Histology_Ross

Histology_Ross.  -Actinin, a short, bipolar, rod-shaped, 190-kilodalton actin-binding protein, bundles thin filaments into parallel arrays and anchors them at the Z line.  Nebulin, an elongated, inelastic, 600-kilodalton protein, is attached to the Z lines and runs parallel to the thin filaments. It helps -actinin anchor thin filaments to Z lines and is thought to regulate the length of thin filaments during muscle development.  Tropomodulin, a small, 40-kilodalton actin-binding protein, is attached to the free portion of the thin filament. This actin-capping protein maintains and regulates the length of the sarcomeric actin filament. Variations in thin filament length (such as those in type I and type IIb muscle fibers) affect the length–tension relationship during muscle contraction and therefore influence the physiologic properties of the muscle.


Histology_Ross

Histology_Ross.  Desmin, a type of 53-kilodalton intermediate filament, forms a lattice that surrounds the sarcomere at the level of the Z lines, attaching them to one another and to the plasma membrane, thus forming stabilizing cross-links between neighboring myofibrils.  Myomesin, a 185-kilodalton myosin-binding protein, holds thick filaments in register at the M line.  C protein, one of possibly several myosin-binding proteins (140 to 150 kilodaltons), serves the same function as myomesin and forms several distinct transverse stripes on either side of the M line.


Histology_Ross

Histology_Ross.  Dystrophin, a large 427-kilodalton protein, is thought to link laminin, which resides in the external lamina of the muscle cell, to actin filaments. Absence of this protein is associated with progressive muscular weakness, a genetic condition called Duchenne’s muscular dystrophy. Dystrophin is encoded on the X chromosome, which explains why only boys suffer from Duchenne’s muscular dystrophy. Recently, characterization of the dystrophin gene and its product has been clinically important (Folder 11.2). When a muscle contracts, each sarcomere shortens and becomes thicker, but the myofilaments remain the same length.


Histology_Ross

Histology_Ross. When a muscle contracts, each sarcomere shortens and becomes thicker, but the myofilaments remain the same length. During contraction the sarcomere and I band shorten, whereas the A band remains the same length. To maintain the myofilaments at a constant length, the shortening of the sarcomere must be caused by an increase in the overlap of the thick and thin filaments. This overlap can readily be seen by comparing electron micrographs of resting and contracted muscle. The H band narrows, and the thin filaments penetrate the H band during contraction. These observations indicate that the thin filaments slide past the thick filaments during contraction. The Contraction Cycle Shortening of a muscle involves rapid contraction cycles that move the thin filaments along the thick filament. Each contraction cycle consists of five stages: attachment, release, bending, force generation, and reattachment.


Histology_Ross

Histology_Ross. Attachment is the initial stage of the contraction cycle; the myosin head is tightly bound to the actin molecule of the thin filament. At the beginning of the contraction cycle, the myosin head is tightly bound to the actin molecule of the thin filament, and


Histology_Ross

Histology_Ross. FIGURE 11.7 • Electron micrograph of skeletal muscle and corresponding molecular structure of a sarcomere. a. This high-magnification electron micrograph shows a longitudinal section of the myofibrils. The I band, which is bisected by the Z line, is composed of barely visible, thin (actin) filaments. They are attached to the Z line and extend across the I band into the A band. The thick filaments, composed of myosin, account for the full width of the A band. Note that in the A band there are additional bands and lines. One of these, the M line, is seen at the middle of the A band; another, the less electron-dense H band, consists only of thick filaments. The lateral parts of the A band are more electron dense and represent areas where the thin filaments interdigitate with the thick filaments. 35,000. b. Diagram illustrating the distribution of myofilaments and accessory proteins within a sarcomere. The accessory proteins are titin, a large elastic molecule that anchors the thick


Histology_Ross

Histology_Ross. 35,000. b. Diagram illustrating the distribution of myofilaments and accessory proteins within a sarcomere. The accessory proteins are titin, a large elastic molecule that anchors the thick (myosin) filaments to the Z line; -actinin, which bundles thin (actin) filaments into parallel arrays and anchors them at the Z line; nebulin, an elongated inelastic protein attached to the Z lines that wraps around the thin filaments and assists -actinin in anchoring the thin filament to Z lines; tropomodulin, an actin-capping protein that maintains and regulates the length of the thin filaments; tropomyosin, which stabilizes thin filaments and, in association with troponin, regulates binding of calcium ions; and myomesin and C proteins, myosin-binding proteins that hold thick filaments in register at the M line. The interactions of these various proteins maintain the precise alignment of the thin and thick filaments in the sarcomere.


Histology_Ross

Histology_Ross.  FOLDER 11.2 Clinical Correlation: Muscular Dystrophies— Dystrophin and Dystrophin-Associated Proteins Dystrophin is a rod-shaped cytoskeletal protein with a short head and a long tail that is located just beneath the skeletal muscle cell membrane. F-actin is bound at the end portion of the tail. Two groups of transmembrane proteins— and -dystroglycans and -, -, -, and -sarcoglycans— participate in a dystrophin–glycoprotein complex that links dystrophin to the extracellular matrix proteins laminin and agrin. Dystroglycans form the actual link between dystrophin and laminin; sarcoglycans are merely associated with the dystroglycans in the membrane. Distribution of dystrophin in healthy individuals is visualized using immunostaining methods (Fig F11.2.1).


Histology_Ross

Histology_Ross. Several forms of muscular dystrophy are attributed to mutations of single genes encoding several proteins of the dystrophin–glycoprotein complex. Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are associated with mutations that affect dystrophin expression (Fig F11.2. 2); different forms of limb FIGURE F11.2.2 • Distribution of dystrophin in a patient girdle muscular dystrophy (LGMD) are caused by muta-with Duchenne muscular dystrophy (DMD). This crosstions in the genes found on the short arm of the X chromo-section of skeletal muscle was obtain from a patient diagnosed some encoding the four different sarcoglycans, and with DMD. Slide preparation similar to Fig F11.2.1. Compare another form of congenital muscular dystrophy (CMD) is the pattern and intensity of the dystrophin distribution within caused by a mutation in the gene encoding the 2 chain of affected muscle fibers to the normal individual. This muscle shows signs of hypertrophy. Some fibers do not have


Histology_Ross

Histology_Ross. distribution within caused by a mutation in the gene encoding the 2 chain of affected muscle fibers to the normal individual. This muscle shows signs of hypertrophy. Some fibers do not have any muscle laminin. Recent research has successfully characexpression of dystrophin; others still express variable levels of terized the dystrophin gene and its products. Most cases dystrophin. 480. (Courtesy of Dr. Andrew G. Engel.) of DMD are caused by a high frequency of gene deletions that create frame shifts, resulting in the absence of dystrophin in affected muscle fibers. This finding in affected individuals opened the way to direct genetic testing and prenatal diagnosis.


Histology_Ross

Histology_Ross. Because of its inheritance as an X-linked recessive trait, DMD primarily affect boys (an estimated 1 in 3,500 boys worldwide). Onset of DMD is between 3 and 5 years of age and progresses rapidly. Most boys become unable to walk by age 12 and by age 20 must use a respirator to breathe. BMD is similar to DMD except that it progresses at a much slower rate. Symptoms usually appear at about age 12, and the ability to walk is lost at an average age of 25 to 30. At the current time, there is no known cure for muscular dystrophies, and available treatment is aimed at controlling symptoms to maximize quality of life. Intensive research efforts are directed to implement gene therapy


Histology_Ross

Histology_Ross. FIGURE F11.2.1 • Distribution of dystrophin in human into treatment of affected patients. One method may lead skeletal muscle. This cross section of skeletal muscle fibers from a healthy individual was immunostained with cells. To achieve that objective, specially engineered forms goat polyclonal antibody against dystrophin using of viruses need to be develop that would carry “normal” immunoperoxydase method. Since dystrophin and associated dystrophin-glycoprotein complexes connect the muscle genes, infect muscle cells, and induce cells to express dyscytoskeleton to the surrounding extracellular matrix through trophin. The other method that might be tried is transplanthe cell membrane, the localization of dystrophin outlines cell tation of “healthy” satellite (muscle stem) cells that can membrane. Note a regular shape of skeletal muscle cells and divide and differentiate into normal muscle cells. Stem cell pattern of dystrophin distribution. 480. (Courtesy of therapy has been tested


Histology_Ross

Histology_Ross. membrane. Note a regular shape of skeletal muscle cells and divide and differentiate into normal muscle cells. Stem cell pattern of dystrophin distribution. 480. (Courtesy of therapy has been tested in laboratory animals and yielded Dr. Andrew G. Engel.) encouraging results.


Histology_Ross

Histology_Ross. ATP is absent. This arrangement is known as the rigor configuration. The muscular stiffening and rigidity that begins at the moment of death is caused by lack of ATP and is known as rigor mortis. In an actively contracting muscle, this step ends with the binding of ATP to the myosin head. Release is the second stage of the cycle; the myosin head is uncoupled from the thin filament. In this stage of the contraction cycle, ATP binds to the myosin head and induces conformational changes of the actin-binding site. This change reduces the affinity of the myosin head for the actin molecule of the thin filament, causing the myosin head to uncouple from the thin filament. STAGE 2: RELEASE Bending is the third stage of the cycle; the myosin head, as a result of hydrolysis of ATP, advances a short distance in relation to the thin filament.


Histology_Ross

Histology_Ross. STAGE 2: RELEASE Bending is the third stage of the cycle; the myosin head, as a result of hydrolysis of ATP, advances a short distance in relation to the thin filament. The ATP-binding site on the myosin head undergoes further conformational changes, causing the myosin head to bend. This movement is initiated by the breakdown of ATP into adenosine diphosphate (ADP) and inorganic phosphate; both products, however, remain bound to the myosin head. In this stage of the cycle, the linear displacement of the myosin head relative to the thin filament is approximately 5 nm. STAGE 3: BENDING Force generation is the fourth stage of the cycle; the myosin head releases inorganic phosphate and the power stroke occurs.


Histology_Ross

Histology_Ross. STAGE 3: BENDING Force generation is the fourth stage of the cycle; the myosin head releases inorganic phosphate and the power stroke occurs. The myosin head binds weakly to its new binding site on the neighboring actin molecule of the thin filament, causing release of the inorganic phosphate. This release has two effects. First, the binding affinity between the myosin head and its new attachment site increases. Second, the myosin head generates a force as it returns to its original unbent position. Thus, as the myosin head straightens, it forces movement of the thin filament along the thick filament. This is the “power stroke” of the cycle. During this stage, ADP is lost from the myosin head. STAGE 4: Reattachment is the fifth and last stage of the cycle; the myosin head binds tightly to a new actin molecule. The myosin head is again tightly bound to a new actin molecule of the thin filament (rigor configuration), and the cycle can repeat.


