A Textbook of Histology - Fawcett - Muscular Tissue PDF

294 MUSCULAR TISSUE the membrane of the longitudinally oriented 100 times a minute throughout life. Its con¬ processes as shear stresses. Moreover, the con¬ traction is myogenic, i.e., it is independent of siderable amplification of the area of the nervous stimulation. All...

294 MUSCULAR TISSUE the membrane of the longitudinally oriented 100 times a minute throughout life. Its con¬ processes as shear stresses. Moreover, the con¬ traction is myogenic, i.e., it is independent of siderable amplification of the area of the nervous stimulation. All cardiac myocytes are membrane across which force is transmitted capable of spontaneous rhythmic depolariza¬ results in a reduction of stress that is propor¬ tion and repolarization of their membrane. tional to the ratio between the force-transmit¬ However, a group of myocytes in the atrium ting area of sarcolemma and the total force¬ constitute the pacemaker and excitation generating cross-sectional area of the myofi¬ spreads from there throughout the myocar¬ brils. This ratio varies from 10 : 1 to 14 : 1 in dium via gap junctions between myocytes. muscles that have been studied. Thus, although made up of separate cellular In transmission electron micrographs of units, cardiac muscle behaves as though it high magnification, the external lamina in¬ were a syncytium. vesting the sarcolemma at the end of the mus¬ cle fiber consists of a 20-nm-thick lamina lu- cida and a 30-nm-thick lamina densa. The HISTOLOGY OF CARDIAC MUSCLE lamina lucida is traversed by fine 2-7-nm fil¬ aments which appear to cross the membrane Under the light microscope, cardiac muscle and mingle with the meshwork of filaments has a pattern of cross-striations similar to that that form the subsarcolemmal dense layer in of skeletal muscle but branching and intercon¬ which the actin filaments of the myofibrils ter¬ nection of neighboring fibers is evident. The minate. It seems likely that these transmem¬ sarcoplasm is more abundant and a longitudi¬ brane thin filaments bind the muscle to the nal striadon is more apparent owing to the type-IV collagen in the lamina densa and to separation of bundles of myofibrils by rows the type-II collagen bundles of the tendon. of mitochondria. The myofibrils diverge The chemical nature of these filaments has around a centrally placed nucleus, outlining not been established. However, a-actimn, talin, a fusiform axial region of sarcoplasm rich in and vinculin have been localized immunocyto- organelles and inclusions. A small Golgi com¬ chemically in this region. Inasmuch as these plex is located near one pole of each elongate proteins have been found to bind intracellular nucleus. Lipid droplets are common in this filaments to desmosomes and other focal con¬ region and, in older animals, granular depos¬ tacts between cells, it is not unreasonable to its of lipochrome pigment may be abundant. speculate that they are also involved in attach¬ In aged humans, such pigment may constitute ment of muscle fibers to tendon. up to 20% of the dry weight of the myocar¬ dium. In smaller animal species, occasional lipid droplets are found throughout the inter- CARDIAC MUSCLE myofibrillar sarcoplasm. A unique feature of cardiac muscle is the Unlike skeletal muscle, cardiac muscle con¬ occurrence of transverse intercalated discs at sists of separate cellular units about 80 /Tin in regular intervals along the length of the fibers. length and 15 /xm in diameter. These cardiac They are relatively inconspicuous in routine myocytes are joined end-to-end at junctional preparations but are heavily stained by iron specializations called intercalated discs (Figs. haematoxylin. A disc may extend straight 10—40 and 10—41). Although the strands so- across the fiber, but, more commonly, seg¬ formed are predominantly parallel, the indi¬ ments of it are slightly offset longitudinally vidual myocytes branch and form oblique in¬ giving it a step-like configuration in section. In terconnections with neighboring strands, re¬ the pattern of cross-striations, the intercalated sulting in a complex three-dimensional discs invariable occur at the I-bands. organization that is quite different from the parallel arrangement of discrete cylindrical fibers of skeletal muscle. Prior to the discovery ULTRASTRUCTURE OF that the intercalated discs are intercellular CARDIAC MUSCLE junctions, the structural units of cardiac mus¬ cle were called “fibers,” as in skeletal muscle. Under the electron microscope a distinctive Although it is questionable whether this term feature of cardiac muscle in cross section is is appropriate for cardiac muscle, it continues the absence of separate myofibrils (Fig. 10— to be used in the contemporary literature. 