Lower capillarization, VEGF protein, and VEGF mRNA response to acute exercise in the vastus lateralis muscle of aged vs. young women.

In humans, the majority of studies demonstrate an age-associated reduction in the number of capillaries surrounding skeletal muscle fibers; however, recent reports in rats suggest that muscle capillarization is well maintained with advanced age. In sedentary and trained men, aging lowers the number of capillaries surrounding type II, but not type I, skeletal muscle fibers. The fiber type-specific effect of aging on muscle capillarization is unknown in women. Vascular endothelial growth factor (VEGF) is important in the basal maintenance of skeletal muscle capillarization, and lower VEGF expression is associated with increased age in nonskeletal muscle tissue of women. Compared with young women (YW), we hypothesized that aged women (AW) would demonstrate 1) lower muscle capillarization in a fiber type-specific manner and 2) lower VEGF and VEGF receptor expression at rest and in response to acute exercise. Nine sedentary AW (70 + 8 yr) and 11 YW (22 + 3 yr) had vastus lateralis muscle biopsies obtained before and at 4 h after a submaximal exercise bout for the measurement of morphometry and VEGF and VEGF receptor expression. In AW compared with YW, muscle capillary contacts were lower overall (YW: 2.36 + 0.32 capillaries; AW: 2.08 + 0.17 capillaries), specifically in type II (YW: 2.37 + 0.39 capillaries; AW: 1.91 + 0.36 capillaries) but not type I fibers (YW: 2.36 + 0.34 capillaries; AW: 2.26 + 0.24 capillaries). Muscle VEGF protein was 35% lower at rest, and the exercise-induced increase in VEGF mRNA was 50% lower in AW compared with YW. There was no effect of age on VEGF receptor expression. These results provide evidence that, in the vastus lateralis of women, 1) capillarization surrounding type II muscle fibers is lower in AW compared with YW and 2) resting VEGF protein and the VEGF mRNA response to exercise are lower in AW compared with YW.     

Capillarization of the myocardium of the yak

Organometric and histometric characteristics of various parts of the heart have been performed in the yak, a permanent inhabitant at altitudes of 3,000-5,000 m above the sea level. Forty hearts of mature animals have been studied. Morphometric investigations of capillaries in the muscle fibers have been carried out in the atrial walls, in the right and left auriculae. in the ventricles and in the papillary muscles. The data are presented for each cardiac part: they describe diameters, density and volume of the capillary network and muscle fibers. In order to estimate the degree of capillarization of the cardiac muscle tissue, a new parameter is suggested to be used, when analyzing the morphometric data. It characterizes ratio of capillaries volume. According to the criterion given, a significant difference is stated between the degree of capillarization in the myocardium of the right and left ventricles. We suppose that this adequately reflects an increased loading experienced by the right cardiac parts under a long-lasting effect of altitude factors.     

Interfiber tension transmission in series-fibered muscles of the cat hindlimb

Several muscles of the cat hindlimb, including biceps femoris and tenuissimus, are composed of short, in-series muscle fibers with tapered intrafascicular terminations. Tension generation and transmission within such muscles requires that active fibers should be mechanically coupled in series via myomyous junctions, specialized connective tissue attachments, or the endomysium. This report establishes that the tapered fibers of the cat biceps femoris and tenuissimus muscles have insignificant numbers of either myomyous or specialized connective tissue junctions. Tension appears to be transmitted in a distributed manner across the plasmalemma of the tapered (and probably the non-tapered) portions of the fibers to the connective tissue of the endomysium, which is therefore an essential series elastic element in these muscles. Subplasmalemmal dense plaques were identified and may play a role in transmembrane force transmission. In addition to the endomysium, passive muscle fibers may also serve to transmit tension between active fibers, and therefore should also be considered to be series elastic elements.     

