If it contracts, it's muscle:
Muscle tissue is categorized on the basis of a functional property: the ability of its cells to contract. In muscle tissue, the bulk of the cytoplasmic volume consists of the contractile protein fibrils actin and myosin. Muscle is responsible for movement of the body and changes in the size and shape of internal organs. Muscle cells are generally referred to as muscle fibres. (Note that the term "fibre" is used both for muscle cells, and for the extracellular elements, eg. collagen, produced by connective tissue cells.) Muscle fibres are typically arranged in parallel arrays, allowing them to work together effectively.
The structure and physiology of muscle will be studied in detail in the Homeostasis and Development Block. Here, we will discuss only the identifying features of the different types of muscle. Additional information on the histology of muscle is found in Ross et al., chapter 10.
The three types of muscle:
Three types of muscle tissue can be identified histologically: skeletal muscle, cardiac muscle and smooth muscle. The fibres of skeletal muscle and cardiac muscle exhibit cross striations at the light microscope level and they are both referred to as striated muscle.
Skeletal muscle constitutes the muscle that is attached to the skeleton and controls motor movements and posture. There are a few instances where this type of muscle is restricted to soft tissues: the tongue, pharynx, diaphragm and upper part of the esophagus. (Some people use the term visceral striated muscle in the foregoing examples, but since it is identical in structure to the muscle that moves the skeleton, we won't bother with the extra term.)
Skeletal muscle fibres (cells) are actually a multinucleated syncytium formed by the fusion of individual small muscle cells or myoblasts, during development. They are filled with longitudinally arrayed subunits called myofibrils. The myofibrils are made up of the myofilaments myosin (thick filaments) and actin (thin filaments). The striations reflect the arrangement of actin and myosin filaments and support structures. The individual contractile units are called sarcomeres. A myofibril consists of many sarcomeres arranged end to end. The entire muscle exhibits cross-striations because sarcomeres in adjacent myofibrils and muscle fibers are in register. The most obvious feature in longitudinal sections of skeletal muscle is the alternating pattern of dark and light bands, called respectively the A (anisotropic) and I (isotropic) band. The I band is bisected by a dense zone called the Z line, to which the thin filaments of the I band are attached.
The nuclei are located peripherally, immediately under the plasma membrane (sarcolemma). The thickness of individual muscle fibres varies (depending for example on location in the body and exercise) but each fibre is of uniform thickness throughout its length. Skeletal muscle fibres do not branch.
Connective tissue elements surround muscle fibres. Individual muscle fibres are surrounded by a delicate layer of reticular fibres called the endomysium. Groups of fibres are bundled into fascicles by a thicker CT layer called the perimysium. The collection of fascicles that constitutes one muscle is surrounded by a sheath of dense CT called the epimysium, which continues into the tendon. Blood vessels and nerves are found in the CT associated with muscle. The endomysium contains only capillaries and the finest neuronal branches.
Summary: Skeletal muscle fibres bear obvious striations, have many peripherally located nuclei, are of the same thickness throughout their length and do not branch.
Figure 1shows a longitudinal section showing parts of three skeletal muscle fibres. The A, I and Z (thinnest) bands are clearly visible. Three nuclei are seen at the peripheries of the muscle fibres.
Figure 2 shows a high power view of skeletal muscle in cross section. One entire muscle fibre is seen surrounded by parts of others. The stippled appearance of the muscle is due to the cut ends of the myofibrils. The muscle fibre at the centre is surrounded by a delicate sheath of connective tissue, the endomysium. The nucleus located in the endomysium (upper left) belongs to a fibroblast. The nucleus at the right may belong to a muscle fibre.
Cardiac muscle is the type of muscle found in the heart, and at the base of the venae cavae as they enter into the heart. Cardiac muscle is intrinsically contractile but is regulated by autonomic and hormonal stimuli.
Cardiac muscle exhibits striations because it also has actin and myosin filaments arranged into sarcomeres. Generally these striations do not appear as well-defined as in skeletal muscle. (At the ultrastructural level, some differences in the arrangement of the sarcoplasmic retiuculum and T tubules can be seen. Cardiac muscle also has a much greater number of mitochondria in its cytoplasm. More details on the anatomy and physiology of muscle will be discussed in H&D and Cardiovascular Blocks.)
At the light microscope level, a number of features distinguish cardiac from skeletal muscle. Cardiac muscle cells have only one or two nuclei, which are centrally located. The myofibrils separate to pass around the nucleus, leaving a perinuclear clear area (not always evident in standard preparations). This clear area is occupied by organelles, especially mitochondria (which are of course not visible in LM). As in skeletal muscle, individual muscle fibres are surrounded by delicate connective tissue. Numerous capillaries are found in the connective tissue around cardiac muscle fibres.