42). It will be recalled that, in similar sections The human heart beats at a rate of 60 to of skeletal muscle, the myofilaments are as-  Figure 10-40. Drawing of a histological section of human cardiac muscle that was stained with thiazin red and toluidine blue to show the intercalated discs, a characteristic feature of this type of striated muscle. (From H Heidenhein 1901 Anat. Anz. 20,1.) Figure 10-41. Photomicrograph of a longitudinal section of cardiac muscle (left) illustrating the variable diameter of the fibers and the central position of their nuclei. In routinely stained preparations, the intercalated discs are not evident. At the right, a cross section of human cardiac muscle. 295 Figure 10-42. Electron micrograph of a small peripheral area of a cardiac muscle cell in cross section. Observe that the myofilaments are not associated in discrete myofibrils with clearly defined limits, as in skeletal muscle, but form a more- or-less continuous mass interrupted by mitochondria and elements of the sarcoplasmic reticulum. Figure 10-43. Micrograph of a portion of a cardiac muscle cell in longitudinal section. The pattern of cross-banding is similar to that of skeletal muscle. Mitochondria occupy fusiform spaces that may appear to subdivide the mass of myofilaments into myofibril-like units of varying width for short distances. 296  MUSCULAR TISSUE 297 sembled into discrete myofibrils of uniform numerous cristae that often show a periodic diameter, each outlined by a thin layer of sar¬ angulation that gives them a zig-zag form. As coplasm containing longitudinal elements of a rule, the mitochondria are about the length the sarcoplasmic reticulum and occasional mi¬ of a sarcomere (2.5 jum), but they may be as tochondria. In cardiac muscle, separate myo¬ long as 7-8 /xm. Glycogen tends to be more fibrils are not distinguishable. Instead, the abundant in cardiac than in skeletal muscle. cross section of a myocyte is occupied by a It occurs in the form of 30-40-nm dense parti¬ continuum of myofilaments, interrupted here cles located in the areas of intermyofilament and there by mitochondria and profiles of sar¬ sarcoplasm that contain mitochondria, but coplasmic reticulum that penetrate into the particles may also be found aligned in rows cylindrical mass of myofilaments from its pe¬ between myofilaments (Fig. 10-44). These are riphery. In longitudinal sections, these incur¬ more numerous in the I-bands than in the A- sions appear as slender fusiform areas of sar¬ bands. Glycogen and lipid are both important coplasm containing mitochondria and circular energy sources for the contractile activity of profiles of sarcotubules (Fig. 10-43). Locally, the myocardium. these may appear to define the lateral limits The T-tubules of cardiac muscle differ sig¬ of myofibrils of varying width, but this is mis¬ nificantly from those of skeletal muscle. They leading because they are of limited longitudi¬ are located at the level of the Z-discs instead nal extent and, at their ends, lateral continuity of at the A-I junctions and there is, therefore, of the mass of myofilaments is again evident. only one per sarcomere. They are of greater Absence of distinct myofibrils is also reported diameter and penetrate deep into the cell in certain slow-acting tonic skeletal muscles where they communicate with occasional tu¬ found mainly in amphibians. bules of slightly smaller diameter that course The mitochondria of cardiac muscle have parallel with the long axis of the cell. Thus, Figure 10-44. (A) Longitudinal section of a small area of cardiac muscle showing a loose network of tubular elements of the sarcoplasmic reticulum. (B) Particles of glycogen are abundant around the mitochondria and may be found between the myofilaments in the I- and H-bands. The muscle in the two figures was fixed in slightly different degrees of relaxation. Note the difference in length of the l-band (indicated by brackets), the A-band is of constant length. 