Muscle Tissue & Review

Atlas:
Ross & Pawlina (6th ed.), Plates 21-26, pgs. 340-351
Text:
Ross & Pawlina (6th ed.), Ch 11, pgs. 310-338, Muscle Tissue

 

Overview:

The goal 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. 

 

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. 

Skeletal Muscle

1. Motor End Plate

Webslide 0102_N: skeletal muscle spread, acetylcholinesterase stain
[Aperio ImageScope] [Aperio WebScope]

Webslide #102 contains a whole 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.

 

2. Sarcomere Organization in Slides

CROSS SECTION

Webslide 0013_N: skeletal muscle, primate, c.s., H&E
[Aperio ImageScope] [Aperio WebScope]

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.

 

LONGITUDINAL SECTION

Webslide 0017_N: rabbit psoas muscle, stretched, long. sect. H&E.
[Aperio ImageScope] [Aperio WebScope]

Note fibers, nuclei, capillaries and connective tissue.

Observe myofibrils (staggering and longitudinal splitting of cross-banded substance) and try to determine the average sarcomere length (use the ruler tool and enter the data into this online calculator). 

UMich slide 058thin: rabbit psoas muscle, long. sect., H&E.
[Aperio ImageScope] [Aperio WebScope]

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).

 

Smooth Muscle

Webslide 0032_G: Ileum, monkey, H&E
[Aperio ImageScope] [Aperio WebScope]

Towards the top of the slide is a typical appearance of smooth muscle in H and E sections. This section from the gut shows the outer longitudinal layer in longitudinal section (LS) and the inner circular layer in cross-section (XS). In longitudinal section, note the serrated edges of the cells which represent cell-cell junctions. 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, stain more lightly.  This makes the contrast with smooth muscle easier to notice.  Note that collagen fibers appear 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.

 

 

Webslide 0098_N: urinary bladder, H&E
[Aperio ImageScope] [Aperio WebScope]

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 #32 to distinguish between collagen fibers and smooth muscle.

 

 

Webslide 0023_N: vena cava, monkey, plastic, c.s., H&E
[Aperio ImageScope] [Aperio WebScope]

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.

 

 

Webslide 0093_N: recto-anal junction, mammal, H&E
[Aperio ImageScope] [Aperio WebScope]

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.

 

Extra slides to study muscle tissue:

 

Smooth Muscle:

alternate UMich slide 155: esophagus-stomach junction, human, l.s., H&E (compare to WebSlide 96 above)
[Aperio ImageScope] [Aperio WebScope]

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.

 

Muscle spindles:

Slide UMich 71-1B: Muscle and muscle spindle, c.s., H&E)
[ImageScope] [WebScope]

Neuromuscular spindles are stretch receptor organs that regulate muscle tone via the spinal stretch reflex. Look at slide #71-1B and identify the neuromuscular spindle in the within the perimysium between fascicles in the belly of the muscle [example]. In this preparation, the sensory nerve fibers of the spindle are NOT visible, but the modified skeletal muscle fibers (intrafusal fibers), which are smaller than the muscle fibers proper (extrafusal fibers), are easily visualized -- 2 to 10 fibers are contained in a fluid-filled space within a discrete, external connective tissue capsule. Note the intrafusal fibers are bundled together by a delicate internal capsule that is not so evident in these sections. The sensory receptors (nerve endings) are activated by stretching of the intrafusal fibers, which opens mechanically-gated ion channels in the nerve endings. This sensory input may evoke a reflex contraction of the extrafusal fibers that is driven by large (alpha) somatic motor neurons (located in the ventral horn) in a two-neuron spinal reflex arc.

It is worth noting that, in addition to being stretch receptors, the intrafusal fibers are functional, contractile muscle cells. They are innervated by special (gamma) motor neurons that set the tone of the intrafusal fibers thus modulating sensitivity of the stretch receptor (contraction of the spindle cells makes them taut and therefore even more sensitive to stretch). This also allows the spindle cells to contract in concert with the extrafusal fibers thus maintaining sensitivity to stretch over the muscle's full range of motion [see explanatory figure].

 

 

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Updated 8/28/19 - Velkey