Page 318 - Atlas of Histology with Functional Correlations

FIGURE 8.15 | Longitudinal and Transverse

Section of Smooth Muscle: Wall of Small Intestine

In the small intestine, smooth muscle fibers that surround the lumen are arranged
in two concentric layers: an inner circular layer and an outer longitudinal layer.

Here, the muscle fibers of one layer are arranged at right angles to the fibers of
the adjacent layer.

The upper smooth muscle fibers of the inner circular layer are cut in
longitudinal section. Smooth muscle fibers (1) are spindle-shaped cells with

tapered ends. The cytoplasm (sarcoplasm) of each muscle fiber stains dark. An
elongated or ovoid single nucleus (7) is present in the center of each smooth
muscle fiber.

The muscles of the adjacent longitudinal layer are cut in transverse section.
Because the spindle-shaped muscle fibers are sectioned at different places along

their length, their central nuclei exhibit different shapes and sizes. Large nuclei
(5) are seen in smooth muscle fibers (5) that have been sectioned through their
center. Muscle fibers not sectioned through the center appear only as deeply

stained areas of clear cytoplasm (sarcoplasm) (3, upper leader; 9, lower leader)
or exhibit only a small portion of their cytoplasm with a section of their nuclei
(3, lower leader; 9, upper leader).

In the small intestine, the smooth muscle layers are close to each other with
only a minimal amount of connective tissue fibers and fibrocytes (4, 8, 10)

present between the two layers. Smooth muscle also has a rich blood supply,
evidenced by the numerous capillaries (6, 11) between individual fibers and
layers. Between the inner circular muscle layer and the outer longitudinal muscle
layer are found numerous neurons of the myenteric nerve plexus (2).

317