Nonuniform changes in fibre types in the soleus muscle of the developing rat

Age-dependent mechanical and microstructural properties of the rabbit soleus muscle

During growth there are serious changes in the skeletal muscles to compensate for the changed requirements in terms of body weight and size. In this study, the age-dependent (between 21 and 100 days) mechanical and microstructural properties of rabbit soleus muscle tissue were investigated. For this purpose, morphological properties (animal mass, soleus muscle mass, tibial length) were measured at 5 different times during aging. On the other hand, fibre orientation-dependent axial and semi-confined compression experiments were realised. In addition, the essential components (muscle fibres, extracellular matrix, remaining components), dominating the microstructure of muscle tissue, were analysed. While the mechanical results show hardly any age-dependent differences, the morphological and microstructural results show clear age-dependent differences. All morphological parameters increase significantly (animal mass by 839.2%, muscle mass 1050.6%, tibial length 233.6%). In contrast, microstructural parameters change differently. The percentage of fibres (divided into slow-twitch (ST) and fast-twitch (FT) fibres) increases significantly (137.6%), while the proportions of the extracellular matrix and the remaining components (48.2% and 46.1%) decrease. At the same time, the cross-sectional area of the fibres increases significantly (697.9%). The totality of this age-dependent information provides a deeper understanding of age-related changes in muscle structure and function and may contribute to successful development and validation of growth models in the future. STATEMENT OF SIGNIFICANCE: This article reports the first comprehensive data set on age-dependent morphological (animal mass, soleus muscle mass, tibial length), mechanical (axial and semi-confined compression), and microstructural (muscle fibres, extracellular matrix, remaining components) properties of the rabbit soleus muscle. On the one hand, the results of this study contribute to the understanding of muscle mechanics and thus to understanding of load transfer mechanisms inside the muscle tissue during growth. On the other hand, these results are relevant to the fields of constitutive formulation of age-dependent muscle tissue.

Consequences of Maternal Essential Fatty Acid and Conjugated Linoleic Acid Supplementation on the Development of Calf Muscle and Adipose Tissue

Common silage and concentrate-based diets in dairy and beef production may deliver insufficient amounts of essential fatty acids (EFA), thereby also reducing conjugated linoleic acids (CLA) in body tissues and milk. An impaired maternal EFA and CLA supply can have an important impact on calf postnatal development. The current study investigates how maternal supplementation with EFA and CLA affects muscle and adipose tissue development in neonatal calves. Holstein cows (n = 40) were abomasaly supplemented with coconut oil (control), CLA or EFA, or both combined during the transition period. Calves were fed their dam's colostrum until slaughter at day 5 of life. Fatty acid composition and tissue morphology were analyzed. In muscle and adipose tissues, EFA, CLA, and metabolites were elevated, indicating the effective transfer of maternally-supplemented FA to the offspring. Muscle fiber types, fiber nuclei, myosin heavy chain isoform distribution, capillarization, and fat cell size of intramuscular and other adipose tissues did not differ among groups. The results confirm that maternal nutrition during the transition period can alter the FA composition of the calf tissues. This could influence the offspring's development and health in the long-term, even though only minor effects were observed in the neonatal calves' tissue morphology.

Mixing it up: the biological significance of hybrid skeletal muscle fibers

Skeletal muscle fibers are classified according to the myosin heavy chain (MHC) isoforms and other myofibrillar proteins expressed within these cells. In addition to 'pure' fibers expressing single MHC isoforms, many fibers are 'hybrids' that co-express two or more different isoforms of MHC or other myofibrillar proteins. Although hybrid fibers have been recognized by muscle biologists for more than three decades, uncertainty persists about their prevalence in normal muscles, their role in fiber-type transitions, and what they might tell us about fiber-type regulation at the cellular and molecular levels. This Review summarizes current knowledge on the relative abundance of hybrid fibers in a variety of muscles from different species. Data from more than 150 muscles from 39 species demonstrate that hybrid fibers are common, frequently representing 25% or more of the fibers in normal muscles. Hybrid fibers appear to have two main roles: (1) they function as intermediates during the fiber-type transitions associated with skeletal muscle development, adaptation to exercise and aging; and (2) they provide a functional continuum of fiber phenotypes, as they possess physiological properties that are intermediate to those of pure fiber types. One aspect of hybrid fibers that is not widely recognized is that fiber-type asymmetries - such as dramatic differences in the MHC composition along the length of single fibers - appear to be a common aspect of many fibers. The final section of this Review examines the possible role of differential activities of nuclei in different myonuclear domains in establishing fiber-type asymmetries.

