The goal of the first part of this lab is to learn how to identify and describe the organization and key structural features of smooth and skeletal muscle in sections. A challenge is to be able to distinguish smooth muscles fibers from the collagen fibers of connective tissue. The last portion of the lab will be devoted to a review of all tissue types in preparation for the lab practical exam.
I. Muscle Tissue
As you go through these slides, refer to this schematic drawing showing the key structural features and relative sizes of skeletal, smooth, and cardiac muscle as you would observe them with the highest objective setting.
Webslide #102 containsawhole mount of the motor end plate (MEP) region of several muscle fibers. Histochemical staining for cholinesterase lets you see the MEP. These are hard to find in routine muscle sections, because there is only 1 per fiber. This prep was taken from the region of a muscle where most of the MEPs are concentrated.
At low magnification, note the range of fiber diameters and shapes; note the multiple, peripheral nuclei associated with each fiber (recall that each muscle fiber is a single cell) and numerous blood vessels in the connective tissue surrounding each fiber. At high magnification, note sizes and shapes of myofibrils within each muscle fiber. Within fibrils, electron microscopy would show an orderly hexagonal filament lattice filling out the many odd shapes. The perfect jigsaw-puzzle fit of polygonal fibrils to one another, and of fibers to one another, is characteristic of well-preserved and life-like muscle. Truly cylindrical fibrils are found only in text-book diagrams and some insect flight muscles.
Learn to distinguish the skeletal muscle fiber nuclei from the more elongated nuclei of capillaries and fibroblasts found in the surrounding connective tissue.
Slide 058thin is similar to 0017_N above, but it was scanned with a higher power objective making it easier to see the fine structure of skeletal muscle such as Z lines (in the middle of the I bands) and, in some areas, H bands (in the middle of the A bands).
A typical appearance of smooth muscle in paraffin H and E sections is found in these transverse sections from the gut shown in Webslide 0096_N , which show the outer longitudinal layer in cross-section (XS) and the inner circular layer in longitudinal section (LS). Dense red bands indicate where excessive contraction has clumped the contractile material within smooth muscle cells. Note how clearly nuclei are seen within fibers, and note how in XS many fibers lack nuclei (remember why?). Collagen fibers, seen here in submucosa, happen to stain more darkly in this slide. This makes the contrast with smooth muscle easier to notice. Note that collagen fibers appearcoarser, looser-packed, and more varied in size and direction than smooth muscle fibers. Note how, among collagen fibers, nuclei are always fewer and lie external to fibers. Smooth muscle occurs in snug parallel bundles, with more nuclei and with nuclei all internal to fibers.
The wall of the urinary bladder contains layers of smooth muscle interspersed with collagen. In this thin section, use the same criteria as described above for Webslide #96 to distinguish between collagen fibers and smooth muscle.
Many longitudinal bundles of smooth muscle are cross sectioned in the outer wall of this large vein. Some bundles of smooth muscle look strange, almost nerve-like, because few or no nuclei appear at section level. At high power, many cell boundaries appear serrated like postage stamps. (In the EM, the bulges would show many subsurface vesicles and caveolae, while the grooves would locate dense adhesion plaques under the surface membrane that anchor the actin filament bundles.)
This is also a great slide for test your ability to distinguish smooth muscle from connective tissue.
As always, start with lowest magnification to get an overall idea how tissues are distributed. This well-stained slide gives excellent and easy practice in distinguishing between 3 types of fibrous tissue, namely collagen, smooth muscle and skeletal muscle. (Why is it not enough just to call it striated muscle?) For now examine only the muscle and connective tissue that is towards the right-hand side of the slide. In the review portion of this lab we will consider other tissues. Note typical fiber diameters, textures, and placement of nuclei. Smooth muscle nuclei in LS appear less elongated than in previous slides, evidently shortened here by contraction.
The alternate UMich slide 155 features very well-preserved and stained tissue and provides another opportunity to observe smooth muscle in several different planes of section. Refer to this orientation image to find the various layers of smooth muscle that can be seen.
In the esophagus:
muscularis mucosae (shown here in longitudinal section) -this is a strip of smooth muscle adjacent to the epithelium of the esophagus.
inner circular layer (shown here in cross section).
outer longitudinal layer (shown here in longitudinal section).
In the stomach:
innermost "oblique" layer (mostly in longitudinal section here) -this layer is thin and poorly organized, so it tends to appear more as strips of differently oriented smooth muscle next to the prominent middle circular layer.
middle circular layer (in cross section here) -this is the most prominent muscle layer in the stomach.
outer "longitudinal" layer (some parts are in longitudinal section; others are in cross section) -this layer is also rather thin and poorly organized in the stomach.
UMich slide 155 is also an excellent specimen to test your ability to differentiate smooth muscle from nearby connective tissue and peripheral nerve fibers.
II. Molecules, Cells, and Tissues Review Materials
Listed below are several different tools available for individual or small group review. We do NOT want to overwhelm you, so please just use the items that are helpful for your review for the Practical Exam. We suggest that a good review exercise is to try to identify unknown cells and tissue from the resources listed below while writing down the key structural characteristics of each cell or tissue. Working with a colleague can be helpful. Ask your instructor for help if you have trouble with identifications.
(2) Review Powerpoints and other materials: Many lab instructors have prepared study notes, reviews, or orientation images for each lab, all of which can be found on BlueDocs in each lab slot. For your convenience, they have also been collated into the drop-down menus below:
Suggested Slides for Review (with tissues they display):
Same as slide 250-1 above, but this slide is stained with trichrome in which connective tissue and nerve stain BLUE; muscle and epithelia stain RED
Example of Possible Exam Format:
Some students wonder about the format of the final exam. The following illustrates a method that we have used in the past, where we label several tissues or cells (“A”, “B”, “C”, etc.) on a webslide of an organ. Using the virtual microscope, the student examines the section and is then asked to identify, describe, and sometimes draw a picture of each tissue or cell. (The student is NOT required to identify the organ, which we will cover in the Normal Body course.)
For items A through H labeled on the slide (see below for an orientation image), identify the predominant tissue type for each label in the image, listing key structural features. For each tissue, provide a drawing labeling the key cell types. Be sure to include a scale marker.