developed by Kira L. Wennstrom
for the Biology students at Shoreline Community College
How are histology slides prepared?
If I'm only looking at a thin slice of an organ or tissue, how do I recognize the structures?
What is the best way to proceed when examining a microscope slide?
How can I tell which tissues are where on the slide?
How do I recognize epithelial tissue? How can I tell what kind it is?
How do I recognize connective tissue? How can I tell what kind it is?
How do I recognize muscle tissue? How can I tell what kind it is?
How do I recognize nervous tissue?
What is histology?
Histology is the study of tissues. We have time in this class for only a very basic introduction to the topic, but it is nevertheless important. Understanding how tissues are built, how they work, and how they are arranged in organs will help you to grasp the more complex structural and functional features of organs and organ systems.
What is a tissue?
Tissues are groups of cells that work together to perform a particular function. There are four major tissue types:
Each of these four has several subtypes, which you will see listed on your "required structures" sheet. One thing to keep in mind, however, is that each of the four major types has certain structural and/or functional characteristics that define them. All epithelia, for example, consist of closely-packed cells resting on a basement membrane, linked to one another by specialized cell-cell junctions. All connective tissues consist of sparse cells embedded in a matrix of protein fibers. All muscle tissues contain cells which can contract. All nervous tissue consists of electrically active neurons and supporting glial cells.
Tissues of various types make up organs. Any organ will contain tissues that belong to all four of the major types.
How are histology slides prepared?
Tissues do not exist in isolation from one another, unless they've been grown in a lab petri dish. The samples you see under the microscope are taken from the organs of mice, humans, and other animals. Any given slide will likely contain examples of all four of the major tissue types found in animals, and sometimes multiple examples of each.
To get the samples on the slides, they are harvested, "fixed", sliced, and then stained. To fix a sample means to immerse it in a solution such as formaldehyde or alcohol or similar chemicals ("fixatives") that denature and crosslink proteins. The crosslinked proteins within cells and within the extracellular matrix form a kind of mesh that hold all the tissue elements in place, much as they would appear in life. The fixative also helps prevent the tissues from decomposing.
Once the sample is fixed, it is embedded in a substance such as plastic resin, paraffin wax, or even ice. The purpose of this process is to make the sample stiff enough to allow very thin slices of it to be cut. To illustrate this concept, think about cutting a thin, even slice from an overripe tomato and compare that to cutting a thin, even slice from an unripe tomato. The "stiffer" unripe tomato allows much thinner slices.
The thin slices are placed on glass microscope slides and then stained. The typical staining process produces pink proteins and purple nucleic acids. This means the cytoplasm and much of the extracellular matrix will be pink because they are rich in proteins, and the nucleus of the cell will be purple because it contains the DNA.
Some slides are prepared as a whole mount (w.m.), meaning that the specimen has not been sliced in section but is placed on the slide in its entirety. Other slides, especially of liquids or tissues with long fibers, may be prepared as a smear. Samples that are sliced (sectioned) may be prepared as a cross section cut perpendicular to the long axis of the organ (c.s.) or a longitudinal section cut parallel to the long axis of the organ (l.s.). For more on planes of section and how they make structures appear, see below.
What is the best way to proceed when examining a microscope slide?
Before you do anything else, you should read the label on the slide. A great deal of information can be obtained this way. The label will usually tell you what sort of organ the sample is from and the type of section that was taken (w.m., c.s., l.s., smear, etc.). It may also tell you what organism the tissue is from (mouse, cat, human, etc), how it was stained, and what region of the organ was sectioned (pyloric region of stomach, esophageal-stomach junction, etc.).
When you are ready to use the microscope, start at the lowest power available (typically the 4X objective) and scan the slide to orient yourself. Use the information you got from the slide label to guide you. While you will most likely not be able to see individual cells at this magnification, you will be able to see most or all of the sample. Consider what you are looking for. Is it an epithelium? Is it a connective tissue? Different tissues will be located in different parts of the sample. For more on this, see the section on tissue arrangement. Your required structures list also offers you guidelines about where to find each of the tissue types on the various slides.
