Introduction
Histology is the study of tissues: groups of similar cells that have specialized function. Organs are formed of two or more types of tissues and carry out a specific function. In histology you will concern yourself with how cells differ in shape and function, how cells are organized within tissues and how tissues are organized within organs.
Learning histology requires careful strategy. You will find the tissues that you are required to recognize and understand on the following pages. The cells are found in the tissues and many are too difficult to see clearly with the magnification levels available on our microscopes. You must begin on low power (i.e., 4x or scanning objective lens) to get an idea of the organization of the tissue. Move to higher and higher power only when you are comfortable with your understanding of the tissue (use the references posted here and/or your textbook to get your bearings).
When looking at histological slides, remember that you are looking at a single, thin section of a three dimensional structure. Depending on how the specimen was sectioned, you may get one of several different views. To illustrate this, think of sectioning a hard-boiled egg. If we were to take a cut through the top of the egg, you would see no yolk. If we cut through the egg lengthwise, but off center, we would get very little yolk showing, but a lot of egg white. Cutting the egg transversely through the center of the yolk would give us a lot of yolk, but little white showing. There are similar problems when cutting through convoluted tubular structures. The key is to look at several examples of the same structure and attempt to mentally recreate it in three dimensions. You will notice that things are stained in various colors, which is a way to detail otherwise impossible to detect. There are a plethora of stains available for histotechnique, so you should not learn things solely on the basis of color. Concentrate on the morphology of the tissues and organs. Note the textures and opaqueness of the structures (include this information in the column marked “description” in the tables you will find in the lab exercise). It is helpful to draw, diagrammatically, what you see to use as a reference later. Diagrams are particularly helpful for tissues that appear different from those in micrographs in your books.
Today, you will again use the virtual microscope we explored last week: https://www.ncbionetwork.org/educational-resources/elearning/interactive-elearning-tools/virtual-microscope
Now, return to the Slide Catalog, and open “Human.” Select “Simple Cuboidal Epithelium.” Focus on the slide, and raise the magnification until you are viewing at 40x. Adjust the light as needed.
- Describe what you see, including the shape and structure of an individual cell, and also the overall structure of the tissue.
In addition to the information listed with the slide, use the internet resources listed below to learn more about this tissue. You will want to find out the location (where in the body it can be found), and also its function.
Internet resources:
- http://www.siumed.edu/~dking2/index.htm
- https://medpics.ucsd.edu/index.cfm?curpage=main&course=hist&mode=browse
- http://www1.udel.edu/biology/Wags/histopage/colorpage/colorpage.htm
- http://www.bu.edu/histology/m/i_main00.htm
- http://videos.med.wisc.edu/events/140
Now, click on the 100x objective lens. At this magnification, you will need to perform an “oil immersion.” This is a technique used to increase the resolving power of a microscope by immersing both the objective lens and the specimen in a transparent oil of high refractive index. In the lab, you would only use an oil immersion on the 100x lens. NOTE: Do not adjust the coarse focus when using the 100x lens (if you don’t believe me, try it on the simulation, and see what happens). 🙂
On paper, sketch what you see. This sketch should include at least three adjacent cells, and show how they are connected to one another.
As you scroll down this assignment, you’ll notice that the information I’ve asked you for up above is what you will collect for each of the tissue types featured in this assignment.
Remove the slide. You’ll notice that you must clean the lens after performing the oil immersion. In the lab this is a crucial step, as the oil can damage the other objective lenses.
For the remainder of this lab, you will explore a wide variety of tissue types. The tissue types that are bolded in the table below may be viewed using the microscope simulator. The other types, along with supplemental information about all tissues, can be researched using the internet resources below. In addition to the written descriptions, make sure to sketch the tissue types. I know that there are photographs available, but you’ll find that you remember the structures much better if you create your own drawings.
Primary Tissue Types
Epithelial Tissues
Epithelial tissues cover body surfaces, line body cavities and hollow organs, and make up glandular tissue throughout the body. Depending on location, epithelial tissues derive from all three primary germ layers: ectoderm (e.g., epidermis of skin), mesoderm (linings of blood vessels & heart) and endoderm (mucosal layer of digestive tract).