The arrangement and function of octopus arm musculature and connective tissue

The morphology of the musculature and connective tissues of the arms of Octopus bimaculoides was analyzed with light microscopy. We also studied O. briareus and O. digueti, which possess relatively more elongate and less elongate arms, respectively. The morphology of the arms was found to be remarkably uniform among species. The arms consist of a densely packed three-dimensional arrangement of muscle fibers and connective tissue fibers surrounding a central axial nerve cord. Three primary muscle fiber orientations were observed: 1) transverse muscle fibers oriented in planes perpendicular to the long axis of the arm; 2) longitudinal muscle fibers oriented parallel to the long axis; and 3) oblique muscle fibers arranged in helixes around the arm. The proportion of the arm cross section occupied by each of these muscle fiber groups (relative to the total cross sectional area of the musculature) remains constant along the length of the arm, even though the arm tapers from base to tip. A thin circular muscle layer wraps the arm musculature on the aboral side only. Much of this musculature has its origin and insertion on several robust connective tissue sheets including a layer surrounding the axial nerve cord and crossed-fiber connective tissue sheets located on the oral and the aboral sides of the arm. An additional thin layer of connective tissue wraps the arm musculature laterally and also serves as a site of origin and insertion of some of the muscle fibers. The fibers of the oral and aboral crossed-fiber connective tissue sheets are arranged oblique to the long axis of the arm with the same fiber angle as the oblique muscle layers that originate and insert on the sheets. The oblique muscle layers and the crossed-fiber connective tissue sheets thus form composite right- and left-handed helical fiber arrays. Analysis of arm morphology from the standpoint of biomechanics suggests that the transverse musculature is responsible for elongation of the arms, the longitudinal musculature is responsible for shortening, and the oblique muscle layers and associated connective tissues create torsion. Arm bending may involve unilateral contraction of longitudinal muscle bundles in combination with resistance to arm diameter increase due to contraction of the transverse musculature or passive stiffness of the arm tissues. The arms may also be bent by a combination of decrease in diameter due to contraction of the transverse musculature and maintenance of constant length on one side of the arm by unilateral activity of longitudinal muscle bundles. An increase in flexural stiffness of the arm may be achieved by cocontraction of the transverse and longitudinal muscle. Torsional stiffness may be increased by simultaneous contraction of both the right- and left-handed oblique muscle layers.     

The muscle layer of the canine gallbladder and cystic duct

The muscle layer of the canine gallbladder wall and cystic duct was found to be a three-dimensional meshwork of smooth muscle bundles which appear loosely and irregularly arranged on the mucosal aspect and consolidate to form a homogeneous plate-like layer on the serosal aspect. The muscle bundles are tightly woven around interspersed pockets of loose connective tissue in the gallbladder wall and gradually become loosely arranged with more prominent amounts of intervening connective tissue in the cystic duct. The muscle layer is thickest in the gallbladder wall and becomes progressively thinner out into the cystic duct. No anatomic sphincter was observed. Ultrastructural organization revealed individual muscle fibers to be of irregular profile, often branching, widely spaced with intervening collagen fibers, and having few cell-to-cell contacts.     

Architectural and histochemical analysis of the biceps brachii muscle of the horse.

The biceps brachii of horses is a complex muscle subdivided into two heads which may subserve distinct functions. The lateral head contains a large percentage of type I myofibers. This region is largely composed of short fibers (5-7 mm long) arranged in a pinnate fashion and heavily invested with connective tissue. The medial head contains fewer type I fibers and is composed of relatively longer myofibers (15-20 mm long), also arranged in a pinnate fashion but less heavily invested with connective tissue. It is hypothesized that the lateral muscle head of biceps brachii contributes to the postural role of the muscle in the forelimb passive stay apparatus. The medial head, with its longer fibers and generally fast fiber population may be most important during dynamic activity such as walking, trotting and running.     

Influence of swim training on cardiac activity,tissue capillarization,and mitochondrial density in muscle tissue of zebrafish larvae

Larval zebrafish (Danio rerio) of two different age classes ("swim-up" larvae, 9 days old; "free-swimming" larvae, 21 days old) were exposed to either an endurance/continuous training or interval training. Control animals were kept in stagnant water. A comparison of cardiac activity of trained (either endurance or interval) and untrained animals at the end of the training regime revealed no differences in heart rate, end-diastolic and end-systolic ventricular volume, and cardiac output. Training also had no influence on the concentration of erythrocytes in the blood. Thus, at the level of total oxygen transport in the blood, training did not provoke any improvement during the first 32 days of development. Significant changes, however, were observed at the tissue level. In free-swimming larvae [i.e., between 21 and 32 days postfertilization (dpf)] endurance training increased the capillarization of both axial muscle caudal to the anus and the tail fin. In addition, mitochondrial density of red and intermediate muscle fibers increased significantly. In contrast to capillarization, even swim-up larvae, trained between 9 and 15 dpf, were affected. The observed increase in mitochondrial content indicates a high demand for oxygen and energy-rich metabolites for oxidative phosphorylation. In older larvae, this is met by the increase in capillarization that improves the blood supply and with it the required oxygen and metabolite supply of muscle tissue. Both of these adaptational changes result in a reduction of diffusion distances (between capillary and muscle fiber as well as mitochondria) and may contribute to a higher resistance toward oxygen deficiency. Furthermore, this study indicates that plasticity of muscle tissue is already established in early stages of development at both the tissue and cellular levels.     