Histology_Ross

Histology_Ross. The myosin head is again tightly bound to a new actin molecule of the thin filament (rigor configuration), and the cycle can repeat. Although an individual myosin head may detach from the thin filament during the cycle, other myosin heads in the same thick filament will attach to actin molecules, thereby resulting in movement. Because the myosin heads are arranged as mirror images on either side of the H band (antiparallel arrangement), this action pulls the thin filaments into the A band, thus shortening the sarcomere. Regulation of contraction involves Ca2, sarcoplasmic reticulum, and the transverse tubular system. Ca2 must be available for the reaction between actin and myosin. After contraction, Ca2 must be removed. This rapid delivery and removal of Ca2 is accomplished by the combined work of the sarcoplasmic reticulum and the transverse tubular system.


Histology_Ross

Histology_Ross. The sarcoplasmic reticulum is arranged as a repeating series of networks around the myofibrils. Each network of the reticulum extends from one A–I junction to the next A–I junction within a sarcomere. The adjacent network of sarcoplasmic reticulum continues from the A–I junction to the next A–I junction of the neighboring sarcomere. Therefore, one network of sarcoplasmic reticulum surrounds the A band, and the adjacent network surrounds the I band (Fig. 11.8). Where the two networks meet, at the junction between A and I bands, the sarcoplasmic reticulum forms a slightly more regular ringlike channel called the terminal cisterna. The terminal cisternae serve as reservoirs for Ca2. To release Ca2 into the sarcoplasm, the plasma membrane of the terminal cisternae contains an abundance of gated Ca2-release channels. Also located around the myofibrils in association with the sarcoplasmic reticulum are large numbers of mitochondria and glycogen granules, both of which are involved in


Histology_Ross

Histology_Ross. of gated Ca2-release channels. Also located around the myofibrils in association with the sarcoplasmic reticulum are large numbers of mitochondria and glycogen granules, both of which are involved in providing the energy necessary for the reactions involved in contraction.


Histology_Ross

Histology_Ross. The transverse tubular system, or T system, consists of numerous tubular invaginations of the plasma membrane; each one is called a T tubule. T tubules penetrate to all levels of the muscle fiber and are located between adjacent terminal cisternae at the A–I junctions (see Fig. 11.8). They contain voltage-sensor proteins, depolarization-sensitive trans-membrane channels that are activated when the plasma


Histology_Ross

Histology_Ross. FIGURE 11.8 • Diagram of the organization of striated muscle fiber. This diagram illustrates the organization of the sarcoplasmic reticulum and its relationship to the myofibrils. Note that in striated muscle fibers, two transverse (T) tubules supply a sarcomere. Each T tubule is located at an A–I band junction and is formed as an invagination of the sarcolemma of striated muscle. It is associated with two terminal cisternae of the sarcoplasmic reticulum that surrounds each myofibril, one cisterna on either side of the T tubule. The triple structure as seen in cross section, where the two terminal cisternae flank a transverse tubule at the A–I band junction, is called a triad. Depolarization of the T tubule membrane initiates the release of calcium ions from the sarcoplasmic reticulum and eventually triggers muscle contraction.


Histology_Ross

Histology_Ross. membrane depolarizes. Conformational changes of these proteins directly affect gated Ca2-release channels located in the adjacent plasma membrane of the terminal cisternae. The complex of T tubule and the two adjacent terminal cisternae is called a triad. The depolarization of the T-tubule membrane triggers the release of Ca from the terminal cisternae to initiate muscle contraction.


Histology_Ross

Histology_Ross. When a nerve impulse arrives at the neuromuscular junction, the release of neurotransmitter (acetylcholine) from the nerve ending triggers a localized plasma-membrane depolarization of the muscle cell. The depolarization, in turn, causes voltage-gated Na channels in the plasma membrane to open, allowing an influx of Na from the extracellular space into the muscle cell. The influx of Na results in general depolarization, which spreads rapidly over the entire plasma membrane of the muscle fiber. When the depolarization encounters the opening of the T tubule, it is transmitted along the membranes of the T system into the depths of the cell. Electrical charges activate voltage-sensor proteins located in the membrane of the T tubule. These proteins have the structural and functional properties of Ca2 channels. During skeletal muscle depolarization, short activation of these sensors is not sufficient to open Ca2 channels. Thus, Ca2 transport from the lumen of the T tubule into the


Histology_Ross

Histology_Ross. of Ca2 channels. During skeletal muscle depolarization, short activation of these sensors is not sufficient to open Ca2 channels. Thus, Ca2 transport from the lumen of the T tubule into the sarcoplasm does not occur and is not essential to trigger the contraction cycle. Instead, activation of these sensors opens gated Ca2-release channels in adjacent terminal sacs of the sarcoplasmic reticulum, causing the rapid release of Ca2 into the sarcoplasm. The increased concentration of Ca2 in the sarcoplasm initiates contraction of the myofibril by binding to the TnC portion of the troponin complex on the thin filaments (see page 316). The change in molecular conformation of TnC causes the TnI to dissociate from the actin molecules, allowing the troponin complex to uncover myosin-binding sites on the actin molecules. The myosin heads are now free to interact with actin molecules to initiate the muscle contraction cycle.


Histology_Ross

Histology_Ross. Simultaneously, a Ca2-activated ATPase pump in the membrane of the sarcoplasmic reticulum transports Ca2 back into the terminal cisternae. The resting concentration of Ca2 is restored in the cytosol in less than 30 milliseconds. This restoration of resting Ca2 concentration near the myo filaments normally causes contraction to stop. Contraction will continue, however, as long as nerve impulses continue to depolarize the plasma membrane of the T tubules. Skeletal muscle fibers are richly innervated by motor neurons that originate in the spinal cord or brainstem. The axons of the neurons branch as they near the muscle, giving rise to twigs or terminal branches that end on individual muscle fibers (Fig. 11.9). FIGURE 11.9 • Photomicrograph of neuromuscular junction.


Histology_Ross

Histology_Ross. FIGURE 11.9 • Photomicrograph of neuromuscular junction. This silver preparation shows a motor nerve and its final branches that lead to the neuromuscular junctions (motor end plates). The skeletal muscle fibers are oriented horizontally in the field and are crossed perpendicularly by the motor nerve fibers. Note that these fibers distally lose their myelin sheath and divide extensively into small swellings, forming a cluster of neuromuscular junctions. 620. The neuromuscular junction is the contact made by the terminal branches of the axon with the muscle fiber.


Histology_Ross

Histology_Ross. The neuromuscular junction is the contact made by the terminal branches of the axon with the muscle fiber. At the neuromuscular junction (motor end plate), the myelin sheath of the axon ends, and the terminal portion of the axon is covered by only a thin portion of the neurilemmal (Schwann) cell with its external lamina. The end of the axon ramifies into a number of end branches, each of which lies in a shallow depression on the surface of the muscle fiber, the receptor region (Fig. 11.10). The axon ending is a typical presynaptic structure and contains numerous mitochondria and synaptic vesicles that contain the neurotransmitter acetylcholine (ACh). Release of acetylcholine into the synaptic cleft initiates depolarization of the plasma membrane, which leads to muscle cell contraction.


Histology_Ross

Histology_Ross. Release of acetylcholine into the synaptic cleft initiates depolarization of the plasma membrane, which leads to muscle cell contraction. The muscle fiber plasma membrane that underlies the synaptic cleft has many deep junctional folds (subneural folds). Specific cholinergic receptors for ACh are limited to the plasma membrane immediately bordering the cleft and at the  FOLDER 11.3 Functional Considerations: The Sliding


Histology_Ross

Histology_Ross. The sliding filament model postulates that the ratchetlike movements of the myosin heads bound to actin produce the movement of the thin filaments relative to the thick fil-aments, which in turn causes the sarcomere to shorten. Although the sliding filament model can explain contrac-tion in a single sarcomere, it cannot adequately explain the shortening of a myofibril of a muscle fiber. Obviously, if the activity just described were to occur simultaneously in adjacent sarcomeres, no contraction could occur. Equal and opposite forces would be exerted on either side of the Z line, and the contraction of any given sar-comere would be prevented by the contraction of its two immediate serial neighbors. Recent studies with ultrahigh-speed photography have demonstrated that an extremely small temporal delay occurs between the contraction of adjacent sarcomeres, so that a wavelike contraction actu-ally occurs in each muscle fibril and, consequently, in each muscle fiber.


Histology_Ross

Histology_Ross. FIGURE 11.10 • Neuromuscular junction. a. Diagram of a neuromuscular junction. An axon is shown making contact with a muscle cell. Note how the junctional folds of the muscle cell augment the surface area within the synaptic cleft. The external lamina extends throughout the cleft area. The cytoplasm of the Schwann cell is shown covering the axon terminal. (Modified from Kelly DE, Wood RL, Enders AC, eds. Bailey’s Textbook of Microscopic Anatomy. Baltimore: Williams & Wilkins, 1984.) b. Electron micrograph of a neuromuscular junction shows the axon ending within the synaptic cleft of a skeletal muscle fiber. An aggregation of mitochondria (M) and numerous synaptic vesicles (SV ) is visible. The portion of the motor axon ending that is not in apposition to the muscle fiber is covered by Schwann cell cytoplasm (S), but no myelin is present. The muscle fiber shows the junctional folds (JF ) and the subneural clefts (SnC) between them. The external lamina of the muscle fiber is barely


Histology_Ross

Histology_Ross. Schwann cell cytoplasm (S), but no myelin is present. The muscle fiber shows the junctional folds (JF ) and the subneural clefts (SnC) between them. The external lamina of the muscle fiber is barely evident within the subneural clefts. Other structures present are the aggregated mitochondria of the muscle fiber (M) in the region of the neuromuscular junction, the nucleus (N) of the muscle fiber, and some myofibrils (MF ). 32,000. (Courtesy of Dr. George D. Pappas.) top of the folds. The external lamina extends into the subneural folds (see Fig. 11.10). The synaptic vesicles of the axon terminal release ACh into the cleft, which then binds to nicotinic ACh receptors on the sarcolemma of the striated muscle. The nicotinic ACh receptor in striated muscles is a transmitter-gated Na channel. Binding of ACh opens


Histology_Ross

Histology_Ross. ACh Na channels, causing an influx of Na into striated muscle Na receptor cell. This influx results in a localized membrane depolariza tion, which in turn leads to the events described above. An enzyme called acetylcholinesterase (AChE) quickly breaks down the acetylcholine to prevent continued stimulation. For more detail description of ACh function, see Chapter 12. The muscle fiber cytoplasm that underlies the junctional folds contains nuclei, many mitochondria, rough-surfaced endoplasmic reticulum (rER), free ribosomes, and glycogen. These cytoplasmic organelles are believed to be involved in the synthesis of specific acetylcholine receptors in the membrane of the cleft, as well as acetylcholinesterase. A neuron along with the specific muscle fibers that it in- nervates is called a motor unit. before depolarization depolarization


Histology_Ross

Histology_Ross. A neuron along with the specific muscle fibers that it in- nervates is called a motor unit. before depolarization depolarization A single neuron may innervate several to a hundred or more muscle fibers. Muscles capable of the most delicate movements have the fewest muscle fibers per motor neuron in their motor units. For example, in eye muscles, the innervation ratio is about one neuron to three muscle fibers; in the postural muscles of the back, a single neuron may innervate hundreds of muscle fibers. The nature of muscle contraction is determined by the number of motor neuron endings as well as by the number of specific types of muscle fibers that are depolarized. Al-voltage sensor though depolarization of a muscle fiber at a single neuromuscular junction is characterized as an “all-or-none” phenomenon, not all nerve terminals discharge at once, FIGURE 11.11 • Summary of events leading to contraction of which allows a graded response to the contractile stimulus.


