Lab 2: Epithelial Tissue
The primary goal of this lab is to learn how to identify the various classes of epithelial tissues found in the body and the apical specializations of specific epithelial types. A second, critical goal is to correlate function with structure. Thus identification of these epithelial types and specializations will provide direct information on the function of the tissue and organ.
As a routine procedure, look at a slide first with one of the 0.16X – 1.25X objective settings, noting any characteristic features and relating them where possible to the gross structure. Then, examine the slide with the 5X or 10X objective setting, noting the histological landmarks and overall architecture. Locate a favorable area containing a structure you wish to study, and look at it in the context of its surroundings with the 40X objective setting.
In today’s lab you will study examples of the following common types of epithelium: simple squamous (Webslide 98), simple cuboidal (Webslide 38), simple columnar (Webslides 32, 54, and 51), non-keratinized stratified squamous (Webslide 54), keratinized stratified squamous (Webslide 65), transitional (Webslide 98), and pseudostratified columnar (Webslide 8). Stratified cuboidal epithelium and stratified columnar epithelium are not as commonly observed; stratified cuboidal epithelium will be seen later in the skin lab in the ducts of sweat glands.
Webslide 0098_G: Urinary Bladder, monkey, H & E
This slide contains a section of the wall lining the urinary bladder. When you look at the webslide, the lumen of the bladder is at the bottom and the body cavity is at the top of the field. At low power, contrast the appearance of the natural “free edges” at the bottom and top of the section where epithelium lines the bladder lumen and the body cavity, respectively, from the sharp “cut edges” at the left and right of the section where the tissue was cut during specimen preparation. First focus on the top surface of the tissue, and note where a simple squamous epithelium called a mesothelium covers the bladder. This cellular layer is quite thin (less than 5 mm). Note the bulging nuclei and attenuated cytoplasm that forms a continuous sheet that is sharply differentiated from the underlying connective tissue and muscle. Some portion of the specimen has lost its mesothelium, so scan along the top edge of the webslide.
Now examine the lower portion of the slide which is the tissue that lines the lumen of the bladder. Notice the characteristics of the transitional epithelium that is found only in the urinary tract: (1) there are several layers of cells, (2) most of the cells in the outer layer touching the lumen are rounded or polygonal and contain spherical nuclei, (3) in this outer layer of cells there is usually a considerable amount of cytoplasm between the nuclei and the apical plasma membrane. These last two characteristics distinguish this epithelium from stratified squamous epithelium where the outer layer of living cells is flattened and has thin nuclei.
Webslide 0038_G: Submandibular gland, monkey, T.B-A.F, 1.5 µm sec.
Most of this section is composed of darkly staining acini (balls or clusters of cells) that comprise the parenchyma (dominant cell type) of this organ. Our focus here is on the relatively small, pale staining ducts that can be found at low power (10X) within the connective tissue separating the acini. Look for rings of pale-staining cells surrounding a small lumen. These ducts are lined by simple cuboidal epithelium. Note the size and shape of these cells and their central, round nucleus.
Webslide 0032_G: Ileum & Peyer’s Patches, monkey, H&E
The absorptive surface lining the lumen (bottom of webslide) is thrown into large folds (villi) which increase the absorptive surface area. Scan the slide to observe the orientation of the simple columnar intestinal epithelial cells that line the lumen as they are cut in different planes. Two types of cells are present in this columnar epithelium – absorptive cells with striated borders and secretory goblet cells. Identify the following and understand their fine structure and function:
- Striated border (composed of microvilli): What is the height of this border?
- Terminal bars: What cell junctions are present here?
- Goblet cells (unicellular glands): What shape are these epithelial cells? What do they secrete?
Webslide 0051_G: Fallopian tube (oviduct isthmus), human H & E.
The lumen of the oviduct (near the center of the slide) is thrown into many folds that are all lined by a simple columnar epithelium. Scan the epithelial surface to find regions where the epithelium is cut in a section perpendicular to the basal lamina. Two distinct types of columnar cells are present in this epithelium, ciliated cells and non-ciliated secretory cells. Measure the height of the cilia and compare to the height of the striated border composed of microvilli in the previous Webslide 32. Can you resolve individual cilia at highest power?
Note that by being able to distinguish the cilia in this slide from the microvilli of the previous Webslide 32 you have learned about the functional properties of the different epithelial surfaces: transport of material (oocytes) in the oviduct and adsorption of food products in the intestine.
Webslide 0054_G: Gastro-esophageal Junction, human, Mallory
Starting with the 5X objective, scan the entire bottom surface of the slide and note the striking change from simple columnar epithelium lining the stomach to the nonkeratinized stratified squamous epithelium lining the esophagus. Consider now only the non-keratinized stratified squamous epithelium which contains several layers of cell. The inner layers (away from the lumen) contain cuboidal or polygonal cells, whereas the outer layers (lining the lumen) contain flattened cells with flattened nuclei (you will have to scan around to find places where the outer layers are intact).
Webslide 0065_G: Skin, Foot, human, H & E
The epidermis of the skin, found at the bottom of the slide, is an example of keratinized stratified squamous epithelium. There is a change in cell shape from cuboidal at the base of the epithelium to flattened at the surface. The outermost layers consist of keratinized plates containing no nuclei.
Webslide 0008_G: Trachea, monkey, AF-TB, 1.5 mm
The epithelium of the trachea is along the bottom of the slide. Note the different levels of the nuclei in this pseudostratified columnar ciliated epithelium. The elongated nuclei of the columnar ciliated cells are farther from the basal lamina than the rounded nuclei of the short basal cells. Goblet cells can also be observed with their aggregated mucinogen granules near the surface of the epithelium. Observe and measure the dimensions of the cilia. Cilia are involved in transport of material along the epithelial surface. What do they transport in the trachea?