Influence of structure on the tissue dynamics of the human soleus muscle observed in MRI studies during isometric contractions

This article investigates how the internal structure of muscle and its relationship with tendon and even skeletal structures influence the translation of muscle fiber contractions into movement of a limb. Reconstructions of the anatomy of the human soleus muscle from the Visible Human Dataset (available from the National Library of Medicine), magnetic resonance images (MRI), and cadaver studies revealed a complex 3D connective tissue structure populated with pennate muscle fibers. The posterior aponeurosis and the median septum of the soleus form the insertion of the muscle and are continuous with the Achilles tendon. The distal extremities of the pennate muscle fibers attach to these structures. The anterior aponeurosis is located intramuscularly, between the posterior aponeurosis and the median septum. It forms the origin of the muscle and contacts the proximal extremities of the soleus muscle fibers. MRI measurements of in vivo tissue velocities during isometric contractions (20% and 40% maximum voluntary contractions) revealed a similarly complex 3D distribution of tissue movements. The distribution of velocities was similar to the distribution of major connective tissue structures within the muscle. During an isometric contraction, muscle fiber contractions move the median septum and posterior aponeurosis proximally, relative to the anterior aponeurosis. The pennate arrangement of muscle fibers probably amplifies muscle fiber length changes but not sufficiently to account for the twofold difference in muscle fiber length changes relative to excursion of the calcaneus. The discrepancy may be accounted for by an additional gain mechanism operating directly on the Achilles tendon by constraining the posterior movement of the tendon, which would otherwise occur due to the increasingly posterior location of the calcaneus in plantarflexeion.     

The microscopic morphology of orthotopic free muscle grafts in rabbits

The flexor digitorum superficialis muscle was free grafted (without neurovascular anastomoses) in 122 rabbit forelimbs. Histologic nd histochemical examinations through 6 months after grafting were performed. An early ischemic necrosis of the entire graft, except for a few percent of fibers at the very surface, was consistently seen. Subsequently, there occurred a regeneration of muscle with reconstitution of up to 100 percent of normal numbers of fibers. There was a wide variation in the numbers of fibers regenerated; however, the fiber-free areas were then being replaced by connective tissue. Muscle grafts in 1-month-old rabbits regenerated faster and yielded muscle with evidence of more extensive reinnervation and less connective tissue than 3-month-old animals. In the early postgraft period, minced grafts appeared to be as good as whole ones, but after 1 month, they developed far more connective tissue. Differentiation into fast-twitch and slow-twitch muscle fibers and into high- and low-oxidative fibers began at 2 to 3 weeks after grafting but was not extensive until 3 months. At 6 months, grafts showed areas of normal-appearing muscle interspersed with areas that lacked signs of reinnervation. The earliest sign of regeneration is the appearance of several very elongated nuclei encircling each previously anucleate necrotic muscle fiber. A small amount of basophilic cytoplasm then appears around each new nucleus. As blood vessels grow into the graft, a centripetal wave of phagocytosis is seen, taking 2 to 3 weeks and leaving a bed of immature muscle fibers. We believe this to be the first documentation of regeneration's commencing prior to and thus independently of phagocytosis.     

