Epithelium

Epithelium, along with connective tissue, muscle, and nerve, is one of the four primary tissue types. It is commonly found as a covering or lining for other types of tissues, as in the case of the epidermis and the lining of the entire gastrointestinal tract, and as a major component of numerous multicellular glands, such as the liver and pancreas. Epithelial tissues, and the epithelial portions of mixed tissues and organs, are characterized by being composed of closely-packed epithelial cells with little or no intercellular space, a conspicuous lack of vascularization (no blood vessels), and some to all of the cells commonly resting on a basal lamina. A basal lamina may or may not be part of a basement membrane attaching epithelium to connective tissue. Basal laminae are not visible in the light microscope (too thin) and basement membranes are just barely visible in good, specially-stained sections.

Histological differences among the various epithelia are based on the shapes of the cells (squamous, cuboidal, columnar), the number of cell layers present (simple [one layer], stratified [two or more], pseudostratified [irregular]), various modifications to the surfaces of the cells that can be seen with the light microscope (microvilli, stereocilia, cilia), and characteristic patterns of reaction with particular dyes (acidophilia, basophilia, special stains). Epithelial tissues have numerous particular functions which you will learn in detail but, in general, their functions are to mediate absorption, excretion, protection, secretion, sensory reception, and transport.

You will find that almost every slide you examine in this lab will contain one or more epithelia. Get in the habit of identifying all the epithelia you come across in all sections; it is always a good idea to pick slides at random out of your slide box and examine them for, in this case, epithelia. You should do the same with the other primary tissue types, also, after you learn their characteristics. Don't limit yourself to those slides  that are listed and assigned each week; there are LOTS of good examples of various cell types on other slides.

PDF files of the epithelium slides. The PDFs are the same as the pictures below but they are labeled the same as the lab lecture slides.

Simple squamous epithelium consists of flattened, irregularly-shaped cells that form a continuous sheet on a surface. The cells generally are attached to a basement membrane. It is found in those places where rapid and unimpeded movement of gases and transported molecules is required, as in blood vessels, lung alveoli, and the linings of the pleural, pericardial, and peritoneal cavities.

This slide is of amphibian skin, surface view. This is actually a  stratified epithelium. Cells tightly apposed to each other. Dark cell borders due to staining of structures involved in adhesion

Higher power of #5. Interdigitated cell borders. Can see outlines of underlying cells. 

Simple squamous, X-sec. Diagram. A simple squamous capillary and a  sheet of cells.

Simple squamous X-sec. Blood vessel. Simple endothelium lines the lumen of the vessel. Endothelial cells are very thin, but bulging nuclei can be seen. The vessel is an artery.

Higher power of blood vessel endothelium

The cells of simple cuboidal epithelia appear square, or nearly so, when sectioned perpendicular to the basement membrane (BM). When viewed from the surface, or sectioned parallel to the BM, the cells are polygonal, mostly hexagonal. Commonly found lining ducts and tubules as in the kidney, salivary glands, and the pancreas, where they function in secretion and absorption.

Simple cuboidal

Longitudinal section of kidney tubule (proximal). Square profiles. Can't see the Basement Membrane.

L-sec, kidney tubule (collecting duct). Note other epithelial types, including squamous endothelium of small vessels.

Columnar cells are considerably taller than wide. There is a continuous range of cell shapes from squamous to columnar; it is a matter of judgement, sometimes, of which is which. That is, a "tall cuboidal" epithelium is about the same as a "low columnar".

The nuclei of columnar cells tend to be in the basal end of the cell, sometimes in the middle, and rarely at the apical end. Columnar cells are found in regions of rapid absorption or secretion, such as the G.I. tract. Apical specializations include microvilli (brush/striated border) and cilia.

Small intestine section, H&E. Simple columnar -- absorptive cells and goblet cells (mucous secretion). Brush border.

Large intestine, section, H&E. Columnar cells lining a tubular invagination (Crypt of Lieberkuhn). Absorptive and many Goblet cells.

Stratified epithelia are found in locations where some resistance to stress is required. The multiple layers of cells offer a thicker tissue that can withstand abrasion, pushing and pulling, stretching, etc. better than can a simple cell layer. The extra layers of cells, however, reduce the ability of stratified epithelia to allow movement of molecules from one side of the epithelium to the other. Therefore, stratified epithelia tend not to be found at sites of secretion or absorption.

Diagram. All stratified epithelia are named according to the shape of the "top" layer of cells, therefore, this is stratified squamous. Basally, the cells are attached to a BM (not shown). The top layers are not keratinized. Cornea -- Note cuboidal basal cells, flattened apical cells, and intermediate shapes. Note attachment "spines", used for lateral attachment cell-to-cell. Cells are very closely interdigitated.