298 MUSCULAR TISSUE of the Z-discs (Figs. 10—46 and 10—47). The total area of junctional contact of these sac¬ cules is considerably less than that of the ter¬ minal cisternae with the T-tubules of skeletal muscle. In both, transduction of excitation from the sarcolemma to the reticulum takes place at rows of intramembrane particles called feet or spanning proteins bridging the gap between the apposed membranes. In addi¬ tion, there are small saccules or cisternae of the superficial reticulum that are connected directly to the sarcolemma by junctional feet. These are sometimes referred to as the corbu- lar sarcoplasmic reticulum. The calcium-binding protein calsequestrin is localized in the junc¬ tional saccules and in the corbular reticulum. As in skeletal muscle, contraction is depen¬ dent on free calcium ions in the sarcoplasm. But cardiac muscle, with relatively small sac¬ cules in place of terminal cisternae, has more limited intracellular reserves of calcium. Dur¬ ing depolarization of the sarcolemma and its invaginations, there is an influx of extracellu¬ lar calcium. This is followed and supple¬ mented by release of intracellular calcium stored in the reticulum. Calcium from these two sources activates sliding of the filaments and consequent contraction. Figure 10-45. Longitudinal section of a small area of a The structure of cardiac muscle in the atria cardiac muscle cell including a cross section of a T-tubule and ventricles of the heart is similar, but the and an adjacent tubule of the sarcoplasmic reticulum. The atrial myocytes have a smaller average diame¬ larger T-tubule is lined with a layer of glycoprotein (at ter and the transverse-axial tubule system is arrows) like that coating the sarcolemma at the surface of the cell. The dense granules in the surrounding sarcoplasm poorly developed. Such tubules are seen only are glycogen. in the largest of the atrial myocytes. It is possi¬ ble that in the more slender myocytes there is less need for transverse tubules for inward channels with a lumen that opens to the extra¬ conduction of excitation. The spread of the cellular space ramify throughout the myocyte. action potential is reported to be more rapid The transverse tubules are lined by a layer in atrial myocytes than in those of the ventri¬ continuous with the external lamina of the cles. The contractile elements are identical in sarcolemma (Fig. 10-45). This system of their ultrastructure, but minor differences branching tubules is called the transverse-axial- have been discovered at the molecular level. tubular system (TATS) to distinguish it from The heavy chains of myosin molecules occur the T-system of skeletal muscle. in two isoforms, «-HMC and /3-HMC. In the The longitudinal sarcoplasmic reticulum is atrium, a-HMC is more abundant, whereas less elaborate than that of skeletal muscle. It in the ventricle, /3-HMC is the predominant consists of a subsarcolemmal network of tu¬ isoform. Myocytes in the sinoatrial and atrio¬ bules 20—35 nm in diameter that extends into ventricular nodes (vide infra) show specific deep clefts within the column of myofila¬ immunoreactivity for a third isoform. The sig¬ ments. Its pattern varies at different levels in nificance of these regional differences in the the sarcomere, being close-meshed adjacent myosin molecules remains obscure-. to A-bands and more loosely organized at I- bands. Terminal cisternae and triads are not found in cardiac muscle. Their functional THE INTERCALATED DISC counterparts are relatively small flattened sac¬ cules that establish junctional contacts with the At the intercalated discs, the conjoined my¬ transverse-axial system of tubules at the level ocytes have a highly irregular surface with  MUSCULAR TISSUE 299 ^- Contacts of SR with T-tubules Figure 10—46. Schematic drawing of the disposition of the T-system and sarcoplasmic reticulum of cardiac muscle. The transverse tubules are much larger than those of skeletal muscle. The relatively simple reticulum has no terminal cisternae and, therefore, there are no triads. Instead, small expansions of some of its tubules end in close apposition to the sarcolemma, either at the surface of the fiber or at its inward extension in the T-tubules. (From Fawcett D W and N S McNutt. 1969. J. Cell Biol. 42:1.) multiple ridges and papillary projections on of sarcoplasm of varying width on the inner the end of one cell fitting into complementary aspects of the apposed membranes. A high grooves and pits on the other (Fig. 10-48). concentration of the actin-binding proteins a- On this interface, one can distinguish areas actinin and vinculin can be demonstrated in identical to desmosomes, other areas that ap¬ this dense layer. These proteins are often pear to be gap junctions, and large areas that found in other cell types at sites of anchorage resemble the zonula adherens of epithelia. In of actin or intermediate filaments to the mem¬ this mosaic of junctional specializations, only brane. In the intercalated disc, they evidently the desmosomes are typical in respect to their serve to bind the ends of the myofilaments