Fine structure of the nephridial muscle layer cells of Phascolosoma granulatum (Sipuncula)

The nephridial muscle layer of Phascolosoma granulatum consists of a network of longitudinal and circular cells separated by connective tissue matrix. The muscle fibers are densely packed with thick and thin myofilaments, among which are scattered cytoplasmic dense bodies. The nucleus and noncontractile cytoplasmic organelles occupy a lateral projection from the contractile portion of the fiber. Cytoplasmic dense bodies are the result of a clustering of an indeterminate number of the thin actin filaments that fill the cytoplasm between thick filaments. Attached to the cytoplasmic face of the cell membrane are membrane-associated electron-dense plaques. These sites are linked to the contractile myofilaments by narrow filamentous bridges. Extracellular narrow filaments extend from these plaques to collagen fibers of the connective tissue matrix. Differences in length of the dense plaques may be related to differences in thick myofilament diameter in three types of muscle fiber, types A, B and C, statistically distinguished by mean fiber size differences. The plaques may serve as connecting links for the transmission of tension from contractile units to the connective tissue of the muscle layer. © 1993 Wiley-Liss, Inc.     

Histological features of endomysium,perimysium and epimysium in rat lateral pterygoid muscle

The distribution of the endomysium, perimysium, and epimysium and their constituent connective tissue fiber types in the mature rat lateral pterygoid muscle was examined with the light microscope. The endomysium and perimysium were relatively thin and consisted mainly of reticular fibers. The epimysium was thicker than the intramuscular sheaths and consisted of both collagen and reticular fibers; however, the thickness and constituent connective tissue fiber types of these sheaths varied regionally. Near the articular capsule and disc, the endomysium, perimysium, and epimysium were all thicker than in other regions of the muscle and consisted of collagen, reticular, and elastic fibers. The perimysium bound the bundles of muscle fibers together and frequently included blood vessels and nerves. As the superior head of the pterygoid muscle approached its insertion, sheaths of perimysium divided this head into smaller and smaller bundles of muscle fibers. In the inferior head, some of the perimysial sheaths and part of the epimysium were aponeurotic, and many muscle fibers attached to them. There were few such aponeurotic regions in the superior head. © 1996 Wiley-Liss, Inc.     

Tenascin-Y, a component of distinctive connective tissues, supports muscle cell growth

Chicken tenascin-Y is an extracellular matrix protein most closely related to the mammalian tenascin-X. It is highly expressed in the connective tissue of skeletal muscle (C. Hagios, M. Koch, J. Spring, M. Chiquet, and R. Chiquet-Ehrismann, 1996, J. Cell Biol. 134, 1499-1512). Here we demonstrate the presence of tenascin-Y in specific areas of the connective tissues in developing lung, kidney, and skin. In skin tenascin-Y shows a complementary expression pattern to tenascin-C, whereas in the lung and kidney the sites of expression are partly overlapping. Tenascin-Y is also present in embryonic skeletal muscle where it is expressed in the developing connective tissue in between the muscle fibers. This connective tissue is also the major site of alpha5 integrin expression. We purified recombinantly expressed tenascin-Y and tested its effect on cell adhesion and its influence on muscle cell growth and differentiation. C2C12 myoblasts were able to adhere to tenascin-Y and showed extensive formation of actin-rich processes without generation of stress fibers. Furthermore, we found that tenascin-Y influenced cell morphology of chick embryo fibroblasts over prolonged times in culture and that it supports primary muscle cell growth and restricts muscle cell differentiation.     

The abdominal muscle receptor organ in Astacus leptodactylus (Crustacea)

The structure of both the slow- and the fast-adapting abdominal muscle receptor organ of Astacus leptodactylus is described with particular reference to differences between the two systems. The receptors are composed of a thin muscle that extends from the front edge of one segment to the front edge of the following and a sensory cell connected with this muscle. In the zone where the sensory cells enter their respective muscle, muscle fibers are reduced (zone of relative muscle exclusion = ZRME) and partly replaced by connective tissue. The occurrence of dendritic processes of both the slow and the fast neurons is confined to this zone. The following differences between the two receptor types are established: (1) The fast receptor muscle reveals a smaller sarcomere length than the slow receptor muscle and a higher myosin/actin filament ratio. (2) Muscle fibers that pass the ZRME are always found at its periphery in the fast system, separated from dendritic processes by layers of connective tissue, while in the slow system muscle fibers frequently are intermingled with the sensory elements. (3) The ZRME of the slow receptor is 20-30% longer than that of the fast receptor. (4) The dendritic varicosities of the slow neuron, on an average, contain many more mitochondria than those of the fast neuron. (5) Dendritic processes (fine twigs as well as varicosities) are juxtaposed to the sarcolemma of the muscle fibers only in the slow system; in the fast system dendrites and muscle are spatially separated by connective tissue. It is assumed that these differences between the two receptor types are at least in part responsible for the different thresholds observed in physiological experiments.     