Histology_Ross

Histology_Ross. FIGURE 11.11 • Summary of events leading to contraction of which allows a graded response to the contractile stimulus. skeletal muscle. See the text for a description of the events Innervation is necessary for muscle cells to maintain their structural integrity. The motor nerve cell not only instructs the muscle cells to contract but also exerts a trophic influence on the muscle cells. If the nerve supply to a muscle is disrupted, the muscle cell undergoes regressive changes known as tissue atrophy. The most conspicuous indication of this atrophy is thinning of the muscle and its cells. If innervation is reestablished surgically or by the slower process of natural regeneration of the nerve, the muscle can regain normal shape and strength. The events leading to contraction of skeletal muscle can be summarized as a series of steps. The events involved in contraction can be summarized as follows (the numbers refer to the numbers in Fig. 11.11): 1.


Histology_Ross

Histology_Ross. The events involved in contraction can be summarized as follows (the numbers refer to the numbers in Fig. 11.11): 1. The contraction of a skeletal muscle fiber is initiated when a nerve impulse traveling along the axon of a motor neuron arrives at the neuromuscular junction. 2. The nerve impulse prompts the release of acetylcholine into the synaptic cleft that binds into ACh-gated Na channels causing local depolarization of sarcolemma. 3. Voltage-gated Na channels open, and Na enters the cell. indicated by the numerals. ACh, acetylcholine. 4. General depolarization spreads over the plasma membrane of the muscle cell and continues via membranes of the T tubules. 5. Voltage sensor proteins in the plasma membrane of T tubules change their conformation. 6. At the muscle cell triads, the T tubules are in close contact with the lateral enlargements of the sarcoplasmic reticulum, where gated Ca2-release channels are activated by conformational changes of voltage-sensor proteins.


Histology_Ross

Histology_Ross. 7. Ca2 is rapidly released from the sarcoplasmic reticulum into the sarcoplasm. 8. Ca2 binds to the TnC portion of the troponin complex. 9. The contraction cycle is initiated, and Ca2 is returned to the terminal cisternae of the sarcoplasmic reticulum. Encapsulated sensory receptors in muscles and tendons are examples of proprioreceptors. These receptors are part of  FOLDER 11.4 Clinical Correlation: Myasthenia Gravis


Histology_Ross

Histology_Ross. During normal function, acetylcholine (ACh) molecules re-leased to the synaptic cleft at the neuromuscular junction bind to the nicotinic ACh receptors on the sarcolemma of the skeletal muscle cell. As discussed earlier in the text, these re-ceptors represent transmitter-gated Nachannels that con-trol the influx of Nanecessary for generating an action potential leading to initiation of muscle contraction. After stimulating their own receptors, ACh molecules are quickly degraded by the enzyme acetylcholinesterase (AChE) into acetic acid and choline, which is taken up by the axon termi-nal and reused for ACh synthesis (see page 363). In a clinical condition called myasthenia gravis, the ACh nicotinic receptors are blocked by antibodies directed to the body’s own receptor protein. Thus, myasthenia gravis is an autoimmune disease caused by the reduced number of functional ACh receptor sites. In addition, other abnormalities within the synaptic cleft (e.g., widening of the synaptic cleft,


Histology_Ross

Histology_Ross. gravis is an autoimmune disease caused by the reduced number of functional ACh receptor sites. In addition, other abnormalities within the synaptic cleft (e.g., widening of the synaptic cleft, disappearance of junctional folds) also occur, further reducing the effectiveness of the muscle fibers. Myasthenia gravis is characterized by noticeable weakening of the muscle fiber response to the nerve stim-ulus. Initially, weakness begins with extraocular muscles, drooping eyelids, double vision, and generalized muscular weakness. Other somatic musculature might be affected, including respiratory muscles. As the disease progresses, the number of neuromuscular junctions is reduced. An effective pharmacologic treatment for myasthenia gravis is administration of AChE inhibitors. These substances rein-force neuromuscular transmission by extending the life of released ACh within the synaptic cleft. In addition to AChE inhibitors, the immunosuppressive treatment and resection of the enlarged


Histology_Ross

Histology_Ross. rein-force neuromuscular transmission by extending the life of released ACh within the synaptic cleft. In addition to AChE inhibitors, the immunosuppressive treatment and resection of the enlarged thymus (if present) are used to slow down the activity of the immune system and rate of production of antibodies against ACh receptors.


Histology_Ross

Histology_Ross. the somatic sensory system that provides information about the degree of stretching and tension in a muscle. Proprioreceptors inform the central nerve system about the body’s position and movement in space. The muscle spindle is the specialized stretch receptor located within the skeletal muscle.


Histology_Ross

Histology_Ross. The muscle spindle is a specialized stretch receptor in muscle; it consists of two types of modified muscle fibers called spindle cells and neuron terminals (Fig. 11.12). Both types of modified muscle fibers are surrounded by an internal capsule. A fluid-filled space separates the internal capsule from an outer external capsule. One type of spindle cell, the nuclear bag fiber, contains an aggregation of nuclei in an expanded midregion; the other type, called a nuclear chain fiber, has many nuclei arranged in a chain. The muscle spindle transmits information about the degree of stretching in a muscle. The sensory (afferent, Ia) nerve fibers that carry information from the muscle spindle have endings that are spirally arranged around the midregion of both types of spindle cells. In addition, spindle cells receive motor (efferent) innervation from the spinal cord and brain via motor (efferent) nerve fibers, which are thought to regulate the sensitivity of the stretch receptor. When


Histology_Ross

Histology_Ross. spindle cells receive motor (efferent) innervation from the spinal cord and brain via motor (efferent) nerve fibers, which are thought to regulate the sensitivity of the stretch receptor. When skeletal muscle is stretched, nerve endings of sensory nerves become activated. They convey their impulses to the central nervous system, which in turn modulates the activity of motor neurons innervating that particular muscle.


Histology_Ross

Histology_Ross. Recent real-time studies with computed tomography (CT) scans of living muscle in different states of contraction suggest that muscle spindles may also represent the axes of functional units within large skeletal muscles. Such functional units precisely regulate contractions of portions of the muscle by creating “fixation points” within the muscle substance. Similar encapsulated receptors, Golgi tendon organs, are found in the tendons of muscle and respond to increased tension on the muscle. These receptors contain only sensory (afferent, Ib) nerve fibers, and they monitor muscle tension (or the force of contraction) within an optimal range. Development, Repair, Healing, and Renewal Development of myogenic stem cell linage depends on expression of various myogenic regulatory factors.


Histology_Ross

Histology_Ross. Myoblasts are derived from a self-renewing population of multipotential myogenic stem cells that originate in the embryo from unsegmented paraxial mesoderm (cranial muscle progenitors) or segmented mesoderm of somites (epaxial and hypaxial muscle progenitors). Early in embryonic development, these cells express MyoD transcription factor, which, along with other myogenic regulatory factors (MRFs), plays a key role in activation of muscle-specific gene expressions and differentiation of all skeletal muscle lineages. A balancing effect on skeletal-muscle development is achieved by the expression of negative regulatory myostatin gene, which leads to synthesis of myostatin, a 26-kilodalton protein belonging to the bone morphogenetic protein/transforming growth factor(BMP/TGF-) protein superfamily. Myostatin exerts an inhibitory effect on muscle growth and differentiation. It is thought that MyoD preferentially upregulates myostatin gene expression and controls myogenesis during not only


Histology_Ross

Histology_Ross. Myostatin exerts an inhibitory effect on muscle growth and differentiation. It is thought that MyoD preferentially upregulates myostatin gene expression and controls myogenesis during not only the embryonic and fetal periods but also postnatal stages of the development. The hypermuscular phenotypes observed on inactivation of the myostatin gene in animals and humans have confirmed the role of myostatin as a negative regulator of skeletal-muscle development. Experimental studies have demonstrated that muscle mass increases through myostatin inhibition, and the myostatin signaling


Histology_Ross

Histology_Ross. FIGURE 11.12 • Muscle spindle. a. Schematic diagram of a muscle spindle. The diameter of the spindle is expanded to illustrate structural details. Each spindle contains approximately two to four nuclear bag fibers and six to eight nuclear chain fibers. In the nuclear bag fibers, the muscle fiber nuclei are clumped in the expanded central portion of the fiber, hence the name bag. In contrast, the nuclei concentrated in the central portion of the nuclear chain fibers are arranged in a chain. Both afferent Ia (sensory) and efferent (motor) nerve fibers supply muscle spindle cells. The afferent nerve fibers respond to excessive stretching of the muscle, which in turn inhibits the somatic motor stimulation of the muscle. The efferent nerve fibers regulate the sensitivity of the afferent endings in the muscle spindle. b. Photomicrograph of a cross section of a muscle spindle, showing two bundles of spindle cells in the encapsulated, fluid-filled receptor. In one bundle, several of the


Histology_Ross

Histology_Ross. in the muscle spindle. b. Photomicrograph of a cross section of a muscle spindle, showing two bundles of spindle cells in the encapsulated, fluid-filled receptor. In one bundle, several of the spindle cells are cut at the level that reveals their nuclei. An internal capsule surrounds the spindle cells. The external capsule of the muscle spindle and the adjacent perimysium can be seen as a faint double-layer boundary of the receptor. Immediately above and outside of the muscle spindle is a nerve that may be supplying the spindle. The several types of nerves associated with the spindle cells as well as the type of spindle cells cannot be distinguished in this H&E–stained section. Near one of the bundles of spindle cells is a small blood vessel. The flocculent material within the capsule consists of precipitated proteoglycans and glycoproteins from the fluid that filled the spindle before fixation. 550.