shape. The areas having a fine structure re¬ to the sarcolemma. The 83 kD polypeptide, sembling that of zonulae adherentes are not plakoglobin, and another adhesive glycopro¬ circumferential, as the term zonula implies, tein (A-CAM) are localized in the narrow cleft but are more-or-less continuous areas of spe¬ between the membranes. cialization extending over much of the contact The fascia adherens, comprising the surface of the cells. The term fascia adherens greater part of the disc, is interrupted in cer¬ has been suggested as a more appropriate des¬ tain areas by typical desmosomes. The myo¬ ignation for this component of the interca¬ filaments diverge at these sites and do not lated disc. In longitudinal thin sections, the terminate in the dense plaque of the desmo¬ opposing cell membranes can be identified as somes, but intermediate filaments of the cy- two parallel dense lines that follow a sinuous toskeleton may attach there. In other small course, separated by a 15—20-nm intercellular areas in the transverse portion of the interca¬ cleft (Fig. 10—49). The myofilaments of the lated disc, the opposing membranes come into conjoined cells terminate in a very dense layer close contact to form small gap junctions. Figure 10-47. Scanning micrograph of the sarcoplasmic reticulum of rat cardiac muscle, showing a dense network of tubules associated with the A- and l-bands of a myofibril. Transverse tubules are also identifiable at the level of the Z- 1*990 Anat ^ecf 228*227 ketween my°fibrils are mitochondria. (Micrograph courtesy of Ogata, T. and Y. Yamasaka. 300 Figure 10-48. A low-power micrograph of cardiac muscle in longitudinal section showing a typical step-like intercalated disc. The transverse portions are highly interdigitated and there is an abundance of dense material at the attachment of the myofilaments to the end of the cell. The longitudinal segments of the disc are smooth, devoid of dense material, and difficult to see at this magnification. Figure 10-49. Micrograph of a transverse segment of an intercalated disc. The portion of the cell junction, where the myofilaments terminate, resembles the zonula adherens of epithelia but is here called the fascia adherens. Between sites of myofilament attachment are typical desmosomes. (From Fawcett, D.W. and N.S. McNutt. 1969. J. Cell Biol. 42:1.) 301 302 MUSCULAR TISSUE More extensive junctions of this kind are tide hormones involved in the regulation of found on the longitudinal segments of the blood volume and the electrolyte composition step-like intercalated discs. These are of great of the extracellular fluid. physiological importance because diffusion of The myoendocrine cells are specialized myo¬ ions through the pores in such junctions per¬ cytes localized mainly in the right and left mits coordination of the activities of the myo¬ atrial appendages, but they are also found cytes. Measurements of current across interca¬ scattered within other areas of the atria and lated discs, in atrial and ventricular muscle, along the conductive system in the ventricular have shown that all parts of the heart are elec¬ septum. They resemble the working myocytes trically coupled. Thus, although made up of in having longitudinally oriented myofila¬ separate cells, cardiac muscle behaves physio¬ ments that diverge around a centrally placed logically as though it were a syncytium. The nucleus and insert into intercalated discs at very firm attachment of myocytes at the inter¬ either end of the cell. The numerous mito¬ calated discs ensures transmission of the trac¬ chondria and the sarcoplasmic reticulum do tion generated by the individual cells through¬ not differ significantly from those of nonen- out the myocardium. docrine myocytes. The Golgi complex is com¬ monly located at one pole of the nucleus but may be paranuclear and, rarely, isolated dicty- MYOCARDIAL ENDOCRINE CELLS osomes may be found near the sarcolemma. The most conspicuous feature distinguish¬ The functions of the cells in the myocar¬ ing myoendocrine cells from other atrial myo¬ dium were formerly believed to be limited to cytes is the presence of membrane-bounded contraction or excitation and conduction, but dense secretory granules 0.3—0.4 /xm in diam¬ in the past two decades, morphological and eter (Fig. 10—50). These are concentrated in biochemical studies have identified myocytes the core of sarcoplasm that extends in either in the atrium that synthesize and secrete pep¬ direction from the poles of the nucleus, but Figure 10-50. Electron micrograph of a cardiac muscle cell from the rat atrium showing a juxtanuclear region containing mitochondria and numerous spherical secretory granules. These granules contain the precursor of the peptide hormone cardiodilatin, also called atrial natriuretic factor. (Micrograph courtesy of J. Hansen.)  