in the respiratory system. Slides 26 (trachea) and 115 (which includes part of the nasal cavity) provide examples of this type of epithelium. Most of the upper respiratory tree is lined with pseudostratified columnar epithelium.
Pseudostratified epithelium is usually ciliated; but sometimes it isn't. There are one or two examples of unciliated pseudostratifed epithelium in mammals, one of them being the conjunctiva of the eye. As if this deviation of the pattern weren't enough in itself, conjunctival epithelium also has goblet cells! You can see this type of epithelium in the exercise dealing with the eye.
Another special epithelial type is urinary epithelium. Unlike pseudo stratified epithelium, this is a true stratified form. As the name implies, it's found only in the urinary tract. If you will examine slide 723, you'll see it. This is the urinary bladder, which is lined with it.
Notice that the cells in the deep layers are cuboidal to columnar; that those of the topmost layer are larger, pale and washed out looking, with small nuclei; and that the border is "scalloped." These are the definitive characteristics of this epithelial type. There may be pink colored inclusion bodies. This epithelium is impermeable to water, and protects the urinary tract from damage caused by the presence of hypertonic urine.You'll find it in the exercise on the urinary system elsewhere on this CD.
One of the features of epithelia which has a great deal of clinical significance is its capacity for regeneration. In most epithelia, production of new cells is a more or less constant process, and one can express the activity of a tissue in terms of it mitotic index, the ratio of dividing cells to non-dividing ones. A good example of the mitotic activity of epithelia is found in the intestine, slides 40 and 41. Deep down in the layers of the epithelium, right near where the luminal covering meets the underlying connective tissue, you will frequently find mitotic figures. These are evidence of cellular proliferation and appear as dense irregular shapes where the nuclei would normally be located. All of the different stages are represented--prophase, anaphase, metaphase, and telophase--but there is no point in trying to pick them out. Much more clearly defined mitotic figures can be seen on slide 54, a developing whitefish embryo.
There is a rule in nuclear medicine which states that a tissue is sensitive to radiation exposure in direct proportion to its rapidity of division (the so-called "Law of Bergonié," named for Jean A. Bergonié (1857-1925) a French physician). It follows from this rule that exposure of rapidly proliferating epithelia to radiation will have marked effects, and this is what you'll see if you overexpose an animal to X-rays or other ionizing radiation. The symptoms include diarrhea, vomiting, and loss of hair. The rapidly dividing cells of the gastrointestinal tract and the hair follicles are most sensitive, and are killed first. It's also the basis for radiation treatment of tumors; tumor tissue tends to be more sensitive than the tissue around it.
Click here to see an example of proliferative epithelium.