Histology_Ross

Histology_Ross. pathway may be a potent therapeutic intervention point in the treatment of muscle-wasting diseases, such as muscular dystrophy, amyotrophic lateral sclerosis (ALS), AIDS, and cancer. Pharmacologic manipulation of myostatin expression could also lead to the development of new therapeutic approaches in a variety of musculoskeletal pathologies. Skeletal muscle progenitors differentiate into early and late myoblasts. Developing muscle contains two types of myoblasts:  Early myoblasts are responsible for the formation of primary myotubes, chainlike structures that extend between tendons of the developing muscle. Primary myotubes are formed by nearly synchronous fusion of early myoblasts. Myotubes undergo further differentiation into mature skeletal muscle fibers. Primary myotubes observed in the light microscope exhibit a chain of multiple central nuclei surrounded by myofilaments.


Histology_Ross

Histology_Ross.  Late myoblasts give rise to secondary myotubes, which are formed in the innervated zone of developing muscle where the myotubes have direct contact with nerve terminals. Secondary myotubes continue to be formed by sequential fusion of myoblasts into the already-formed secondary myotubes at random positions along their length. Secondary myotubes are characterized by a smaller diameter, more widely spaced nuclei, and an increased number of myofilaments (Fig. 11.13). In the mature multinucleated muscle fiber, the nuclei are all in the peripheral sarcoplasm, just inside the plasma membrane. Some nuclei that appear to belong to the skeletal muscle fiber are nuclei of satellite cells.


Histology_Ross

Histology_Ross. Some nuclei that appear to belong to the skeletal muscle fiber are nuclei of satellite cells. FIGURE 11.13 • Photomicrograph of developing skeletal muscle myotubes. This photomicrograph shows a cross section (on the left) and a longitudinal section (on the right) of developing skeletal muscle fibers in the stage of secondary myotubes. These myotubes are formed by sequential fusion of myoblasts, forming elongated tubular structures. Note that the myotubes have a small diameter and widely spaced, centrally positioned nuclei that gradually become displaced into the cell periphery by the increased number of newly synthesized myofilaments. In the mature multi-nucleated muscle fiber (upper left), all nuclei are positioned in the peripheral sarcoplasm, just inside the plasma cell membrane. 220.


Histology_Ross

Histology_Ross. Satellite cells are interposed between the plasma membrane of the muscle fiber and its external lamina. They are small cells with scant cytoplasm. The cytoplasm typically blends in with the muscle cell sarcoplasm when viewed in the light microscope, thus making them difficult to identify. Each satellite cell has a single nucleus with a chromatin network denser and coarser than that of muscle cell nuclei. Satellite cells are responsible for the skeletal muscle’s ability to regenerate, but their regenerative capacity is limited. These myogenic precursors of muscle cells are normally quiescent and do not express myogenic regulatory factors. However, after injury of muscle tissue, some satellite cells become activated, reenter the cell cycle, and begin to express MRFs. They proliferate and give rise to new myoblasts. As long as the external lamina remains intact, the myoblasts fuse within the external lamina to form myotubes, which then mature into a new fiber. In contrast, if the


Histology_Ross

Histology_Ross. and give rise to new myoblasts. As long as the external lamina remains intact, the myoblasts fuse within the external lamina to form myotubes, which then mature into a new fiber. In contrast, if the external lamina is disrupted, fibroblasts repair the injured site, with subsequent scar tissue formation.


Histology_Ross

Histology_Ross. Muscular dystrophies are characterized by progressive degeneration of skeletal muscle fibers, which places a constant demand on the satellite cells to replace the degenerated fibers. Ultimately, the satellite cell pool is exhausted. New experimental data indicate that, during this process, additional myogenic cells are recruited from the bone marrow and supplement the available satellite cells. The rate of degeneration exceeds the rate of regeneration, however, resulting in loss of muscle function. A future treatment strategy for muscular dystrophies may include the trans-plantation of satellite cells or their myogenic bone marrow counterparts into damaged muscle. FIGURE 11.14 • Photomicrograph of longitudinally sectioned cardiac muscle. The arrows point to the intercalated discs. The disc represent specialized cell-to-cell attachments of the cardiac muscle cells. Also note the apparent branching of the muscle fibers. 360.


Histology_Ross

Histology_Ross. Cardiac muscle has the same types and arrangement of contractile filaments as skeletal muscle. Therefore, cardiac muscle cells and the fibers they form exhibit cross-striations evident in routine histologic sections. In addition, cardiac muscle fibers exhibit densely staining cross-bands, called intercalated discs, that cross the fibers in a linear fashion or frequently in a way that resembles the risers of a stairway (Fig. 11.14 and Plate 24, page 346). The intercalated discs represent highly specialized attachment sites between adjacent cells. This linear cell-to-cell attachment of the cardiac muscle cells results in “fibers” of variable length. Thus, unlike skeletal and visceral striated muscle fibers that represent multinucleated single cells, cardiac muscle fibers consist of numerous cylindrical cells arranged end to end. Furthermore, some cardiac muscle cells in a fiber may join with two or more cells through intercalated discs, thus creating a branched fiber.


Histology_Ross

Histology_Ross. terminal cisterna of sarcoplasmic reticulum FIGURE 11.15 • Diagram of the organization of cardiac muscle fiber. The T tubules of cardiac muscle are much larger than the T tubules of skeletal muscle and carry an investment of external lamina material into the cell. They also differ in that they are located at the level of the Z disc. The portion of the sarcoplasmic reticulum adjacent to the T tubule is not in the form of an expanded cisterna but rather is organized as an anastomosing network. Structure of Cardiac Muscle The cardiac muscle nucleus lies in the center of the cell.


Histology_Ross

Histology_Ross. The central location of the nucleus in cardiac muscle cells is one feature that helps distinguish them from multinucleated skeletal muscle fibers, whose nuclei lie immediately under the plasma membrane. The transmission electron microscope (TEM) reveals that the myofibrils of cardiac muscle separate to pass around the nucleus, thus outlining a biconical juxtanuclear region in which the cell organelles are concentrated. This region is rich in mitochondria and contains the Golgi apparatus, lipofuscin pigment granules, and glycogen. In the atria of the heart, atrial granules measuring 0.3 to 0.4 m in diameter are also concentrated in the juxtanuclear cytoplasm. These granules contain two polypeptide hormones: atrial natriuretic factor (ANF) [L. natrium, sodium] and brain natriuretic factor (BNF). Both hormones are diuretics, affecting urinary excretion of sodium. They inhibit renin secretion by the kidney and aldosterone secretion by the adrenal gland. They also inhibit contractions of


Histology_Ross

Histology_Ross. (BNF). Both hormones are diuretics, affecting urinary excretion of sodium. They inhibit renin secretion by the kidney and aldosterone secretion by the adrenal gland. They also inhibit contractions of vascular smooth muscle. In congestive heart failure, levels of circulating BNF increase.


Histology_Ross

Histology_Ross. Numerous large mitochondria and glycogen stores are adjacent to each myofibril. In addition to the juxtanuclear mitochondria, cardiac muscle cells are characterized by large mitochondria that are densely packed between the myofibrils. These large mitochondria often extend the full length of a sarcomere and contain numerous, closely packed cristae (Fig. 11.15). Concentrations of glycogen granules are also located between the myofibrils. Thus, the structures that store energy (glycogen granules) and the structures that release and recapture energy (mitochondria) are located adjacent to the structures (myofibrils) that use the energy to drive contraction. The intercalated discs represent junctions between cardiac muscle cells.


Histology_Ross

Histology_Ross. As previously noted, the intercalated disc represents the attachment site between cardiac muscle cells. In the light microscope, the disc appears as a densely staining linear structure that is oriented transversely to the muscle fiber. Often it consists of short segments arranged in a steplike fashion (Fig. 11.16). When the site of the intercalated disc is examined with the TEM, the densely staining structure seen in the light microscope can be attributed to the presence of a transverse component that crosses the fibers at a right angle to the myofibrils. The transverse component is analogous to the risers of the stairway. A lateral component (not visible in the light microscope) occupies a series of surfaces perpendicular to the transverse component and lies parallel to the myofibrils. The lateral component is analogous to the steps of the stairway. Both components of the intercalated disc contain specialized cell-to-cell junctions between adjoining cardiac muscle cells:  Fascia


Histology_Ross

Histology_Ross. The lateral component is analogous to the steps of the stairway. Both components of the intercalated disc contain specialized cell-to-cell junctions between adjoining cardiac muscle cells:  Fascia adherens (adhering junction) is the major constituent of the transverse component of the intercalated disc and is responsible for its staining in routine H&E


Histology_Ross

Histology_Ross. FIGURE 11.16 • Structure of cardiac muscle fiber. a. This scanning electron micrograph shows the heart muscle tissue preparation obtained from the monkey right ventricle. The sample was ultrasonicated within the sodium hydroxide that resulted in the digestion of collagen fibers and separation of cardiac myocytes at the intercalated discs. Note the branching pattern of myocytes and clearly visible transverse and lateral components of intercalated disc. 32,000. b. Three-dimensional drawing of an intercalated disc, which represents a highly specialized attachment site between adjacent cardiac muscle cells. The intercalated disc is composed of the transverse component (blue area) that crosses the fibers at a right angle to the myofibrils (analogous to the risers of a stairway) and a lateral component (pink area) that occupies a series of surfaces perpendicular to the transverse component and parallel to the myofibrils (analogous to the steps of a stairway). The fascia adherens is the


Histology_Ross

Histology_Ross. component (pink area) that occupies a series of surfaces perpendicular to the transverse component and parallel to the myofibrils (analogous to the steps of a stairway). The fascia adherens is the major constituent of the transverse component. It holds the cardiac muscle cells at their ends and serves as the attachment site for thin filaments. The maculae adherentes reinforce the fascia adherens and are also found in the lateral components. The gap junctions are found only in the lateral component of the intercalated disc. c. This electron micrograph reveals portions of two cardiac muscle cells joined by an intercalated disc. The line of junction between the two cells takes an irregular, steplike course, making a number of nearly right-angle turns. In its course, different parts of the intercalated disc are evident. These include the transverse components (fascia adherens and maculae adherentes) and lateral components (gap junctions and maculae adherentes). The macula adherens (MA) is


Histology_Ross

Histology_Ross. disc are evident. These include the transverse components (fascia adherens and maculae adherentes) and lateral components (gap junctions and maculae adherentes). The macula adherens (MA) is enlarged in inset 1 (62,000). The fascia adherens (FA) is more extensive than the macula adherens, being disposed in a larger area of irregular outline. The fascia adherens is enlarged in inset 3 (62,000). The fascia adherens of the intercalated disc corresponds to the zonula adherens of other tissues. The gap junction (GJ) is enlarged in inset 2 (62,000). Other features typical of cardiac muscle are also present: mitochondria (Mi), sarcoplasmic reticulum (SR), and components of the sarcomere, including Z lines (Z), M line (M), and myofilaments. This particular specimen is in a highly contracted state, and consequently, the I band is practically obscured. 30,000. (Part A reprinted with permission from Zhang L, Ina K, Kitamura H, Campbell GR, Shimada T. The intercalated discs of monkey myocardial


Histology_Ross

Histology_Ross. and consequently, the I band is practically obscured. 30,000. (Part A reprinted with permission from Zhang L, Ina K, Kitamura H, Campbell GR, Shimada T. The intercalated discs of monkey myocardial cells and Purkinje fibers as revealed by scanning electron microscopy. Arch Histol Cytol 1996;59:453–465.) preparations. It holds the cardiac muscle cells at their ends to form the functional cardiac muscle fiber (see Fig. 5.20, page 131). It always appears as a transverse boundary between the cardiac muscle cells. The TEM reveals an intercellular space between the adjacent cells that is filled with electron-dense material resembling the material found in the zonula adherens of epithelia. The fascia adherens serves as the site at which the thin filaments in the terminal sarcomere anchor onto the plasma membrane. In this way, the fascia adherens is functionally similar to the epithelial zonula adherens, where actin filaments of the terminal web are also anchored.