Epithelial tissues are composed of closely packed cells, with little intercellular material, arranged in a continuous sheet. They are avascular (i.e., they contain no blood vessels) and obtain their nourishment through diffusion of substances from blood vessels in the underlying connective tissue. It is supported atop a thin basement membrane that separates it from the connective tissue below.
Epithelial tissues are capable of rapid regeneration, particularly in areas exposed to high abrasive forces. Since epithelial tissue always has one free surface, a polarity is established between its free side (the apical surface) and the side adjacent to the basement membrane (basal surface). The plasma membranes that compose the two surfaces can possess very different ingredients (e.g., receptors, microvilli) as warranted by the function of the particular tissue. Epithelial tissues are classified based on the number of cell layers (simple or stratified) and the shape (squamous, cuboidal, columnar, pseudostratified, transitional) of cells on the apical surface.
| Tissue Type | Description/Sketch | Location | Function |
|---|---|---|---|
| Simple squamous epithelium | |||
| Simple cuboidal epithelium | |||
| Simple columnar epithelium | |||
| Stratified squamous epithelium | |||
| Pseudostratified columnar epithelium* | |||
| Transitional epithelium* | |||
| Stratified cuboidal epithelium* |
Connective Tissue
This is a very diverse group of tissues. All connective tissues have an abundant extracellular matrix that surrounds relatively few cells.
Connective tissues perform various functions, including: support, binding, storage, etc. Special cells, called fibroblasts synthesize and release the various fibers found in the matrix.
Three specific fibers are recognized: collagenous fibers (abundant, strong, inelastic, made of the protein collagen; occur in bundles), elastic fibers (long, threadlike, branching, made of the protein elastin; they can stretch and return to their original length) and reticular fibers (short, thin, branching; form the internal framework (stroma) of glands). These fibers may only be visible with certain stain techniques. Ground substance makes up the rest of the extracellular matrix and varies from a fluid to a semi-solid gel.
Connective tissues are classified according to the amount and organization of the different fibers and ground substance. Although blood cells do not produce intercellular substances, they are classified as connective tissues because they develop from mesoderm like the other connective tissues.
| Tissue Type | Description/Sketch | Location | Function |
|---|---|---|---|
| Mesenchyme* | |||
| Areolar | |||
| Adipose | |||
| Reticular* | |||
| Dense regular | |||
| Dense irregular | |||
| Hyaline cartilage | |||
| Elastic cartilage* | |||
| Fibrocartilage* | |||
| Bone (osseus tissue) | |||
| Blood |
Muscle Tissue
These tissues are composed of cells elongated into thin fibers that are capable of contracting when properly stimulated. All are richly supplied with blood vessels and nerves. All muscle tissue is derived from mesoderm. Three distinct types exist: skeletal, smooth, cardiac.
Skeletal muscles are long multinucleated cells with a distinctive striated appearance and each cell (myofiber) is directly innervated by a motor neuron.
Smooth muscle cells, found in the GI tract, blood vessels and many visceral organs, are uninucleate and lack the striations of skeletal muscle. Cardiac muscle is found only in the heart. It is derived from smooth muscle embryologically, is uninucleate and has striations similar to skeletal mm. Individual cardiac muscle cells are electrically joined by distinctive structures known as intercalated discs.
| Tissue Type | Description/Sketch | Location | Function |
|---|---|---|---|
| Skeletal | |||
| Cardiac | |||
| Smooth |
Nervous Tissue
These tissues are composed of (1) neurons, highly specialized cells with elongated cytoplasmic processes (= dendrites, axons) and (2) neuroglial (= supporting) cells that support and protect neurons. Nervous tissue is capable of generating and conducting electrical signals for the purpose of information relay. Derived from ectoderm, it is richly supplied with blood vessels. Neuroglia are a fraction of the size of the relatively enormous neurons.
| Tissue Type | Description/Sketch | Location | Function |
|---|---|---|---|
| Nervous tissue (multipolar neuron) |