Another thing to consider is the topography of the sample itself. Often, especially if the sample was taken from a human, it consists of only a tiny piece of an organ. That means it had to be cut. You can recognize the cut edges and distinguish them from the edges of a lumen in two ways: i) cut edges are generally straight, and ii) cut edges are not lined by a neat row of cells of the same general shape and size.
Once you've found a likely spot to search for the tissue of interest, move up to the 10X objective. At this magnification, you should be able to see nuclei more clearly and identify individual cells. If necessary, move up to the 40X objective to get a closer look at the detailed structure of the tissue.
If I'm only looking at a thin slice of an organ or tissue, how do I recognize the structures?
This can be tricky! Essentially, you will be learning to recognize structures in two dimensions (2-D) when you are used to seeing or imagining them in three dimensions (3-D). It can help if you practice making these mental adjustments by looking at pictures of everyday objects. For instance, do you recognize the objects below just from a section taken through the middle? Some objects are easier to recognize than others, and it is easier to recognize certain objects in some planes compared to other planes. Roll your mouse over the pictures below to see if you've identified them correctly.
One thing that complicates your task is that in your tissue sections, objects won't always be sectioned perfectly. Rather than an ideal cross section or longitudinal section, you will often be looking at an oblique section (one taken at an angle). Below, see how a single shape (a tube) can look different when sectioned in these different ways. Roll your mouse over the images below to see how the tube would look in cross section, longitudinal section, and oblique section.
Keep this in mind as you are looking at your slides. Always pay attention to the plane in which the section was taken and try to imagine how the structures you are examining would appear in 3-D.
How can I tell which tissues are where on the slide?
The diagram below illustrates the typical arrangement of tissues within a tube-shaped organ. In this case, the organ is the small intestine, but the arrangement here is similar to that of other digestive organs, blood vessels, respiratory organs, and organs in the urinary and reproductive systems. Clicking on the name of a structure will bring up a description of it in a small window.
If you are asked, for example, to find epithelial tissue, you should look at the area surrounding the lumen. Connective tissue will be found underlying the epithelial tissue, though sometimes this layer of connective tissue is very thin--practically invisible. Blood vessels and nerves will often be found within the connective tissue. Muscle will be found in the outer walls of the organs, and there may be another thin layer of connective tissue on the outermost surface of the organ.
How do I recognize epithelial tissue? How can I tell what kind it is?
All epithelial tissues serve as linings for the inside of organs or of body cavities, or as protection for areas directly in contact with the air. Therefore, you should look for the part of your specimen that represents these locations. It will usually be located on one edge of the specimen, or you may find small, individual lumens (see the section on tissue arrangement if you don't know what a lumen is) inside a larger specimen.
Once you find such an area, look for a regular, even lining of cells. If you don't see such a lining, you might have mistaken the cut edge of a specimen for the edge of a lumen! Once you've found the lining of cells, ask yourself two questions:
If there are multiple layers, you have a stratified epithelium. If there is one layer, you have a simple epithelium. If the both the cells and the nuclei inside them are flattened like a fried egg, it is a squamous epithelium. If the cells are about as tall as they are wide and the nuclei are round, it is a cuboidal epithelium. If the cells are taller than they are wide and the nuclei are oval, it is a columnar epithelium. Take a look at the image below. What kind of epithelium is it? You can roll your mouse over the image for some help.
This epithelium has a single layer of cells (see how the nuclei line up with one another?) that are about four times taller than they are wide. The nuclei are oval in shape. This is a simple columnar epithelium.
There are examples of epithelia that don't quite follow these rules, but we'll discuss them in class.
How do I recognize connective tissue? How can I tell what kind it is?
It can be more difficult to recognize connective tissue by sight than to recognize an epithelium. Connective tissues are made up of cells suspended in a matrix composed of ground substance and protein fibers. You can click the link to see a picture of connective tissue arrangement. Compare that picture to the one below.