Histology_Ross

Histology_Ross.  Maculae adherentes (desmosomes) bind the individual muscle cells to one another. Maculae adherentes help prevent the cells from pulling apart under the strain of regular repetitive contractions. They reinforce the fascia adherens and are found in both the transverse and lateral components of the intercalated discs.  Gap junctions (communicating junctions) constitute the major structural element of the lateral component of the intercalated disc. Gap junctions provide ionic continuity between adjacent cardiac muscle cells, thus allowing informational macromolecules to pass from cell to cell. This exchange permits cardiac muscle fibers to behave as a syncytium while retaining cellular integrity and individuality. The position of the gap junctions on the lateral surfaces of the intercalated disc protects them from the forces generated during contraction. The sER in cardiac muscle cells is organized into a single network along the sarcomere, extending from Z line to Z line.


Histology_Ross

Histology_Ross. The sER in cardiac muscle cells is organized into a single network along the sarcomere, extending from Z line to Z line. The sER of cardiac muscle is not as well organized as that of skeletal muscle. It does not separate bundles of myofilaments into discrete myofibrils. The T tubules in cardiac muscle penetrate into the myofilament bundles at the level of the Z line, between the ends of the sER network. Thus, there is only one T tubule per sarcomere in cardiac muscle. Small terminal cisternae of the sER are in close proximity to the T tubules to form a diad at the level of the Z line (see Fig. 11.15). The external lamina adheres to the invaginated plasma membrane of the T tubule as it penetrates into the cytoplasm of the muscle cell. The T tubules are larger and more numerous in cardiac ventricular muscle than in skeletal muscle. They are less numerous, however, in cardiac atrial muscle.


Histology_Ross

Histology_Ross. Passage of Ca2 from the lumen of the T tubule to the sarcoplasm of a cardiac muscle cell is essential to initiate the contraction cycle. As discussed in the section on skeletal muscle, depolarization of the T tubule membrane activates voltage-sensor proteins, which are similar in structure and function to Ca2 channels. In contrast to skeletal muscle, long-lasting depolarization in cardiac muscle activates these sensors and prompts their slow conformation change into functional Ca2 channels (Fig. 11.17). Thus, in the first stage of the cardiac muscle contraction cycle, Ca2 from the lumen of the T tubule is transported to the sarcoplasm of cardiac muscle, which opens gated Ca2-release channels in adjacent terminal sacs of the FIGURE 11.17 • Movement of the calcium ions after depola rization of the plasma membrane in cardiac muscle.


Histology_Ross

Histology_Ross. FIGURE 11.17 • Movement of the calcium ions after depola rization of the plasma membrane in cardiac muscle. Depolarization of the T tubule membrane activates voltage-sensor proteins that function as Ca2 channels. Initially, Ca2 is transported from the lumen of the T tubule through channels in voltage-sensor proteins into the sarcoplasm of the cardiac muscle (illustrated next to the upper terminal sac of the sER). Next, Ca2 activates gated Ca2-release channels in adjacent terminal sacs of the sarcoplasmic reticulum. This causes the massive release of sequestrated Ca2 from the sER into the sarcoplasm and initiates the contraction cycle.


Histology_Ross

Histology_Ross. sarcoplasmic reticulum. This calcium-triggered calcium release mechanism causes a massive and rapid release of additional Ca2 that initiates subsequent steps of the contraction cycle, which are identical to those in skeletal muscle. The differences between initiation of cardiac and skeletal muscle contractions—the longer-lasting membrane depolarization and activation of voltage-sensitive Ca2 channels in the wall of the T tubule—account for an approximately 200-millisecond delay from the start of a depolarization in a cardiac muscle twitch (see Fig. 11.11). Cardiac muscle cells exhibit a spontaneous rhythmic contraction.


Histology_Ross

Histology_Ross. The intrinsic spontaneous contraction or beat of cardiac muscle is evident in embryonic cardiac muscle cells as well as in cardiac muscle cells in tissue culture. The heartbeat is initiated, locally regulated, and coordinated by specialized, modified cardiac muscle cells called cardiac conducting cells (Plate 25, page 348). These cells are organized into nodes and highly specialized conducting fibers called Purkinje fibers that generate and rapidly transmit the contractile impulse to various parts of the myocardium in a precise sequence. Both parasympathetic and sympathetic nerve fibers terminate in the nodes. Sympathetic stimulation accelerates the heartbeat by increasing the frequency of impulses to the cardiac conducting cells. Parasympathetic stimulation slows down the heartbeat by decreasing the frequency of the impulses. The impulses carried by these nerves do not initiate contraction but only modify the rate of intrinsic cardiac muscle contraction by their effect at the nodes.


Histology_Ross

Histology_Ross. the frequency of the impulses. The impulses carried by these nerves do not initiate contraction but only modify the rate of intrinsic cardiac muscle contraction by their effect at the nodes. The structure and functions of the conducting system of the heart are described in Chapter 13, Cardiovascular System.


Histology_Ross

Histology_Ross. A localized injury to cardiac muscle tissue that results in the death of cells is repaired by replacement with fibrous connective tissue. Consequently, cardiac function is lost at the site of injury. This pattern of injury and repair is seen in nonfatal myocardial infarction (MI). Confirmation of suspected MI in the individual can be made through the detection of specific markers in the blood. These markers are the structural subunits TnI and TnT of the cardiac troponin complex. They are usually released into the bloodstream within 3 to 12 hours after an MI. TnI levels remain elevated for up to 2 weeks from the time of the initial injury; thus, it is regarded as an excellent marker for diagnosing MI that has recently occurred. Mature cardiac muscle cells are able to divide.


Histology_Ross

Histology_Ross. Mature cardiac muscle cells are able to divide. In the past, it was thought that once cardiac muscle cells are destroyed, they cannot be replaced by new muscle cells. Recent studies of hearts removed from individuals who had received transplants reveal nuclei undergoing mitosis. Although the number of dividing nuclei in these hearts is low (0.1%), it suggests that damaged cells can potentially be replaced. Perhaps in the future, a method might be developed that could induce human cardiac muscle to regenerate into healthy tissue.


Histology_Ross

Histology_Ross. Smooth muscle generally occurs as bundles or sheets of elongated fusiform cells with finely tapered ends (Fig. 11.18 and Plate 26, page 350). The cells, also called fibers, range in length from 20 m in the walls of small blood vessels to about 200 m in the wall of the intestine; they may be as large as 500 m in the wall of the uterus during pregnancy. Smooth muscle cells are interconnected by gap junctions, the specialized communication junctions between the cells (Fig. 11.19). Small molecules or ions can pass from cell to cell via these junctions and provide communication links that regulate contraction of the entire bundle or sheet of smooth muscle.


Histology_Ross

Histology_Ross. Smooth muscle cytoplasm stains rather evenly with eosin in routine H&E preparations because of the concentrations of actin and myosin that these cells contain. The nuclei of smooth muscle cells are located in the center of the cell and often have a corkscrew appearance in longitudinal section. This characteristic is a result of contraction of the cell during fixation and is often useful in distinguishing smooth muscle cells from fibroblasts in routine histologic sections. In the noncontracted cell, the nucleus appears as an elongated structure with tapering ends, lying in the center axis of the cell. When the nucleus is included in a cross section of a smooth muscle fiber, it appears as a round or circular profile whether the cell is contracted or relaxed. The TEM shows that most of


Histology_Ross

Histology_Ross. FIGURE 11.18 • Photomicrograph of smooth muscle from a human colon. The smooth muscle shown in this micrograph is arranged in two layers. On the left, the muscle cells are cut in longitudinal section; on the right, they are cut in cross section. Smooth muscle cells are elongate and have tapering ends. Note that the nuclei in the longitudinally sectioned muscle cells appear elongate and also exhibit tapering ends, thus matching the shape of the cell. In contrast, the nuclei in the cross-sectioned muscle cells are circular in profile. Also, some of the cross-sectioned cells appear to lack a nucleus, a reflection that the section passed through one of the ends of the cell. Also note that the longitudinally sectioned muscle cells are not easily delineated from one another, which is on account of the way they lie over one another within the thickness of the section. 400.


Histology_Ross

Histology_Ross. the cytoplasmic organelles are concentrated at each end of the nucleus. These include numerous mitochondria, some cisternae of the rER, free ribosomes, glycogen granules, and a small Golgi apparatus. Structure of Smooth Muscle Smooth muscle cells possess a contractile apparatus of thin and thick filaments and a cytoskeleton of desmin and vimentin intermediate filaments. The remaining sarcoplasm is filled with thin filaments that form a part of the contractile apparatus. Thick myosin filaments are scattered throughout the sarcoplasm of a smooth muscle cell. They are extremely labile and tend to be lost during tissue preparation. Special techniques can be used, however, to retain the structural integrity of the thick filaments and thus demonstrate them with the TEM. The thin FIGURE 11.19 • Electron micrograph of smooth muscle cells.


