Chapter Summary

I. SKELETAL MUSCLE

A. Longitudinal Section

1.Connective tissue elements of perimysium contain nerves, blood vessels, collagen, fibroblasts, and occasionally other cell types. Endomysium is composed of fine reticular fibers and basal lamina, neither of which are normally evident with the light microscope.

2.Skeletal muscle cells appear as long, parallel, cylindrical fibers of almost uniform diameter. Nuclei are numerous and peripherally located. Satellite cell nuclei may be evident. Cross-striations, A, I, and Z, should be clearly noted at higher magnifications, and with oil immersion (or even high dry), the H zone and M disc may be distinguished in good preparations.

B. Transverse Section

1.Connective tissue elements may be noted, especially nuclei of fibroblasts, cross sections of capillaries, other small blood vessels, and nerves.

2.Muscle cells appear as irregular polygon-shaped sections of fibers of more or less uniform size. Myofibrils present a stippled appearance inside the fiber, frequently clustered into distinct but artifactual groups known as Cohnheim’s fields. Peripherally, a nucleus or two may be noted in many fibers. Fasciculi are closely packed, but the delicate endomysium clearly outlines each cell.

II. CARDIAC MUSCLE

A. Longitudinal Section

1.Connective tissue elements are clearly identifiable because of the presence of nuclei that are considerably smaller than those of cardiac muscle cells. The connective tissue is rich in vascular components, especially capillaries. The endomysium is present but indistinct.

2.Cardiac muscle cells form long, branching, and anastomosing muscle fibers. Bluntly oval nuclei are large, are centrally located within the cell, and appear somewhat vesicular. A and I bands are present but are not as clearly defined as in skeletal muscle. Intercalated

discs, marking the boundaries of contiguous cardiac muscle cells, may be indistinct unless special staining techniques are used. Purkinje fibers are occasionally evident.

B. Transverse Section

1.Connective tissue elements separating muscle fibers from each other are obvious, since nuclei of these cells are much smaller than those of cardiac muscle cells.

2.Cross-sectional profiles of muscle fibers are irregularly shaped and vary in size. Nuclei are infrequent but are large and located in the center of the cell. Myofibrils are clumped as Cohnheim’s fields (an artifact of fixation) in a radial arrangement. Occasionally, Purkinje fibers are noted, but they are present only in the subendocardium of the ventricles.

III. SMOOTH MUSCLE

A. Longitudinal Section

1.Connective tissue elements between individual muscle fibers are scant and consist of fine reticular fibers. Larger bundles or sheets of muscle fibers are separated by loose connective tissue housing blood vessels and nerves.

2.Smooth muscle cells are tightly packed, staggered, fusiform structures whose centrally located nuclei are oblong in shape. When the muscle fibers contract, their nuclei assume a characteristic corkscrew shape.

B. Transverse Section

1.A very limited amount of connective tissue, mostly reticular fibers, may be noted in the intercellular spaces. Sheets and bundles of smooth muscle are separated from each other by loose connective tissue in which neurovascular elements are evident.

2.Since smooth muscle cells are tightly packed, staggered, fusiform structures, transverse sections produce circular, homogeneous-appearing profiles of various diameters. Only the widest profiles contain nuclei; therefore, in transverse section, only a limited number of nuclei will be present.

CHAPTER OUTLINE

Graphics

Graphic 7-1 Spinal Nerve Morphology p. 156 Graphic 7-2 Neurons and Myoneural Junctions