MUSCULAR TISSUE 303 they can also be found among the myofila¬ node, at the junction of the superior vena cava ments and occasionally near the sarcolemma. with the right atrium; in the atrioventricular The granules contain the precursor of a fam¬ node situated in the lower part of the interatrial ily of biologically active polypeptides collec¬ septum; in internodal tracts connecting the si¬ tively called cardiodilatins (CDD) or atrial natri¬ noatrial and atrioventricular nodes; and in the uretic polypeptides (ANP). atrioventricular bundle (bundle of His) which As in other peptide-hormone-secreting originates in the atrioventricular node and en¬ cells, the product is synthesized as a large pre¬ ters the fibrous portion of the interventricular cursor molecule and reduced to the active septum where it divides into right and left bun¬ form by an endopeptidase. The prohormone, dle branches that ramify beneath the endocar¬ a polypeptide of 126 amino acids (CDD 1- dium of the right and left ventricles, establish¬ 126) is cleaved during or immediately after ing communicating junctions with the exocytosis to peptides of lower molecular unspecialized working myocytes. weight, including a peptide of 28 amino acids All myocytes are autonomously excitable (CDD 99—126) which is the only product that cells that undergo rhythmic depolarization circulates in the blood. and repolarization independent of nervous The hormone causes vasodilatation, low¬ influences, but the inherent rate of this activity ering of blood pressure and decreased blood in myocytes of the atria is greater than that volume. Some of its effects are mediated by of the ventricles. The rhythm of cells of the its inhibition of arginine-vasopressin secre¬ sinoatrial node is still more rapid and their tion by the posterior pituitary and of aldoste¬ depolarization, propagated over tracts of my¬ rone production by the adrenal cortex. It ocytes specialized for conduction, overrides causes constriction of the efferent arteriole of the slower rhythm of the working myocar¬ the renal glomeruli resulting in diuresis and dium. The sinoatrial node is, therefore, the increased excretion of sodium. It is the most site of initiation of excitation and the “pace¬ potent endogenous natriuretic agent discov¬ maker” of the heart. ered to date. Injection of an extract of cardiac The sinoatrial node is 10 to 20 mm long, 3 atria into an experimental animal may result mm wide, and about 1 mm in thickness. It is in a 30-fold increase in sodium excretion. The made up of pale-staining branched cells en¬ regulation of fluid balance in the central ner¬ meshed in a framework of collagen. These vous system is regulated by the epithelium of nodal myocytes contain relatively few myofila¬ the choroid plexus that secretes the cerebro¬ ments, aggregated into inconsistently ori¬ spinal fluid; by the arachnoid villi of the dural ented myofibrils of varying diameter in sar¬ sinuses that are the outflow system for this coplasm rich in mitochondria. They are joined fluid, and by the cerebral capillary endothe¬ to like cells and to other myocytes by conspicu¬ lium that constitutes the blood-brain barrier. ous gap junctions. The nature of the cellular All of these structures have been shown to constituents of the internodal tracts is dis¬ possess receptors for atriopeptides. It is likely, puted. They are identified by some as transi¬ therefore, that the atriopeptides also have a tional myocytes and are described as being more role in maintenance of fluid balance in the slender than ordinary atrial myocytes and brain. having more myofibrils than nodal myocytes. Little is known about their conduction veloc¬ ity. Interposed between the nodal myocytes CONDUCTION SYSTEM OF THE HEART and the rapidly conducting distal portions of the conduction system, it is speculated that The heart does not contract synchronously they may be relatively slow and, thus, contrib¬ throughout the myocardium. To function ef¬ ute to the atrioventricular delay essential for fectively as a pump, the contraction of the optimal filling of the ventricles. Transitional atria must be completed slightly before the myocytes are also the principal cellular ele¬ onset of ventricular contraction. The precise ments of the atrioventricular node which con¬ timing and coordination of events in the car¬ tains a relatively small population of nodal diac cycle depends