Cardiac muscle cells are joined to one another in a linear array. The boundary between two cells abutting one another is called an intercalated disc. Intercalated discs consist of several types of cells junctions whose purpose is to facilitate the passage of an electrical impulse from cell to cell and to keep the cells bound together during constant contractile activity. Unlike skeletal muscle fibres, cardiac muscle fibres branch and anastomose with one another. Although made up of individual fibres, heart muscle acts as a functional syncytium during contraction for the efficient pumping of blood.
Specialized fibres, called Purkinje fibres, arise from the atrioventricular node and travel along the interventricular septum toward the apex of the heart, sending branches into the ventricular tissue. Purkinje fibres are of larger diameter than ordinary cardiac fibres, with fewer myofibrils and an extensive, well-defined clear area around the nucleus. They conduct impulses at a rate about four times faster than that of ordinary cardiac fibres and serve to coordinate the contraction of the atria and ventricles.
Summary: Cardiac muscle fibres are striated, have one or two centrally located nuclei, branch and anastomose with other fibres, and are joined to one another by intercalated discs.
Figure 3 shows a longitudinal section of cardiac muscle. Two nuclei belonging to cardiac muscle fibres can be clearly seen (lower right and middle left). Both have a prominent nucleolus and a delicate pattern in the remainder of the nucleus. The perinuclear area is also evident around both nuclei. The other two nuclei are not very clear, but appear to lie in connective tissue and probably belong to fibroblasts. Fibroblast nuclei tend to be more flattened and darker staining. Two intercalated discs are indicated by asterisks, others, not quite so prominent, can also be seen. To the left of the upper intercalated disc indicated, a muscle fibre appears to be branching.
Figure 4 shows a section of cardiac muscle in which some fibres are cut transversely (in cross section) while most are cut more tangentially. The endocardium, the connective tissue lining the inner surface of the heart, is indicated by an asterisk. The area shown contains Purkinje fibres, but is not necessary to distinguish Purkinje fibres from other cardiac fibres at this time. In the fibre at the extreme (arrow), the nucleus with its well-defined perinuclear area can be seen. Note the central location of the nucleus. As with skeletal muscle, cardiac muscle in cross section has a stippled appearance due to cut myofibrils.
Smooth muscle is the intrinsic muscle of the internal organs and blood vessels. It is also found in the iris and ciliary body of the eye and associated with hair follicles (arrector pili). No striations are present in smooth muscle due to the different arrangement of actin and myosin filaments. (The arrangement of the filaments and mechanism of contraction is described on pg. 230-234, ch. 10 of Ross et al.) Like cardiac muscle, smooth muscle fibres are intrinsically contractile but responsive to autonomic and hormonal stimuli. They are specialized for slow, prolonged contraction.
Smooth muscle fibres are generally arranged in bundles or sheets. Each fibre is fusiform in shape with a thicker central portion and tapered at both ends. The single nucleus is located in the central part of the fibre. Fibres do not branch. They range enormously in size, from 20 (in wall of small blood vessels) to 500 (in wall of uterus during pregnancy) micrometers. Smooth muscle fibres lie over one another in a staggered fashion (tapered part of one fibre over thicker part of another). In longitudinal sections, it is often not possible to distinguish the fibre boundaries, and smooth muscle may closely resemble connective tissue (bundles of collagen). Where smooth muscle bundles are interlaced with bundles of connective tissue (eg. in the uterus), one can distinguish the smooth muscle by the orientation of the nuclei (all oriented in the same direction), and the greater abundance of nuclei per unit area (every smooth muscle cell has a nucleus, fibroblast nuclei are more scattered in bundles of CT). Also, smooth muscle nuclei often have a corkscrew shape in longitudinal section due to contracton of the muscle fibre during fixation. In cross section, smooth muscle appears as profiles of various sizes, depending on whether the cut went through the thick central part or tapered end of any individual fibre. Nuclei are seen only in the thicker profiles.
One distinguishing physiological feature of smooth muscle is its ability to secrete connective tissue matrix. In the walls of blood vessels and the uterus in particular, smooth muscle fibres secrete large amounts of collagen and elastin.
Summary: Smooth muscle fibres are fusiform with tapered ends, have a single centrally located nucleus, and do not exhibit striations.
Figure 5 shows a whole mount of teased smooth muscle cell. One whole, spindle-shaped cell is visible at the top of the field of view. Note the dark nucleus at the centre of the cell.
Figure 6 shows a layer of smooth muscle in longitudinal section lying between two layers in cross section. Notice how the shape of the fibres in longitudinal section is harder to distinguish than when individual fibres are seen, as in Figure 5. Some nuclei are clearly seen lying in their muscle fibre, other nuclei are harder to assign to a particular fibre (these might not be in the plane of section). Note however, the abundance of nuclei per unit area and their common orientation. In the cross sections, only some of the circular profiles have nuclei, which stain blue. Those fibres have been sectioned through the central part.
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