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Part 7: Muscle |
The cells of muscular tissue lie parallel to each other and, therefore, can be cut in either
cross or longitudinal section and have to be distinguished from each other in both planes. They
must also be distinguished from cuts of nerve and tendon. Watch for diagnostic features as you go
along through these slides.
Longitudinal sections:
Smooth muscle - long, slender central nuclei, lying within narrow, fusiform cells that lie
parallel to each other in a smooth arrangement. (Muscle cells are often referred to as muscle
fibers because of their narrowness and length.)
Smooth muscle - with cells more separated so as to see their extent and shape better, and the
central position of their nuclei. A loose, irregular connective tissue (endomysium) lies between
the cells. Nuclei seen in this c.t. belong to fibroblasts mainly.
Smooth muscle with wrinkled nuclei due to contraction of cells.
EM of smooth muscle showing typical "hairy" look of primarily filaments in the cytoplasm. Part of
the cytoplasm is clear of filaments and shows mitochondria and polyribosomes. The cell membrane is
at the lower right of the field and shows a few pinocytotic vesicles toward the extreme right. The
left-hand extent of that same membrane seems darker and denser: probably a plaque, where filaments
attach. The fuzzy density just outside the cell membrane is the basal lamina.
Skeletal muscle cells (fibers), with cross-striations and peripheral nuclei.
Higher power of skeletal muscle for details of cross-striations. Notice thin Z discs and heavy
A bands. From one Z disc to the next is a sarcomere, the unit of muscle contraction. In the upper
muscle cell notice shadowy myofibrils running longitudinally.
EM of several myofibrils running longitudinally through skeletal muscle cell. Between individual
myofibrils lie the mitochondria (M) and glycogen (G) of the cytoplasm. Within each myofibril are
the typical striations: A= A band; I= I band; Z= Z line; and H= H band. The banding is formed by
the arrangement of myosin and actin filaments.
Cardiac muscle with cross-striations, dark intercalated discs, and centrally located nuclei.
Notice too that the nuclei are stubby in appearance, and that they lie in a rather granular
cytoplasm. Some of the intercalated discs form a straight line across muscle fibers; others
make a step-like arrangement.
EM of intercalated disc between the ends of two cardiac muscle cells. Both desmosomes (1) and
fasciae adheretes (2) are identified. Notice mitochondria and glycogen particles lying between
myofibrils.
Another view of cardiac muscle showing wavy connective tissue (endomysium) between muscle cells.
Also, notice capillaries with r.b.c.'s; muscle is a highly vascularized tissue. Some yellow granular
cytoplasm can be seen inside the lower muscle cells, where myofibrils are parted. This picture also
gives some indication of the branching of cardiac fibers.
This is a longitudinal section of peripheral nerve, for comparison with the three types of muscle.
The foamy, pale look is due to the dissolving out of lipids from the myelin sheath. Note also the
rounded constrictions of nodes of Ranvier.
Another comparison, this time with tendon (dense, regular, collagenous c.t.). Here you see very
thin fibroblast nuclei compressed between collagen fibers and lined up in rows ("box-car").
Dense, fairly regular, collagenous tissue with mostly fibers and very few cells. Not as neatly
arranged as the previous tissues.
Cross-sections:
Smooth muscle. Since the muscle cells are spindle-shaped, with tapered ends, the diameters of
cross-cuts of individual cells vary considerably. Nuclei are central but appear only when the
section goes through the widest part of the cell. Compare diameters of these cells with those in
the next two slides, which are at the same magnification.
Cardiac muscle, with central nuclei surrounded by proportionally greater amounts of cytoplasm than
previous smooth muscle. The "graininess" of the cytoplasm is due to cut ends of myofibrils.
Remember that a very fine connective tissue endomysium lies between the individual muscle cells in
all three types of muscle; often it is not well preserved because it collapses during fixation.
Skeletal muscle -- large, rounded cross-cuts of muscle cells, packed so full of myofibrils that
nuclei are displaced to the periphery. (There is a capillary filled with pink rbc's in the upper
middle field.)
A cross-cut of nerve for comparison. The pale central axons are surrounded by myelin sheaths that
seem to have radiating lines in them due to the way the protein component of the sheath is
preserved. All nuclei lie between nerve processes rather than in them.
A cross-cut of tendon to show fibroblasts compressed between thick pale collagenous fibers that
they look stellate in shape. The cells look as if they are lying in "cracks" between the fibers;
notice this on the right side of field particularly.
Further details of muscle:
The inner surface of the heart showing large, pale-staining Purkinje fibers lying across the
mid-portion of the picture. They are modified cardiac muscle fibers and seem mostly free of
myofibrils except at the cell periphery, so that each cross-cut seems to have a darker pink rim
and a pale center. The normal cardiac muscle fibers lie below in this micrograph and appear much
smaller and more darkly stained than the Purkinje fibers.
Cross-cut of skeletal muscle to show connective tissue partitioning of muscle into groups or
bundles of fibers. Endomysium is very delicate and lies between individual fibers, while perimysium
is more visible and lies around a group of fibers. Epimysium is not seen here but ensheaths a whole
muscle. In this picture notice the presence of small blood vessels in both perimysium and
endomysium. Notice also the cross-cuts of myofibrils within the muscle cells, making them look
grainy.
Longitudinal view of skeletal muscle cell with unusually clear cross-striations. This muscle is
stretched, so that the A band is widely split.
- a)Z disc
- b)A band, split -- with pale H band in the middle
- c)the line lies right in an H band
- d)width of I band, with Z disc in the middle
- e)pointing to a practically invisible thin line, the sarcolemma (or cell membrane), which
lies outside the pale peripheral nucleus seen to the right.
Diagram of contraction of skeletal muscle. On the left is the view with light microscopy. On
the right are the thin actin filaments and thick myosin filaments seen in EM. Notice that the
total width of the A band stays the same throughout and that the sliding in or out of the actin
filaments determines the width of the H band. Consider which filaments you would see if you cut
the muscle cross-wise through the I band, A band, or H band.
EM of cross-cut cardiac muscle showing thick myosin and thin actin filaments
in a highly geometric arrangement.
Drawing of relationship (at EM level) of myofibrils to sarcoplasmic reticulum (smooth ER) and
T-tubules in skeletal muscle. In this drawing the sarcoplasmic reticulum is labelled "sarcotubules"
and "terminal cisternae". Notice that T-tubules are extensions of the sarcolemma (cell membrane,
seen at right-hand edge), so that depolarization can spread along this part of the sarcolemma as
well. (See diagrams and further explanation in your textbook.)
Same kind of diagram, this time for cardiac muscle. Note differences between the two in:
- their amount and arrangement of sarcoplasmic reticulum
- the presence or near-absence of terminal cisterns (next to the T-tubules)
- the position of T-tubules in relation to the A, I, and Z bands seen at the left.
A triad consists of two terminal cisterns with a T-tubule in the middle. When the cisterns are
not well developed, a true triad does not exist. A diad means two elements are together, as with
one T-tubule and a neighboring bit of sarcoplasmic reticulum. NOTE: sarcoplasmic reticulum is just
a form of smooth endoplasmic reticulum (SER). In muscle it is particularly associated with the
release of calcium ions needed for contraction.