Occurrence of a dense plexus of sensory nerve fibers immunoreactive to calcitonin-gene-related peptide in the cauda epididymidis of rats

The occurrence, distribution and ontogeny of nerves displaying calcitonin-gene-related peptide (CGRP)-like immunoreactivity were studied in the male reproductive tract of rats. A marked regional difference in number of CGRP-immunoreactive nerve fibers was observed in the epididymis. The immunoreactive nerve fibers were particularly numerous in the cauda epididymidis, where the nerves were found in the capsular and interstitial connective tissue and further in the smooth muscle layer and the subepithelial connective tissue surrounding the duct. In the remaining portions of the reproductive tract proximal and distal to the epididymis, CGRP-immunoreactive nerve fibers were scarcely found in the connective tissues surrounding the duct, although a small number of the CGRP-immunoreactive nerve fibers was constantly found adjacent to small blood vessels throughout the male reproductive tract. CGRP-immunoreactive nerve fibers in the epididymis were first detected at embryonic day 18 when thin bundles or single fibers were evenly distributed in the interstitial connective tissue of the entire epididymal duct. A marked regional difference in number of CGRP-immunoreactive nerve fibers seen in the adult epididymis was established by postnatal day 14. In the epididymis of young rats treated neonatally with capsaicin, CGRP-immunoreactive nerve fibers were almost completely absent. This finding together with the distribution pattern of CGRP-immunoreactive nerve fibers different from that of the autonomic nerves so far reported strongly suggests that the immunoreactive nerves were sensory in nature.     

Fine structure of the epineural connective tissue sheath of the subesophageal ganglion in Helix aspersa

Summary The epineural connective tissue sheath investing the subesophageal ganglion of Helix aspersa consists of a superficial region and a deeper region. The superficial region contains masses of globular cells intermingled with smooth muscle cells and nerve fibers all embedded in a connective tissue matrix. The histochemical and fine structural features of the globular cells show seasonal changes. During autumn to winter glycogen accumulates in their cytoplasm; this accumulation is accompanied by the appearance of dense, cytoplasmic globules which fuse together and ultimately form large pools of granular material. All the organelles and cytoplasm are displaced towards the cell periphery. Various cell-membrane invaginations containing dense material are prominent but there is no direct evidence to link these structures with the uptake of metabolites for glycogenesis. In winter there is a concentration of homogeneous, membrane-bound inclusions in the vicinity of the Golgi bodies. It is suggested that these inclusions constitute a lipid store. They decrease in number during summer. The globular cells do not bear any intimate relation to neurons and there is no reason to include these cells in the neuroglia. The muscle cells often weave around the globular cells but there is no direct contact. Nerve fibers innervate at least some of the muscle cells. The connective tissue consists of large and small diameter fibers suggesting that maturation of the fibrous components of the intercellular matrix is taking place in the superficial regions of the epineural sheath.This work has been supported by the Australian Research Grants Committee.     

Mechanical integration of muscle, tegument, and subtegumental tissues by anchoring fibrils and microfibrils in the cestode Hymenolepis diminuta

Electron microscopic examination of the epidermal basement membrane region of the rat tapeworm, Hymenolepis diminuta, has revealed specialized connective tissue structures that appear to anchor the epidermis, or tegument, to the parenchymal tissues of the helminth, as well as interconnect subtegumental muscle fibers, tegument, and parenchyma. Anchoring fibrils-cross-banded bundles of ca. 3 nm diameter filaments--were observed to directly interlink tegument and muscle, muscle and muscle, and tegument, muscle, and parenchymal connective tissue. Anchoring fibrils therefore appear to mechanically integrate epidermal tissue movements in response to subtegumental muscle contraction. A well-developed stratum of microfibrils, forming the lamina reticularis of the tegumental basement membrane, may also help anchor the tegument as well as to serve as a flexible, reinforcing sheath that protects parenchymal tissues from excessive radial displacement due to muscle contraction.     

