Objectives:

1. Be able to distinguish the three types of muscle at the light and electron microscope levels, including distinctive features of each (e.g. intercalated disks in cardiac muscle).


2. Describe the structural basis of muscle striation.


3. Know the structural elements that harness muscle contraction (i.e., the shortening of myofibrils) to the movement of a body part (i.e., via connection to bone).


4. Know the function and organization of the connective tissue in muscle (endo-, peri-, and epimysium).

There are three major types of muscle, and their structure reflects their function.  Skeletal and cardiac muscle cells are called striated muscle because of the very regular arrangement of their intracellular contractile units, sarcomeres, at the light microscope (LM) and electron microscope (EM) levels.  This regular arrangement imparts a cross-striated (or striped) appearance.  Such an arrangement is not seen in smooth muscle cells.  Skeletal muscle is also called voluntary muscle, because its contraction is under conscious neural control.  In contrast, cardiac and smooth muscles are called involuntary muscles because their contractions are either spontaneously generated or are under the control of the autonomic nervous system.

Each of these three types of muscle has a characteristic appearance in both cross and longitudinal sections.  You should be able to recognize each type of muscle in both planes of section.

I. SKELETAL MUSCLE

!! If you are a WINDOWS USER on campus, remember to map a drive to the file server (click here), and USE THE [WinLab] LINKS !!

A. Cytology of skeletal muscle cells, (W. pgs. 102-5, 6.2-7)
Slide 58L (skel. muscle, long. sect., H&E) [WinLab] [Mac] [WinHome]
Slide 58Lex (skel. muscle, long. sect., H&E) [WinLab] [Mac] [WinHome]
Slide 58thin (skel. musc., l.s., H&E, 83x) [WinLab] [Mac] [WinHome]
Slide 58T (skel. muscle, cross sect., H&E) [WinLab] [Mac] [WinHome]

In longitudinal sections of skeletal muscle (Slide 58, odd-numbered slide boxes), observe the following :

1. Nonbranching, cylindrical shape of the cell (also referred as muscle fiber). These cells are very long; you cannot see their ends.

2. Be sure you can identify the borders of the muscle cell. Why do some muscle cells seem wider than others? (MU1)

3. Peripheral position of the elongate nuclei just inside of the sarcolemma (plasma membrane). Note that each cell contains large numbers of nuclei. You might see occasional nuclei which appear to be centrally located, but aren't. Why?(MU2)

4. Cross striations can be seen, and are due to the structure of the sarcomere (W pg 102, 6.8) A sarcomere consists of the structures between two Z lines. You should recognize:
(Using your microscope and glass slides may help to see these fine structures)

a. The dark A band

b. The lighter H zone which bisects the A band

c. The light I band

d. The dark Z line which bisects the I band.

It is admittedly difficult to see the Z line and especially the H zone with the light microscope. However, they can be seen clearly on some areas of almost all slides, and it is just necessary to do some looking around for a favorable area on your slide. Here are some examples showing cross striations:

[example 1]
[example 2]
[example 3]

5. A less regular longitudinal striping can sometimes be seen within the muscle fibers and is due to the bundling together of the thick and thin filaments into myofibrils, which are arranged parallel to the long axis of the cell. Note that sometimes the cross striations are not aligned all the way across the cell. This is because different myofibrils may not be aligned.

In transverse sections of skeletal muscle cells (slide 58, even slide boxes), observe the cylindrical shape of the cells (fibers) and the peripherally-located nuclei. Note also the the cytoplasm of the muscle cells has a stippled, punctate appearance which is due to the bundling of thick and thin filaments into myofibrils as mentioned above (W pgs 103-4, 6.3a, 6.5). What exactly is a muscle cell and what's the difference between a muscle fiber and a muscle fibril? (MU3)

B. Perimuscular Connective Tissue (W. pgs. 102-3, 6.1-3)
Slide 59-3 (skel. muscle, cross sect., trichrome) [WinLab] [Mac] [WinHome] [Orientation]
Slide 59-1 (skel. muscle, cross sect., H&E) [WinLab] [Mac] [WinHome] [Orientation]

In slide 59, stained with trichrome (even slide boxes) or H&E (odd slide boxes), three layers of connective tissue sheaths are visible. This is a transverse section of an entire fetal forearm, and contains many elements in addition to the muscle, such as nerves and tendons, both of which are, unfortunately, simlarly bundled into fascicles and therefore easily confused with muscle. So, make sure you're looking at the muscles [see 59-3 orientation] [see 59-1 orientation]

1. Endomysium - thin connective tissue sheath, consisting of basal lamina, some reticular fibers and capillaries around each muscle fiber. In fetal muscle, endomysium is not always clearly defined, so it is best to use the trichrome-stained slide (slide 59-3) to this --it is the connective tissue immediately surrounding the individual cells and very delicate perhaps visible only as a bluish tinge.