Histology_Ross

Histology_Ross. FIGURE 11.19 • Electron micrograph of smooth muscle cells. This electron micrograph shows parts of three smooth muscle cells. The nucleus of one cell is in the lower part of the micrograph. The bulk of the cytoplasm is occupied by thin (actin) filaments, which are just recognizable at this magnification. The -actinin–containing cytoplasmic densities, or dense bodies, are visible among the myofilaments (arrows). Elements of the sarcoplasmic reticulum (SR) and the pinocytotic vesicles (PV ) are also indicated. The other two cells in the middle and upper part of the micrograph possess visible gap junctions (GJ) that allow communication between adjacent cells. The small dark particles are glycogen. 25,000. Inset. Enlargement of the gap junction. Note the presence of pinocytotic vesicles. 35,000.


Histology_Ross

Histology_Ross. filaments in a smooth muscle cell are attached to cytoplasmic densities or dense bodies that are visible among the filaments (Fig. 11.20). These structures are distributed throughout the sarcoplasm in a network of intermediate filaments containing the protein desmin. Intermediate filaments are part of the cytoskeleton of the cell. Note that vascular smooth muscle contains vimentin filaments in addition to desmin filaments.


Histology_Ross

Histology_Ross. The components of the contractile apparatus in smooth muscle cells are the following.  Thin filaments contain actin, the smooth muscle isoform of tropomyosin, and two smooth muscle–specific proteins, caldesmon and calponin. No troponin is associ ated with smooth muscle tropomyosin. Actin is involved in the force-generating interaction with myosin II molecules. Research suggests that the tropomyosin position on the actin filament is regulated by phosphorylation of myosin heads. Caldesmon (120 to 150 kilodaltons) and calponin (34 kilodaltons) are actin-binding proteins that block the myosin-binding site. The action of these proteins is Ca2-dependent and is also controlled by the phosphorylation of myosin heads.


Histology_Ross

Histology_Ross.  Thick filaments containing myosin II differ slightly from those found in skeletal muscle. They, too, are composed of two polypeptide heavy chains and four light chains. However, the structure of thick filaments in smooth muscle is different than in skeletal muscle. Rather than a bipolar arrangement, myosin II molecules are oriented in one direction on one side of the filament and in an opposite direction on the other side of the filament. In this arrangement, myosin molecules are staggered in parallel between two immediate neighbors and are also bound to an antiparallel partner via a short overlap at the very tip of their tails (Fig. 11.21). The polarity of the myosin heads is the same along the entire length of one side of the filament and the opposite on the opposite side. This side-polar myosin filament also has no central “bare zone,” but instead has asymmetrically tapered bare ends. This organization maximizes the interaction between thick and thin filaments, allowing the


Histology_Ross

Histology_Ross. myosin filament also has no central “bare zone,” but instead has asymmetrically tapered bare ends. This organization maximizes the interaction between thick and thin filaments, allowing the overlapped thin filaments to be pulled over the entire length of the thick filaments.


Histology_Ross

Histology_Ross. Several more proteins are associated with the contractile apparatus and are essential to initiation or regulation of the smooth muscle contractions.  Myosin light chain kinase (MLCK) is a 130to 150kilodalton enzyme that is important in the mechanism of contraction in smooth muscle. It initiates the contraction cycle after its activation by Ca2–calmodulin complex. Active MLCK phosphorylates one of the myosin regulatory light chains, enabling it to form a cross-bridge with actin filaments.  Calmodulin, a 17-kilodalton Ca2-binding protein, is related to the TnC found in skeletal muscle, which regulates the intracellular concentration of Ca2. A Ca2 calmodulin complex binds to MLCK to activate this enzyme. It may also, with caldesmon, regulate its phosphorylation and release from F-actin.  -actinin, a 31-kilodalton protein, provides structural component to dense bodies. Dense bodies provide an attachment site for thin filaments and intermediate filaments.


Histology_Ross

Histology_Ross.  -actinin, a 31-kilodalton protein, provides structural component to dense bodies. Dense bodies provide an attachment site for thin filaments and intermediate filaments. Dense bodies contain a variety of attachment plaque proteins, including -actinin, which anchors both thin filaments and intermediate filaments either directly or indirectly to the sarcolemma. They play an important role in transmitting contractile forces generated inside the cell to the cell surface, altering the cell’s shape (Fig. 11.22). Dense bodies are intracellular analogs of the striated muscle Z lines. In support of this concept is the finding that dense bodies, although frequently appearing as small, isolated, irregular, electron-dense bodies,


Histology_Ross

Histology_Ross. FIGURE 11.20 • Electron micrographs showing the cytoplasmic densities in vascular smooth muscle cells. Upper inset. The plane of section includes only the smooth muscle cells in the vascular wall. The rectangle in the inset shows portions of three smooth muscle cells that appear at higher magnification in the large electron micrograph. The -actinin–containing cytoplasmic densities (single arrows) usually appear as irregular masses, some of which are in contact with, and attached to, the plasma membrane. The cell in the center of the micrograph has been cut in a plane closer to the cell surface and reveals these same densities as a branching structure (double arrows). A three-dimensional model of the cytoplasmic densities would reveal an anastomosing network. BL, basal (external) lamina; PV, pinocytotic vesicles. 27,000. Lower inset. Higher magnification of cytoplasmic densities attached to the plasma membrane from the area indicated by the rectangle. Note that each cell possesses a


Histology_Ross

Histology_Ross. PV, pinocytotic vesicles. 27,000. Lower inset. Higher magnification of cytoplasmic densities attached to the plasma membrane from the area indicated by the rectangle. Note that each cell possesses a basal (external) lamina. In addition, the pinocytotic vesicles can be observed in different stages of their formation. 49,500.


Histology_Ross

Histology_Ross. FIGURE 11.21 • Comparison of myosin filaments of skeletal muscle and smooth muscle. This drawing shows the different arrangements of myosin thick filaments. a. Bipolar thick filaments are present in skeletal and cardiac muscle. They have a helical parallel–antiparallel arrangement of myosin molecules with their globular heads projecting from both ends of the filament. This filament has a “bare zone” in the middle of the filaments that does not have globular heads. b. Side-polar nonhelical thick filaments are present in smooth muscle. In these filaments, myosin molecules are staggered in parallel by two immediate neighbors and are also bound to an antiparallel partner via a short overlap at the very tip of their tails. The polarity of the myosin heads is the same along the entire length of one side of the filament and the opposite on the opposite side. There is no central “bare zone”; instead, the filament b has asymmetrically tapered bare ends.


Histology_Ross

Histology_Ross. -actinin-containing cytoplasmic densities (dense bodies) actin-myosin filaments nucleus actin filament actin tropomyosin myosin filament intermediate filament (desmin, vimentin) dense body -actinin FIGURE 11.22 • A suggested model for smooth muscle cell contraction. Bundles of myofilaments containing thin and thick filaments, shown in dark brown, are anchored on cytoplasmic densities, shown in beige. These densities, in turn, are anchored on the sarcolemma. Cytoplasmic densities are intracellular analogs of striated muscle Z lines. They contain the actin-binding protein -actinin. Because the contractile filament bundles are oriented obliquely to the long axis of the cell, their contraction shortens the cell and produces the “corkscrew” shape of the nucleus.


Histology_Ross

Histology_Ross. may also appear as irregular linear structures. In fortuitous sections, they exhibit a branching configuration consistent with a three-dimensional anastomosing network that extends from the sarcolemma into the interior of the cell (see Fig. 11.20). Contraction in smooth muscles is initiated by a variety of impulses, including mechanical, electrical, and chemical stimuli. The mechanisms that cause contraction of smooth muscle cells are very different from those of striated muscle. Smooth muscle has diverse signal transduction pathways that initiate and modulate smooth muscle contraction. They all lead to elevation of the intracellular concentration of Ca2, which is directly responsible for muscle contraction. Thus muscle contraction can be triggered by the following.  Mechanical impulses, such as passive stretching of vascular smooth muscle, activate mechanosensitive ion channels, leading to initiation of spontaneous muscle contraction (myogenic reflex).


Histology_Ross

Histology_Ross.  Mechanical impulses, such as passive stretching of vascular smooth muscle, activate mechanosensitive ion channels, leading to initiation of spontaneous muscle contraction (myogenic reflex).  Electrical depolarizations can occur, such as those during neural stimulation of smooth muscle. The release of the neurotransmitters acetylcholine and norepinephrine from their synaptic nerve endings stimulates receptors located in the neuronal plasma membrane and changes the membrane potential. This causes opening of voltage-sensitive Ca2 channels (see below).


Histology_Ross

Histology_Ross.  Chemical stimuli, such as those elicited by angiotensin II, vasopressin, or thromboxane A2, act on specific cell membrane receptors, leading to muscle contraction. These substances use second-messenger pathways that do not require the generation of an action potential and cell depolarization to trigger contraction. The most common second-messenger pathways used by smooth muscle are inositol 1,4,5-trisphosphate (IP3), G-protein-coupled, and nitric oxide (NO)-cGMP pathways. Smooth muscle cells lack a T system.


Histology_Ross

Histology_Ross. Smooth muscle cells lack a T system. A characteristic feature of smooth muscle cells is the presence of large numbers of invaginations of the cell membrane that resemble caveolae (see Fig. 11.19). Beneath the plasma membrane, and often in proximity to the sparse profiles of the sER, are cytoplasmic vesicles. It is thought that the invaginations of the cell membrane and the underlying vesicles along with the sER function in a manner analogous to the T system of striated muscle to deliver Ca2 to the cytoplasm. Intracellular Ca2 concentrations are very important in regulating smooth muscle contraction.


Histology_Ross

Histology_Ross. An elevation of intracellular Ca2 levels in smooth muscle is achieved either by depolarization of the cell membrane with subsequent activation of voltage-sensor proteins or by direct activation of gated Ca2-release channels in the sER by a second-messenger molecule, most commonly IP3. The IP3 receptor is located in the sER membrane and has properties similar to those of gated Ca2-release channels. The amount of Ca2 entering the cell after activation of the voltage-sensor protein is usually insufficient to initiate smooth muscle contraction and needs to be supplemented by release of Ca2 from the sER. The Ca2 then binds to calmodulin, which activates phosphorylation of the myosin light chain kinase to initiate contraction. After the contraction cycle commences, Ca2 is removed from the sarcoplasm by ATP-dependent calcium pumps and resequestered in the sER or delivered to the extracellular environment.


Histology_Ross

Histology_Ross. Contraction of smooth muscle is regulated by the Ca2–calmodulin–myosin light chain kinase system.