In vitro attachment of skeletal muscle fibers to a collagen gel duplicates the structure of the myotendinous junction

The myotendinous junction (MTJ) and its associated cells and connective tissue are important structures involved in transmission of contractile force from skeletal muscle to tendon. A model culture system was developed to investigate the formation of the MTJ and its attachment to collagen fibers. Skeletal muscle cells were cultured in a well modeled from two layers of a native gel of type I collagen. Muscle cells cultured in this manner formed attachments to the collagen gel and developed into highly contractile multinucleated muscle fibers with the development of extensive terminal invaginations of the sarcolemma. In addition, the subsarcolemma at the ends of muscle fibers showed areas of increased electron density which corresponded well with the termini of myofibrils. The results indicate that the development of sarcolemmal invaginations at the end of a muscle fiber probably occurs intrinsically during muscle development in vivo. The direct association of collagen fibers with the basal lamina at the end of muscle fibers was only occasionally observed in culture, suggesting that other fibrils or proteins may also be involved in the attachment of collagen fibers to the basal lamina of muscle fibers at the MTJ.     

AUTORADIOGRAPHIC LOCALIZATION OF NEW RNA SYNTHESIS IN HYPERTROPHYING SKELETAL MUSCLE

Work-induced growth of rat soleus muscle is accompanied by an early increase in new RNA synthesis. To determine the cell type(s) responsible for the increased RNA synthesis, we compared light autoradiographs of control and hypertrophying muscles from rats injected with tritiated uridine 12, 24, and 48 h after inducing hypertrophy. There was an increased number of silver grains over autoradiographs of hypertrophied muscle. This increase occurred over connective tissue cells; there was no increase in the number of silver grains over the muscle fibers. Quantitative studies demonstrated that between 70 and 80% of the radioactivity in the muscle that survived fixation and washing was in RNA. Pretreatment of the animals with actinomycin D reduced in parallel both the radioactivity in RNA and the number of silver grains over autoradiographs. Proliferation of the connective tissue in hypertrophying muscle was evident in light micrographs, and electron micrographs identified the proliferating cells as enlarged fibroblasts and macrophages; the connective tissue cells remained after hypertrophy was completed. Thus, proliferating connective tissue cells are the major site of the increase in new RNA synthesis during acute work-induced growth of skeletal muscle. It is suggested that in the analysis of physiological adaptations of muscle, the connective tissue cells deserve consideration as a site of significant molecular activity.     

Ultrastructure of striated muscle fibers in the middle third of the human esophagus

Striated muscle fibers and their spatial relationship to smooth muscle cells have been studied in the middle third of human esophagus. Biopsies were obtained from 3 patients during surgery. In both the circular and longitudinal layers, the muscle coat of this transition zone was composed of fascicles of uniform dimension (100-200 microns of diameter); some of these bundles were made up of striated muscle fibers, others were pure bundles of smooth muscle cells and some were of the mixed type. Striated muscle fibers represented three different types, which were considered as intermediate, with certain structural features characteristic of the fast fiber type. Of these, the most frequently-found fibers were most similar to the fast fiber type. Satellite cells were numerous; in mixed fascicles they were gradually replaced by smooth muscle cells. The gap between striated muscle fiber and smooth muscle cells was more than 200 nm wide. It contained the respective basal laminae and a delicate layer of amorphous connective tissue. No specialized junctions were formed between consecutive striated muscle fibers, or between striated muscle fibers and smooth muscle cells. Interstitial cells of Cajal were never situated as close to striated muscle fibers as to smooth muscle cells.     

On the So-called Mantle Muscle Scars on Shells of the Margaritiferidae (Mollusca, Pelecypoda), with Observations on Mantle-Shell Attachment in the Unionoida and Trigonioida

Mantle attachment scars on the inner surface of shells of the Margaritiferidae have been traditionally regarded as sites of mantle muscle attachment; however, the actual occurrence of muscle tissue at points of attachment has never been verified. Mantle attachment scars occur on shells of seven examined species of margaritiferids. Gross and histological investigation of the mantle of five species reveals that mantle attachment involves modification of mantle epithelial cells and associated connective fibers within the mantle. The connective fibers within the mantle do not appear to have contractile properties, but along with the attachment cells probably provide support for the mantle. Mantle-shell attachment scars similar to those of margaritiferids also occur in shells of recent trigonids, thus strengthening arguments for a phylogentic link between the Unionoida and Trigonioida.