on myocytes that are spe¬ myocytes in its center. At its periphery, there cialized for initiation of excitation and its con¬ are many Purkinje myocytes. These uninucleate duction to different regions of the cylindrical cells, associated end-to-end in long myocardium at a rate that will ensure their rows, continue from the node into the atrio¬ activation in the correct sequence. These spe¬ ventricular bundle. The long strands or tracts cialized myocytes are located in the sinoatrial that they form were traditionally called “Pur- 304 MUSCULAR TISSUE mixture of cellular elements. Transitional my¬ ocytes extend from the node into its initial portion, but more distally, Purkinje myocytes predominate. The common bundle and its right and left bundle branches are ensheathed by a layer of connective tissue that appears to insulate the conducting tissue from the sur¬ rounding cardiac muscle, but where the con¬ duction system terminates in profuse suben¬ docardial plexuses, functional contacts between Purkinje cells and the ventricular myocardium are common. Lesions of the conduction system may cause asynchrony in the beating of the ventricles or disorders in the timing of atrial and ventricu¬ lar contraction that result in impaired effi¬ ciency of the heart. INNERVATION OF THE MYOCARDIUM Although the initiation of each heartbeat is myogenic, the heart is innervated and its rate is modulated by the autonomic nervous sys¬ tem. Parasympathetic nerve fibers from the vagus and fibers from the sympathetic trunk form extensive plexuses at the base of the Figure 10-51. Photomicrograph of the specialized conduc¬ heart. Ganglion cells and numerous nerve ax¬ tion tissue of the human atrioventricular bundle. The large Purkinje fibers seen in cross section, at the left of the figure, ons are found in the wall of the right atrium, can be compared with the smaller unspecialized heart mus¬ especially in the regions of the sinoatrial and cle cut longitudinally at the right. atrioventricular nodes. The heart rate is slowed by stimulation of the vagus and accel¬ erated by sympathetic nerve stimulation. The kinje fibers” before their multicellular nature autonomic nervous system acts on the myocar¬ was revealed by the electron microscope. Pur¬ dium indirectly by modifying the inherent kinje myocytes are relatively short (—50 /zm) rhythm of the pacemaker. compared to ordinary myocytes (—80 p,m) but Light and electron microscopic observa¬ are nearly twice their diameter (30 p,m)(Figs. tions confirm the presence of many unmyelin¬ 10-51 and 10—52). In cross section, they are ated axons among the specialized myocytes of often quite irregular in outline with one cell the nodes and conduction pathways (Fig. 10— partially surrounding another or extending 53). The nerves do not form specialized end¬ large processes into conforming concavities in ings comparable to the myoneural junctions the neighboring cell (Figs. 10-53 and 10-54). of skeletal muscle but merely pass close to the Their irregular shape increases the area of specialized myocytes. They are identifiable as cell-to-cell contact. Intercalated discs are not functional endings only by the presence of found but there are large gap junctions both local aggregations of small vesicles identical at the ends and sides of the cells. Their ultra¬ to those found in synapses elsewhere in the structure and membrane properties favor body. Dense-cored vesicles, found in some of rapid impulse conduction. In the “Purkinje these axons, identify them as sympathetic fibers” of the bovine heart, which are excep¬ nerve endings. Similar endings occasionally tionally large and have been well studied, the observed in close relation to ordinary working conduction velocity is said to be 2-3 m/s com¬ myocytes suggest that there may also be a di¬ pared to 0.6 m/s in the unspecialized myo¬ rect action of the nerves on the myocardium, cardium. but this has yet to be convincingly established The atrioventricular bundle contains an ad¬ in physiological studies. Figure 10-52. Photomicrographs of the very large Purkinje fibers in the moderator band of the bovine heart. In the figure at the left, the fibers are cut longitudinally and in the figure at the right they are cut transversely. In both, it is evident that the myofilaments occupy only a small part of the sarcoplasm. The large clear areas are rich in glycogen, not stained here. Figure 10-53. Micrograph of portions of two adjacent Purkinje fibers and an accompanying nerve from the atrioventricular bundle of the cat heart. Mitochondria are abundant and myofilaments occur only in scattered bundles. 305