Can you see how the cells (you can really only make out the nuclei) are far apart from one another? The space between them is filled with fluid and long strands of protein. The thicker pink strands are collagen and the thin dark strands are elastin. Roll your mouse over the image if you need help making this out.
Other connective tissues have very different appearances, but looking for cells that are sparse (widely separated) is a good way to start. One exception to this rule is adipose tissue, in which the adipocytes (fat cells) are often packed close together. However, adipose tissue has a very distinctive appearance, which makes it fairly easy to recognize.
The large fat droplets inside the adipocytes push the nucleus off to the edge of the cell. If you roll your mouse over the image, you will see the adipocytes labeled "A" and one of their nuclei pointed out. Sometimes these nuclei are so squashed they are difficult to see.
How do I recognize muscle tissue? How can I tell what kind it is?
Muscle tissue has a distinctive appearance. The cells are full of contractile fibers (actin and myosin) that tend to stain quite pink. Muscle cells are also quite long. There are three types of muscle cells: skeletal, cardiac, and smooth. Smooth muscle is the type of muscle found in the walls of tube-shaped organs. Skeletal muscle is the muscle that moves your joints, and is also found in parts of the digestive system and in your diaphragm. Cardiac muscle is found in your heart. Sometimes you can use this location information to help you figure out which of the three types you are looking at.
The three types of muscle tissue also have structural differences. Skeletal and cardiac muscle are striated, meaning that they have thin, perpendicular stripes. Roll your mouse over the image below to see the striations labeled.
Skeletal and cardiac muscle differ in the shape, location, and number of nuclei; the diameter of the cells; and the length of the cells. Skeletal muscle cells have very narrow, cigar-shaped nuclei that lie on the periphery of the cell. Skeletal muscle cells are multinucleated and are very long--they extend the entire length of many muscles. Cardiac muscle cells have rounder, centrally placed, single nuclei and the cells themselves are short and branched. They are linked to one another by cellular junctions called intercalated discs. You can see these features in the images below. Roll your mouse over the images to see the labels.
Smooth muscle is less distinctive than skeletal and cardiac muscle. The cells are far smaller in diameter (not much larger than the diameter of the nucleus), relatively short (about 3-4 times longer than the nucleus), and lack striations. The nucleu are cigar-shaped and often have visible nucleoli. See the left-hand picture below. It may often be easier to identify smooth muscle by its location in the walls of hollow organs, where you will frequently find double or even triple layers of smooth muscle running perpendicular to one another. Roll your mouse over the right-hand image below to see these layers labeled.
How do I recognize nervous tissue?
This is one of the easiest tissues to recognize because the cells are very distinctive. Neurons are large cells with many projections called axons and dendrites. Neurons have large nuclei with prominent nucleoli. Glial cells are very small cells which are much more numerous than neurons. The nuclei of glial cells are about the same size as the nucleoli of the neurons. Roll your mouse over the image below to see neurons and glia.
The list says I'm supposed to be looking at (fill in the blank) tissue, but I see (something else) instead.
Remember that the samples on the microscope slide come from organs. That means they will contain all four of the major tissue types. If you're looking for a columnar epithelium but find a structure that seems to have simple squamous epithelium, it doesn't mean you're looking at the wrong slide. All organs, for example, contain blood vessels, and all blood vessels are lined with simple squamous epithelium. Even the slide labeled "jejunum", where you are intended to find columnar epithelium, will contain some simple squamous epithelium because it contains blood vessels. To find the columnar epithelium you should find the lumen of the jejunum (a segment of the small intestine).
You should also keep in mind that the pictures in your atlas and in your textbooks are only examples. Tissues may appear different under the microscope due to differences in magnification, staining techniques, angle of section, the region of the organ that was sampled, and the quality of the preparation. With practice, you will learn to recognize the features that make each tissue distinctive. Your best bet for achieving this is to spend a lot of time observing the slides through the microscope!
