Beneath the endothelium is a middle layer consisting of dense connective tissue (DICT), in which elastic fibers are present as well as some smooth muscle cells. The third layer, the deepest part of the endocardium (Ec’), consists of more irregularly arranged connective tissue with blood vessels and occasional fat cells. At the bottom of the micrograph is myocardium (My). Note how darkly stained the cardiac muscle fibers are compared to those of the Purkinje fibers.
cell. The cytoplasm in the inner portion of the cell appears unstained. Where the nuclei are included in the section of the cell, they are surrounded by the clear cytoplasm. In the lower portion of the figure, several longitudinally sectioned Purkinje fibers can be seen. Note the intercalated discs (ID) when seen in this profile. The inset reveals to advantage the intercalated discs and the myofibrils with their cross-banding. Note the clear area or unstained cytoplasm surrounding the nuclei.
PLATE 25 • CARDIAC MUSCLE, PURKINJE FIBERS EtEtDICTDICTPFPFEc’Ec’EcEcDICTDICTMyMyEtDICTPFEc’EcDICTMyEtCEtCSMSMSMSMIDIDMMMMMMEtCSMSMIDMMM
Smooth muscle is the intrinsic muscle of the alimentary canal, blood vessels, the genitourinary and respiratory tracts, and other hollow and tubular organs. It is also a component of the nipple, scrotum, skin (arrector pili muscle) and parts of the eye (iris). In most locations, smooth muscle consists of bundles or layers of elongate fusiform cells. They may range in length from 20 m in the walls of small blood vessels to about 200 m in the intestinal wall. In the case of the uterus, they may become as large as 500 m during pregnancy. The smooth muscle cells are joined by gap junctions that allow small molecules or ions to pass from cell to cell and allow regulation of contraction of the entire bundle or sheet of smooth muscle. The cytoplasm of smooth muscle cells stains uniformly with eosin in routine H&E preparations because of the concentration of actin and myosin that these cells contain. The nucleus of the cell is located in its center and is elongate with tapering ends, matching the shape of the cell. When the cell is maximally contracted, the nucleus displays a corkscrew shape. During lesser degrees of contraction, the nucleus may appear to have a slight spiral shape. Often in H&E preparations, smooth muscle stains much the same as dense connective tissue. A distinguishing feature relative to smooth muscle is that nuclei are considerably more numerous and they tend to look the same, appearing as elongate profiles when smooth muscle is longitudinally sectioned and as circular profiles when smooth muscle is cross-sectioned. In contrast, the nuclei of dense connective tissue, though fewer in number per unit area, may appear in varying profiles in a given section.
Smooth muscle, small intestine, human, H&E, ×256.
This low power micrograph reveals part of the wall of the small intestine, the muscularis externa. The left side of the micrograph shows two bundles, both are longitudinally sectioned (LS), whereas on the right side, smooth muscle bundles are seen in cross section (CS). Note that the nuclei of
Smooth muscle, small intestine, human, H&E, ×512.
This higher magnification photomicrograph shows a bundle of smooth muscle cells (SMC). Note how the nuclei exhibit an undulating or wavy form indicating that the cells are partially contracted. The nuclei seen in the dense connective tissue the smooth muscle cells in the longitudinally sectioned bundles are all elongate; in contrast the nuclei in the cross-sectioned smooth muscle bundles appear as circular profiles. Intermixed between the bundles is dense irregular connective tissue (DICT). While both the smooth muscle cells and the dense connective tissue stain with eosin, the dense connective tissue exhibits a paucity of nuclei compared to the smooth muscle cell bundles.