Muscle Tissue

Epimysium is the dense connective tissue that surrounds an entire muscle tissue.

Perimysium is the connective tissue that surrounds each bundle of muscle fibers (fascicles).

Endomysium is the delicate, loose connective tissue that surrounds each single muscle fiber/cell.

Axial section: Nuclei are located at the periphery of each cell and are round in profile. There may be more than one visible in each cross-sectioned fiber.

Longitudinal section: Nuclei are elongated, relatively pale-staining, and are usually located at the edges of the fiber (though sometimes the plane of section is such that it looks like it’s in the middle). You will almost certainly see more than one nucleus per cell in longitudinal section. See if you can also find a fibrocyte nucleus and distinguish it from the muscle cell (it will be skinnier and darker, and located in between adjacent muscle cells.

Z-bands are considered the ends of a single sarcomere, with the H-band in the center. As the muscle contracts, the Z-bands move closer together and the I-band and H-band shorten in length as the actin thin filaments are moved along the myosin thick filaments. The A band remains the same length because the length of the myosin unit does not change.

A sarcomere is the basic contractile unit of a muscle cell and runs from z-disc to z-disc.  Repeating sarcomeres comprise a myofibril.

I band: Shortens

H band: Shortens

A band: Stays the same length

In a muscle fiber cut in cross-section, you can see nuclei at the periphery of the fiber, but in one that is sectioned longitudinally, the nuclei can be anywhere.

Capillaries and peripheral nerves are located in the perimysium in between fascicles (bundles).

Intercalated discs differentiate cardiac muscle from skeletal muscle. They stain darkly and are perpendicular to the myofibrillar striations. Cardiac muscle cells may also exhibit branching, and their nuclei are located more centrally when viewed in longitudinal section.

Cardiac muscle cells are highly active and require a great deal of oxygen and other nutrients, brought via blood vessels.

Intercalated discs support synchronized contraction of cardiac muscle (more detail below).

Gap junctions allow action potentials to spread between cardiac cells by permitting the passage of ions.

Desomsomes hold the cells together during forceful contraction of the heart muscle.

(Fascia adherens are the anchoring sites for actin, and connect the intercalated disc to the nearest sarcomere.)

The inner layer encircles the lumen of the colon, so these fibers have been sectioned axially. The outer longitudinal layer runs parallel to the lumen of the colon, so these fibers have been sectioned longitudinally.

Boundaries between individual fibers are somewhat indistinct. The nucleus is elongate/ovoid. The nucleus may also appear “corkscrew” shaped due to the contraction of myofilaments during preparation.

Your comparison should mainly center around the fact that in a longitudinal vs. axial section of the gut tube, the two layers of muscle fiber will be oriented differently. I.e., individual muscle cells in the circular layer will be sectioned axially in a longitudinal section vs. longitudinal in an axial section, and vice versa.

Smooth muscle fibers are somewhat “cigar-shaped” and as such, the plane of section will hit some at narrower parts, and others at wider parts. They do not have nuclei along their entire length.

In the skeletal muscle, note the strong acidophilia, peripheral nuclei, and larger size. Smooth muscle will have many “empty” cells in which the nucleus is out of the plane of section. In cross-section note that the skeletal muscle fibers resemble rounded polygons separated by the endomysium with the nuclei clearly visible at the periphery, whereas smooth muscle cells are more rounded and may or may not have nuclei, which may be located anywhere in the cytoplasm. In longitudinal sections of skeletal muscle, try to identify cross-striations.