Histology_Ross

Histology_Ross. A modified version of the sliding filament model described on page 323 can explain contraction in both striated and smooth muscle (see Fig. 11.22). As in striated muscle, contraction is initiated by an increase in the Ca2 concentration in the cytosol, but the contraction does not act through a troponin–tropomyosin complex on the thin filament. Rather, in smooth muscle, an increase in Ca2 concentration stimulates a myosin light chain kinase (MLCK) to phosphorylate one of the two regulatory light chains of the myosin molecule. The Ca2 binds to calmodulin to form the Ca2–calmodulin complex, which in turn binds to MLCK to activate the phosphorylation reaction of the regulatory light chain of myosin (Fig. 11.23). When the light chain is phosphorylated, the actin-binding site of the myosin head is activated and attaches to actin. In the presence of ATP, the myosin head bends, producing contraction. When it is dephosphorylated, the myosin head dissociates from actin. This phosphorylation


Histology_Ross

Histology_Ross. is activated and attaches to actin. In the presence of ATP, the myosin head bends, producing contraction. When it is dephosphorylated, the myosin head dissociates from actin. This phosphorylation occurs slowly, with maximum contraction often taking up to a second to achieve.


Histology_Ross

Histology_Ross. Smooth muscle cell myosin hydrolyzes ATP at about 10% of the rate of skeletal muscle, producing a slow cross-bridging cycle that results in slow contraction of these cells. Thus, smooth muscle cells, and nonmuscle cells that contract by this same mechanism, are capable of sustained contractions over long periods of time while using only 10% of the ATP that would be used by a striated muscle cell performing the same work. The force of smooth muscle contraction may be maintained for long periods in a “latch state.”


Histology_Ross

Histology_Ross. The force of smooth muscle contraction may be maintained for long periods in a “latch state.” In addition to normal phosphorylation of the regulatory light chains of myosin, smooth muscle cells possess a secondary mechanism that allows them to maintain long-term contraction with minimal expenditure of ATP. This mechanism is detected in vascular smooth muscles, for example, and is used to maintain the force of contraction (tone of blood vessels) for an extended time. This so-called latch state of smooth muscle contraction occurs after the initial Ca2-dependent myosin phosphorylation. The myosin head attached to the actin molecule becomes dephosphorylated, causing its ATPase activity to decrease. As a result of the decrease in ATP activity, the myosin head is unable to detach from the actin flament, which maintains the contracted state. The latch state is comparable in many ways to rigor mortis in striated muscle. Functional Aspects of Smooth Muscle


Histology_Ross

Histology_Ross. Functional Aspects of Smooth Muscle Smooth muscle is specialized for slow, prolonged contraction. As noted previously, smooth muscle cells may enter the latch state and remain contracted for long periods of time without fatiguing. They may contract in a wavelike manner, producing peristaltic movements such as those in the gastrointestinal tract and the male genital tract, or contraction may occur along the entire muscle, producing extrusive movements (e.g., those in the urinary bladder, gallbladder, and uterus). Smooth muscle exhibits a spontaneous contractile activity in the absence of nerve stimuli. Contraction of smooth muscle is usually regulated by post-synaptic neurons of the autonomic nervous system (ANS); most smooth muscle is directly innervated by both sympathetic and parasympathetic nerves. In the gastrointestinal tract, the third component of the ANS, the enteric division, is the primary source of nerves to the muscular layers.


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Histology_Ross. FIGURE 11.23 • Schematic diagram illustrating steps leading to initiation of smooth muscle contraction. An increase in the Ca2 level concentration within the cytosol is necessary to initiate smooth muscle contraction. This increase is achieved either by initial depolarization of the cell membrane or hormonal stimulation of cell surface receptors. The intracellular Ca2 binds to calmodulin (4 Ca2 per 1 molecule of calmodulin) to form the Ca2–calmodulin complex. This complex then binds to myosin light chain kinase (MLCK) to phosphorylate one of the two regulatory light chains of the myosin molecule. When phosphorylated, the myosin changes its conformation and the actin-binding site on the myosin head is activated, allowing it to attach to actin. In the presence of ATP, the myosin head bends, producing contraction. sER, smooth endoplasmic reticulum.


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Histology_Ross. Although Ca2 enters the cytoplasm during depolarization by voltage-gated Ca2 channels, some Ca2 channels, called ligand-gated Ca2 channels, are activated by hormones via their second-messenger pathways (see Fig. 11.23). Thus, smooth muscle contraction may also be initiated by certain hormones secreted from the posterior pituitary gland (e.g., oxytocin and, to a lesser extent, antidiuretic hormone [ADH]). In addition, smooth muscle cells may be stimulated or inhibited by hormones secreted by the adrenal medulla (e.g., epinephrine and norepinephrine). Also oxytocin is a potent stimulator of smooth muscle contraction, and its release by the posterior pituitary plays an essential role in uterine contraction during parturition. It is often used to induce or enhance labor. Many peptide secretions of enteroendocrine cells also stimulate or inhibit smooth muscle contraction, particularly in the alimentary canal and its associated organs.


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Histology_Ross. Nerve terminals in smooth muscle are observed only in the connective tissue adjacent to the muscle cells. Nerve fibers pass through the connective tissue within the bundles of smooth muscle cells; enlargements in the passing nerve fiber, or bouton en passant (see page 359), occur adjacent to the muscle cells to be innervated. The enlargements contain synaptic vesicles with neuromuscular transmitters. However, the neuromuscular site is not comparable to the neuromuscular junction of striated muscle. Rather, a considerable distance, usually 10 to 20 m (in some locations, up to 200 m), may separate the nerve terminal and the smooth muscle. The neurotransmitter released by the nerve terminal must diffuse across this distance to reach the muscle.


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Histology_Ross. Not all smooth muscle cells are exposed directly to the neurotransmitter, however. As discussed above, smooth muscle cells make contact with neighboring cells by gap junctions. As in cardiac muscle, contraction is propagated from cell to cell via gap junctions, thus producing coordinated activity within a smooth muscle bundle or layer. The gap junction between two smooth muscle cells was originally designated a nexus, a term still in use. Smooth muscle cells also secrete connective tissue matrix.


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Histology_Ross. Smooth muscle cells also secrete connective tissue matrix. Smooth muscle cells have organelles typical of secretory cells. A well-developed rER and Golgi apparatus are found in the perinuclear zone. Smooth muscle cells synthesize both type IV (basal lamina) collagen and type III (reticular) collagen as well as elastin, proteoglycans, and multiadhesive glycoproteins. Except at the gap junctions, smooth muscle cells are surrounded by an external lamina. In some locations, such as the walls of blood vessels and the uterus, smooth muscle cells secrete large amounts of both type I collagen and elastin. Renewal, Repair, and Differentiation Smooth muscle cells are capable of dividing to maintain or increase their number.


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Histology_Ross. Smooth muscle cells may respond to injury by undergoing mitosis. In addition, smooth muscle contains regularly replicating populations of cells. Smooth muscle in the uterus proliferates  FOLDER 11.5 Functional Considerations: Comparison of the Three Muscle Types 337 Cardiac muscle shares structural and functional charac-teristics with skeletal muscle and smooth muscle. In both cardiac and skeletal muscle, the contractile elements— thick and thin filaments—are organized into sarcomeres surrounded by sER and mitochondria. Both cardiac and smooth muscle cells retain their individuality, although both are in functional communication with their neighbors through gap junctions. In addition, cardiac and smooth muscle cells have a spontaneous beat that is regulated but not initiated by autonomic or hormonal stimuli. Both have centrally located nuclei and perinuclear organelles. These common characteristics suggest that cardiac mus-cle may have evolved in the direction of skeletal muscle from


Histology_Ross

Histology_Ross. or hormonal stimuli. Both have centrally located nuclei and perinuclear organelles. These common characteristics suggest that cardiac mus-cle may have evolved in the direction of skeletal muscle from the smooth muscle of primitive circulatory systems. A summary of major characteristics of all three muscle types is provided in the table below. Comparison of the Three Muscle Types Skeletal Cardiac Smooth Structural features Muscle cell Large, elongate cell, 10–100 m in diameter, up to 100 cm in length (sartorius m.) Short, narrow cell, 10–15 m in diameter, 80–100 m in length Short, elongate, fusiform cell, 0.2–2 m in diameter, 20–200 m in length Location Muscles of skeleton visceral striated (e.g., tongue, esophagus, diaphragm) Heart, superior and inferior vena cava, pulmonary veins Vessels, organs, and viscera Connective tissue components Epimysium, perimysium, endomysium Endomysium (subendocardial and subpericardial connective tissue) Endomysium, sheaths, and bundles Fiber Single


Histology_Ross

Histology_Ross. organs, and viscera Connective tissue components Epimysium, perimysium, endomysium Endomysium (subendocardial and subpericardial connective tissue) Endomysium, sheaths, and bundles Fiber Single skeletal muscle cell Linear branched arrangement of several cardiac muscle cells Single smooth muscle cell Striation Present Present None Nucleus Many peripheral Single central, surrounded by juxtanuclear region Single central T tubules Present at A-I junction (triad: with two terminal cisternae), two T tubules/sarcomere Present at Z lines (diad: with small terminal cisternae), one T tubule/sarcomere None, well-developed sER, many invaginations and vesicles similar to caveolae Cell-to-cell junctions None Intercalated discs containing 1. Fasciae adherentes 2. Macula adherens (desmosome) 3. Gap junctions Gap junctions (nexus) Special features Well-developed sER and T tubules Intercalated discs Dense bodies, caveolae, and cytoplasmic vesicles Functions Type of innervation Voluntary Involuntary


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Histology_Ross. Gap junctions (nexus) Special features Well-developed sER and T tubules Intercalated discs Dense bodies, caveolae, and cytoplasmic vesicles Functions Type of innervation Voluntary Involuntary Involuntary Efferent innervation Somatic Autonomic Autonomic Type of contraction “AlI or none” (type I and type II fibers) “All or none” rhythmic (pacemakers, conductive system of the heart) Slow, partial, rhythmic, spontaneous contractions (pacemakers of stomach) Regulation of contraction By binding of Ca2to TnC, causes tropomyosin movement and exposes myosin-binding sites on actin filaments By binding of Ca2to TnC, causes tropomyosin movement and exposes myosin-binding sites on actin filaments By phosphorylation of myosin light chain by myosin light chain kinase in the presence of Ca2-calmodulin complex Growth and regeneration Mitosis None None (in normal condition) Present Response to demand Hypertrophy Hypertrophy Hypertrophy and hyperplasia Regeneration Limited (satellite cells and myogenic


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Histology_Ross. Growth and regeneration Mitosis None None (in normal condition) Present Response to demand Hypertrophy Hypertrophy Hypertrophy and hyperplasia Regeneration Limited (satellite cells and myogenic cells from bone marrow) None (in normal condition) Present chapter 11 Muscle Tissue SMOOTH M USCLE 337 during the normal menstrual cycle and during pregnancy; both activities are under hormonal control. The smooth muscle cells of blood vessels also divide regularly in the adult, presumably to replace damaged or senile cells; the smooth muscle of the muscularis externa of the stomach and colon regularly replicates and may even slowly thicken during life.