Esophagus, section, H&E. Non-keratinized, stratified squamous epithelium. The top layer cells are squamous, basal ones are cuboidal. Squamous cells are abraded off as food passes through esophagus. Species that eat very rough food (eg, rodents and many herbivores) often have keratinized esophageal epithelium. Don't use the lack of keratinization as an absolute diagnostic criterion for esophagus! See capillaries in the Loose Connective Tissue under the epith.

Vagina, section, H&E. The basal layer cells tend to be more darkly stained and more basophilic than the upper layers because the basal cells are stem cells and thus highly active in protein synthesis, having lots of ribosomes and mRNA. The upper layer cells are unusually swollen and poorly stained. Not good prep technique.

Surface view of strat. squamous epithelium. Amphibian skin, stained to reveal cell borders. The intermediate filaments of the numerous desmosomes stain well with Silver salts.

Stratified squamous keratinized epithelium. Skin, sec, H&E. Basal cuboidal cells, apical squamous. Keratinization involves the replacement of a normal cytoplasm with a very dense, cross-linked matrix of hydrophobic proteins like keratin. All cell organelles are destroyed before keratinization. Top layers of cells are not tightly adherent and tend to flake off.

Thick skin, sec, H&E. Much thicker keratinized layer, cells more tightly adherent. The dark, granular layer is the last layer to contain cellular organelles; the layers above the granular layer are keratinized

Skin, sec, H&E.  Stratified cuboidal epithelium lines the ducts of a sweat gland. The duct is two layers thick with the "top", or inner, layer being clearly cuboidal. In tubes, the apical layer is the lumenal layer. Stratified cuboidal usually consists of only 2-3 layers of cells, the basal being cuboidal, as is the top layer. The nuclei are centrally located, a characteristic of cuboidal epithelia. Often found in ducts of secretory glands like  pancreas and sweat glands. 

Diagram. Stratified Columnar epithelium from male urethra. Cuboidal cells at BM, columnar at lumen

Diagram. In pseudostratified columnar all the cells touch the BM but not all extend to the lumen. It is an incompletely stratified tissue. There is neither a single row of nuclei, as in simple columnar, nor are there double or triple rows of nuclei, as in stratified columnar. It is in between. This diagram is of trachea so the cells have cilia at their apical end. Cilia are 5-10X longer than microvilli; therefore, a ciliated border is more easily seen than is a brush border. Note lack of a clear row of nuclei.

Trachea, sec, H&E. Irregular nucleus layers. Note small, dense nuclei basally, elongated, paler nuclei centrally. No clear layers of nuclei, therefore it is not a stratified epithelium. When trying to decide if an epithelium is simple, pseudostratified, or stratified, examine as much of the tissue as you can. That is, look along the entire length of it to try to find regions of a clean X-sec. Oblique sections tend to make all epithelia look stratified.

Trachea, sec.  Same as above but with different stain. Irregular nuclear layers. 

Diagram. Found in the urinary tract, transitional epithelium undergoes changes in shape of especially the top layer of cells when the bladder or ureters change from contracted to distended or vice versa. When distended, the top layer is essentially squamous; when contracted, the top layer cells are domed and extend out into the lumen.

Contracted (empty) bladder, sec. Domed apical cells, cuboidal basal cells, clear basal layer of nuclei as in all stratified epithelia.

Contracted bladder.

Distended bladder, sec, H&E. Looks a lot like stratified squamous but with fewer layers than stratified (usually).

Diagram. Terminal bars are junctional complexes found in particular epithelia, usually simple columnar. Terminal bars encircle each cell and occupy the space between the tightly apposed cells. When stained well, the bars outline the hexagonal shape of most columnar cells. The stained portion of the terminal bar is the zonula adherens. The tight junction (zonula occludens) and macula adherens (desmosome) don't stain with light microscope stains.

Section of probably intestine. Some columnar cells are sectioned through the terminal bar. Can see that bar does circle the cell. The hexagonal pattern reflects the arrangement of the cells in the tissue.

Section stained for terminal bars (Iron-hematoxylin, probably). Cross-sectioned bars appear as dots.

Simple cuboidal with brush border. Kidney, sec. Tubules with enough microvilli to form brush borders.

Small intestine, section. Clear brush border. Note that each cell extends to the BM and that there is a single row of nuclei. Therefore, it is a simple epithelium. The microvilli of the brush border serve to greatly increase the surface area of apical PM and to greatly increase the rate of absorption across the PM as a result. Absorptive cells tend to have brush borders, secretory cells tend not to have BBs. Note the dark nuclei of lymphocytes migrating into this epithelium.

Small intestine absorptive cells with brush border.

Epididymis, section. Simple columnar with stereocilia. Sterocilia are very long microvilli, not cilia. They are often matted down on to the tops of the cells and are hard to see except when extending out from the cell as they are in especially the left side of this picture.