2. Perimysium - loose connective tissue sheath consisting of type I collagen fibers found around fascicles (bundles of muscle fibers). Note that the tissue in these specimens underwent some shrinkage during preparation so the separation between fascicles is somewhat exaggerated. Similarly, much of the perimysium has been damaged; however there are still some areas where it can be clearly seen (e.g. around many of the blood vessels found within the belly of the muscles).

3. Epimysium - the dense irregular connective tissue sheath around the entire muscle = deep fascia in gross anatomy; contains even larger blood vessels and nerves.

Click here to see an area where all three layers can be seen in slide 59-3 [see orientation]

II. CARDIAC MUSCLE
Slide 57 (cardiac muscle, H&E) [WinLab] [Mac] [WinHome]
Slide 98-1 (right heart wall, H&E) [WinLab] [Mac] [WinHome]
Slide 98-N (right heart wall, trichrome) [WinLab] [Mac] [WinHome]
Slide 305 (cardiac muscle, H&E) [WinLab] [Mac] [WinHome] (note: this slide NOT in glass slide collection)

Cardiac muscle will be studied in the wall of the ventricle of the heart. In comparison with skeletal muscle, note the following differences (W pgs 116-8, 6.21-24).

Cardiac muscle cells branch and form a three-dimensional network. These branch points can sometimes be seen in your sections, and you should also note that the muscle fibers are less parallel than in skeletal muscle.

A. In longitudinal sections, observe:

1. Intercalated discs which are dark lines which cross the cell transversely. Look around in slide 57 [see example] or slide 305 [see example] for lightly stained regions of the slide. There are artifactual transverse breaks in some of the muscle fibers, which are NOT intercalated discs. Intercalated discs are also fairly easy to find in the areas where muscle fibers are longitudinally oriented in slide 98-1 [see example] and/or slide 98-N [see example] (be sure to look at both H&E and trichrome-stained sections).

2. The nucleus is centrally located in a fiber.

3. Longitudinal striping due to myofibrils can sometimes be seen.

B. In transverse sections, observe:

1. The myofibrils are coarse and give rise to the nonhomogeneous, punctate appearance of the sarcoplasm.

2. The nuclei are centrally-located. You won't see a nucleus in every fiber cross-section.

3. The extensive network of capillaries in the endomysium --heart muscle is ALWAYS beating and therefore always in demand of oxygen and nutrients delivered via the blood.

MAKE SURE YOU CAN DIFFERENTIATE BETWEEN CARDIAC AND SKELETAL MUSCLE IN BOTH LONGITUDINAL AND TRANSVERSE SECTIONS!

III. SMOOTH MUSCLE
Slide 29 (intestine, cross sect., H&E) [WinLab] [Mac] [WinHome]
Slide 169 (jejunum, cross sect., H&E) [WinLab] [Mac] [WinHome]
Slide 155 (gastro-esophageal junction, long. sect., H&E) [WinLab] [Mac] [WinHome]
Slide 250-2 (vagina, trichrome) [WinLab] [Mac] [WinHome]
Slide 250-1 (vagina, H&E) [WinLab] [Mac] [WinHome]

Smooth muscle (W pgs 112-5, 6.14-6.17) may be studied using slide #29 [see example] again in section of intestine or slide #169 [see example], also in the intestine  (W pg 274, 14.16b; see also diagram 14.2 on pg 264 for general orientation).  To find the muscle layer, look at the at slide at the lowest power (this is about the same as looking at the glass slide with the naked eye).  The purple layer is largely the epithelium and the lamina propria filled with plasma cell and lymphocytes.  Next to that you see a lighter region of connective tissue (the submucosa you looked at to see loose connective tissue and fibroblasts), then a darker pink region which is made up of the two layers of smooth muscle you want to look at.  Slide 29 is a cross section of the intestine, so the inner, circular layer of muscle will have cells oriented longitudinally (or, in places, the cells may appear to be oriented more obliquely).  Move further out to see the outer sheet of smooth muscle which runs longitudinally along the intestine, and will therefore be seen in cross section.

A. In longitudinally cut smooth muscle cells, observe the following points:

1. Cells are small and spindle-shaped (fusiform); this may be hard to appreciate because the cell membrane is indistinct.

2. Myofibirils and cross striation cannot be seen.

3. Nuclei are narrow, elongated and sometimes kinked or spiraling. They are centrally located.

B. In transversely cut smooth muscle cells, observe the following points:

1. The cell has a small diameter.

2. The nucleus is located centrally, but will not be seen in every cross section.

3. Myofibrils cannot be seen.

4. Cross-sectional diameters vary due to the spindle shape of the cells.

Now, look at slide #250 and see if you can distinguish between small fascicles of smooth muscle and collagen fibers in the lamina propria (it’s easier if you look first at the trichrome-stained section, which stains the muscle pink(ish) and the collagen blue) [see example]. It's more challenging to make this distinction in the H&E-stained section [see example], but you should note that smooth muscle is pink wheras collagen is a bit more orange-red. Also, smooth muscle tissue is mostly cellular (and therefore more nuclei are present) whereas the connective tissue is mostly extracellular collagen fibers with many fewer cells.