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Histology_Ross. New smooth muscle cells have been shown to differentiate from undifferentiated mesenchymal stem cells in the adventitia of blood vessels. Differentiation of smooth muscle progenitor cells is regulated by a variety of intracellular and environmental stimuli, and developing muscles exhibit a wide range of different phenotypes at different stages of their development. To date, no transcription factors have been identified that are characteristic for the smooth muscle cell lineage. However, serum response factor (RF), a member of the MADS–box transcription factor family, has been shown to regulate most smooth muscle differentiation marker genes. Smooth muscle cells have also been shown to develop from the division and differentiation of endothelial cells and pericytes during the repair process after vascular injury.


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Histology_Ross. Vascular pericytes are located within the basal lamina of capillaries and postcapillary venules. They function as multi-potential mesenchymal progenitor cells. In capillaries, their cytoplasmic morphology is difficult to distinguish from that of the endothelial cell. In postcapillary venules and pericytic venules, they may form a nearly complete investment of the vessel with cells that resemble smooth muscle cells (see Chapter 13, Cardiovascular System).


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Histology_Ross. Fibroblasts in healing wounds may develop morphologic and functional characteristics of smooth muscle cells (myofibroblasts; see page 179). Epithelial cells in numerous locations, particularly sweat glands, mammary glands, salivary glands, and the iris of the eye, may acquire the characteristics of smooth muscle cells (myoepithelial cells). Myoid cells of the testis have a contractile function in the seminiferous tubules, and cells of the perineurium, a concentric layer of connective tissue that surrounds groups of nerve fibers and partitions peripheral nerves into distinct fascicles, function as contractile cells as well as transport barrier cells. This page intentionally left blank.


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Histology_Ross. 340 Muscle tissue is classified on the basis of the appearance of its contractile cells. Two major types are recognized: striated muscle, in which the cells exhibit a cross-striation pattern when observed at the light microscope level; and smooth muscle, in which the cells lack striations. Striated muscle is further subclassified based on location namely skeletal muscle, visceral striated muscle, and cardiac muscle. Skeletal mus-cle is attached to bone and is responsible for movement of the axial and apendicular skeleton, and for maintenance of body position and pos-ture. Visceral striated muscle is morphologically identical, but is restricted to soft tissues including the tongue, pharynx, upper part of the esophagus, and the diaphragm. Cardiac muscle is a type of striated muscle found in the heart and the base of the large veins that empty into the heart. The cross-striations in striated muscle are due to the organization of the contractile elements that occur in the muscle cell,


Histology_Ross

Histology_Ross. in the heart and the base of the large veins that empty into the heart. The cross-striations in striated muscle are due to the organization of the contractile elements that occur in the muscle cell, namely thin fila-ments composed largely of the protein actin and thick filaments composed of the protein myosin II. The two types of myofilaments occupy bulk of the cytoplasm. The skeletal and visceral striated muscle cells, more commonly called fibers, are a multinucleated syncytium formed during development by the fusion of individual small muscle cells called myoblasts. Surrounding each fiber is a delicate mesh of collagen fibrils referred to as endomysium. In turn, bundles of muscle fibers that form func-tional units within a muscle are surrounded by a thicker connective tissue layer. This connective tissue is referred to as perimysium. Lastly, a sheath of dense connective tissue that surrounds the muscle, is referred to as epimysium. The force generated by individual muscle fibers is


Histology_Ross

Histology_Ross. connective tissue is referred to as perimysium. Lastly, a sheath of dense connective tissue that surrounds the muscle, is referred to as epimysium. The force generated by individual muscle fibers is transferred to the collagenous elements of each of these connective tissue elements to end in a tendon.


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Histology_Ross. Skeletal muscle, human, H&E, ×33. but are not individually discernable. However, the small blue dot-like structures are nuclei of the fibers. Between the fascicles, though difficult to see at This low power micrograph shows a longitudinal section of this magnification, is connective tissue, the perimysium (P). Also evidentstriated muscle. The muscle tissue within the muscle is arin the micrograph is a nerve (Nv). ranged in a series of fascicles (F). The individual muscle fibers within a fascicle are in close proximity to one another, Skeletal muscle, human, H&E, ×33.


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Histology_Ross. ranged in a series of fascicles (F). The individual muscle fibers within a fascicle are in close proximity to one another, Skeletal muscle, human, H&E, ×33. This micrograph reveals part of a muscle that has been cut in cross section. Again, individual bundles of muscle fibers or fascicles (F) can be readily identified. In contrast to the previous micrograph, even at this low magnification, upon careful examination, individual muscle fibers (MF) can be identified in many of the fascicles. Each is bounded by connective tissue, which constitutes the perimysium (P). Also identifiable in this micrograph is a dense connective tissue surrounding the muscle, namely epimysium (E). Skeletal muscle, human, H&E, ×256; inset ×700.


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Histology_Ross. This higher magnification of a longitudinal section of a muscle reveals two muscle fibers (MF). At this magnification, the cross-banding pattern is just perceptible. With few exceptions, the nuclei (N), which tend to run in linear arrays, belong to the muscle fibers. Also evident in this micrograph is a small blood vessel (BV). The inset, taken from a gluteraldehyde fixed plastic embedded specimen, is a much higher magnification of a portion of two muscle fibers. The major bands are readily identifiable at this magnification and degree of specimen preservation. The thick, dark stained band is the A-band. Between A-bands is a lightly stained area, the I-band, which is bisected by the Z-line. The two elongate nuclei (N) belong to the muscle fibers. Below them are a capillary (C) and a portion of an endothelial cell nucleus (End). At this higher magnification, the endothelial nuclei, as well as the nuclei of the fibroblasts, can be distinguished from the muscle cell nuclei by their


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Histology_Ross. portion of an endothelial cell nucleus (End). At this higher magnification, the endothelial nuclei, as well as the nuclei of the fibroblasts, can be distinguished from the muscle cell nuclei by their smaller size and heterochromatin giving them a dark stain. The muscle cell nuclei (N) exhibit more euchromatin with a speckling of heterochromatin, thus given them a lighter staining appearance.


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Histology_Ross. Skeletal muscle, human, H&E, ×256. In this cross section, individual muscle fibers (MF) are readily discernable in contrast to identifying individual muscle fibers in longitudinal sections. For example, if one imagines a cut crossing a number of cells (see dashed line), the close proximity of the muscle cells can mask the boundary between individual cells within a fascicle when observed in the opposite or longitudinal plane. The connective tissue (CT) that is readily apparent here belongs to the perimysium, which separates fascicles. The nuclei of the individual fibers are located at the periphery of the cell. At this magnification, it is difficult to distinguish between occasional fibroblasts that belong to the endomysium from the nuclei of the muscle cells. KEY BV, blood vessel C, capillary CT, connective tissue E, epimysium End, endothelial cell nucleus F, fascicles MF, muscle fibers N, nuclei Nv, nerve P, perimysium


Histology_Ross

Histology_Ross. The myofibril is the structural and functional subunit of a muscle fiber. They are best seen at higher magnification in the light microscope in a cross section of the cell where they appear as dot-like structures. The overall effect is a stippled appearance of the cytoplasm. Each myofibril is composed of a bundle of myofilaments that are of two types. One type is the myosin II thick filament. The other is actin and its associated proteins that make up the thin filaments. It is the arrangement of the thick and thin filaments that produce density differences that in turn cre-ate the cross-striations of the myofibril when viewed in longitudinal section (see diagram). As evident in the diagram, the site of overlap of thin and thick filaments produces the dark A-band. The light appearing I-band contains the thin filaments. Careful examination of the A-band in the light microscope reveals a light staining area in the middle of the A-band. This is referred to as the H-band. It is an area


Histology_Ross

Histology_Ross. contains the thin filaments. Careful examination of the A-band in the light microscope reveals a light staining area in the middle of the A-band. This is referred to as the H-band. It is an area that is occupied by thin filaments and is devoid of thick filaments. At the middle of each I-band is the thin dense Z-line to which the thin filaments are attached. The distance between Z-lines is referred to as a sarcomere. When a muscle contracts, the sarcomere and I-band shorten. The filaments, however, maintain a constant length, thus the contraction is produced by an increase in the overlap between the two filament types.


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Histology_Ross. Skeletal muscle, human, H&E, ×512; inset ×985. This micrograph reveals a cross section of a muscle fascicle. The individual muscle fibers (MF) exhibit a polygonal shape, but vary only slightly in width. Of the many nuclei that can be observed in this plane of section, only some belong to the muscle fibers. The muscle fiber nuclei (MFN) appear to be embedded within the extreme periphery of the fiber. In contrast, fibroblast nuclei (FN) that belong to the endomysium lie clearly outside of the muscle fiber, are typically smaller and exhibit greater density than the nuclei of Skeletal muscle, human, H&E, ×512; inset ×985.


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Histology_Ross. Skeletal muscle, human, H&E, ×512; inset ×985. This micrograph, a longitudinal section of a gluteraldehyde fixed, plastic embedded specimen reveals four myofibers (M). Although they appear to be markedly different in width, the difference is due mainly to the plane of section through each of the fibers. Because the nuclei of the myofibers are located at the periphery of the cell, their location is variable when observed in a longitudinal section. For example, three nuclei (N) are seen in what appears to be a central location of a fiber. This is due to the section grazing the periphery of this fiber. The clear space at either end of two of these nuclei represents the cytoplasmic portion of the cell that contains organelles and is devoid of Skeletal muscle, human, electron micrograph, ×5,000.


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Histology_Ross. The low power electron micrograph shown here should be compared to the inset of the longitudinally sectioned myofibers above. It reveals portions of three myofibers (M), two of which exhibit a nucleus (N). Between cells, various amounts of collagenous fibers are present representing the endomy sium (E). The micrograph illustrates the banding pattern of the myofibrils to advantage. In contrast to the longitudinally sectioned muscle in the inset above, individual myofibrils (My) can be identified in this electron the muscle fibers. Also present between the muscle fibers are capillaries (C). The endothelial cell nuclei (ECN) are also relatively dense. Other nuclei that may be present, but are very difficult to identify belong to satellite cells. The inset which shows the boxed area reveals several nuclei, two of which belong to the muscle fibers (MF). The small, very dense nucleus (FN) probably belongs to a fibroblast of the endomysium. Also clearly evident here is a cross-sectioned


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Histology_Ross. several nuclei, two of which belong to the muscle fibers (MF). The small, very dense nucleus (FN) probably belongs to a fibroblast of the endomysium. Also clearly evident here is a cross-sectioned capillary (C). The more striking feature at this magnification is the appearance of the muscle cells’ myofibrils which appear as the punctate or dot-like structures.