The table below compares the differences in the morphology of the three types of muscle.

MAJOR HISTOLOGICAL CHARACTERISTICS OF THE THREE TYPES OF MUSCLE AS SEEN WITH THE LIGHT MICROSCOPE

	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
1.	Prominet cross- striations	Prominet cross- striations	No cross- striations
2.	Fiber diameter large	Fiber diameter medium	Fiber diameter small
3.	Nuclei usually peripheral	Central nucleus	Central nucleus
4.	Unbranched	Branched	Unbranched
5.	No intercalated discs	Intercalated discs	No intercalated discs
6.	Longitudinal striation (myofibrils)	Longitudinal striation	No longitudinal striation

 

Electron Micrograph Wall Charts

#41 SKELETAL MUSCLE (longitudinal secion) [WinLab] [Mac] [WinHome]
Find the skeletal muscle nuclei and note their peripheral location. Note the intimate contact between capillaries and muscle cells, and be sure you can tell where one muscle cell or fiber stops and another begins (you can see parts of four fibers in this picture). Make sure you known which is the longitudinal axis of the cell. Identify sarcomeres, A bands, I bands, Z lines and H zones. Note that, as you saw at the LM level, the individual myofibrils do not line up perfectly across the fiber.

#42 SKELETAL MUSCLE (cross section) [WinLab] [Mac] [WinHome]
Note location of muscle fiber nuclei. You can see cross sections of A bands (darker) and I bands (lighter) side by side in the same cell because of the fact that the myofibrils don't line up perfectly. Identify the approximate outline of a single myofibril.

#43 SKELETAL MUSCLE (longitudinal section) [WinLab] [Mac] [WinHome]
Identify a sarcomere. Relate the sarcomeric structure seen in the LM to the structure seen here. Note that there is also lots of glycogen in the region between the two myofibrils in this picture, a storage form for glucose (which is metabolized to provide energy for muscle contraction). At the border of the I and A-bands, note the triads consisting of a central T (transverse) tubule and flanking cisternae of the sarcoplasmic reticulum.

#44 SKELETAL MUSCLE (cross section) [WinLab] [Mac] [WinHome]
Review the reasons for the fact that you can see only cross-sectioned thin filaments in the I bands, whereas the A bands may contain both thin and thick filaments. Thus, you should be able to tell at which level each of these myofibrils was cut. Note the abundance of the sarcoplasmic reticulum.

#45 CARDIAC MUSCLE (longitudinal section) [WinLab] [Mac] [WinHome]
Note central location of muscle nuclei. Note the "stacks" of mitochondria between myofibrils. Cardiac muscle is even richer than skeletal muscle in mitochondria (again, important for energy production). An intercalated disc is present in the upper left region of the picture.

#46 CARDIAC MUSCLE (intercalated disc) [WinLab] [Mac] [WinHome]
Note the irregular course of the intercalated disc. In this preparation, the I bands are very short, indicating that the sarcomere is in a contracted state. Review the types of junctions present in an intercalated disc and their functions.

#48 SMOOTH MUSCLE [Winlab] [Mac] [WinHome]
Study the orientation of the smooth muscle layers in the intestine. The diagram will help you understand the patterns which arise from longitudinal and cross-sections.

#50 SMOOTH MUSCLE [WinLab] [Mac] [WinHome]
Here you can see the filaments in cross-section, appearing as dots. Also, the dark areas which are membrane-associated are called dense plaques and are sites of filament attachment.

Review Question Answers

MU1: Why do some skeletal muscle cells seem wider than others?

answer

MU2: You might see occasional nuclei which appear to be centrally located, but aren’t. Why?

answer

MU3: So what exactly is a muscle cell and what’s the difference between a muscle fiber and a muscle fibril?

answer

Practice Questions

1. Caveolae in smooth muscle are functionally analogous to:

1. transverse portion of intercalated disks in cardiac muscle.
2. lateral portion of intercalated disks in cardiac muscle.
3. Z lines in skeletal muscle.
4. sarcoplasmic reticulum in skeletal muscle.
5. T tubules in skeletal muscle.

ANSWER

Select either the MAC link or PC link to view image.
2. The predominant tissue in this slide:

1. contracts rapidly and voluntarily.
2. interacts directly with nerves via motor end plates.
3. consists of regularly arranged sarcomeres.
4. can readily regenerate and/or proliferate via cell division.
5. ALL of the above

ANSWER

Select either the MAC link or PC link to view image.
3. Identify the tissue.

1. skeletal muscle
2. cardiac muscle
3. smooth muscle
4. epimysium
5. dense regular connective tissue (tendon)

ANSWER

Click here to view image
4. Which of the lines in the electron micrograph indicates a single sarcomere?

1. A
2. B
3. C
4. D
5. E

ANSWER