Age-associated changes of the intrinsic nervous system in relation with interstitial cells in the pre-weaning goat rumen

In this study, we investigated the neural changes and their relationships with interstitial cells (ICs) in the rumen of pre-weaning goats by transmission electron microscopy, western blot and immunofluorescence (antibody: general neuronal marker-Protein Gene Product (PGP9.5)/ IC marker-vimentin). The immunofluorescence results showed that PGP9.5-positive reaction was widely distributed in neuronal soma (NS) and nerve fibre (NF). The NSs were observed in the ganglia of the myenteric plexus (MP) but not in the submucosal plexus. The mean optical density (MOD) of the whole of PGP9.5-positive nerves and the protein expression level of PGP.5 in the rumen wall both decreased significantly with age. However an obvious increase MOD of PGP.5-positive NFs within the rumen epithelium were observed. In the MP, the nerves and ICs were interwoven to form two complex networks that gradually tightened with age. Furthermore, NSs and nerve trunks were surrounded by a ring-boundary layer consisting of several ICs that became physically closer with aging. Moreover, ICs were located nearby NFs within the ML, forming connections between ICs, smooth muscle cells and axons. This study describes the pattern of neural distribution and its association with ICs in the developing rumen which shed light on the postpartum development of ruminants.

A continuum of myofibers in adult rabbit extraocular muscle: force, shortening velocity, and patterns of myosin heavy chain colocalization

Extraocular muscle (EOM) myofibers do not fit the traditional fiber typing classifications normally used in noncranial skeletal muscle, in part, due to the complexity of their individual myofibers. With single skinned myofibers isolated from rectus muscles of normal adult rabbits, force and shortening velocity were determined for 220 fibers. Each fiber was examined for myosin heavy chain (MyHC) isoform composition by densitometric analysis of electrophoresis gels. Rectus muscle serial sections were examined for coexpression of eight MyHC isoforms. A continuum was seen in single myofiber shortening velocities as well as force generation, both in absolute force (g) and specific tension (kN/m(2)). Shortening velocity correlated with MyHCIIB, IIA, and I content, the more abundant MyHC isoforms expressed within individual myofibers. Importantly, single fibers with similar or identical shortening velocities expressed significantly different ratios of MyHC isoforms. The vast majority of myofibers in both the orbital and global layers expressed more than one MyHC isoform, with up to six isoforms in single fiber segments. MyHC expression varied significantly and unpredictably along the length of single myofibers. Thus EOM myofibers represent a continuum in their histological and physiological characteristics. This continuum would facilitate fine motor control of eye position, speed, and direction of movement in all positions of gaze and with all types of eye movements-from slow vergence movements to fast saccades. To fully understand how the brain controls eye position and movements, it is critical that this significant EOM myofiber heterogeneity be integrated into hypotheses of oculomotor control.

Long-term reinnervation effects after sciatic nerve lesions in adult rats

Transection of the sciatic nerve in adult rats induces drastic changes in hindleg muscles. Earlier, we demonstrated that the reinnervated soleus (SOL) muscle, 21 weeks after a transection mainly contains type II fibers. This is in striking contrast to normal muscle, which consists to 80% of type I muscle fibers. Also we observed 13.9% of the fibers to be polyneurally innervated. The problem of the present study is whether these changes are reversible after Longer survival periods. Therefore, the SOL was studied 60 weeks after transection and reconstruction by an autologous nerve graft. In six rats, we studied muscle fiber distributions by monoclonal antibodies, and innervation patterns by cholinesterase staining and AgNO3 impregnation. Still at 60 weeks, only 20% of the muscle fibers are of type I and this is similar to results at 21 weeks, indicating that no recovery to the normal has been reached by that age. Furthermore, 20% of the endplates in the reinnervated SOL were polyneurally innervated, but we also observed this in 10% of the endplates on the control side. These increases, compared to data at 21 weeks, are interpreted as an aging effect.

Proximo-distal organization and fibre type regionalization in rat hindlimb muscles

Five muscles of the rat's lower hindlimb were compared with regard to their histochemical fibre type distribution at seven different proximo-distal levels. The muscles were: extensor digitorum longus (ED), flexor digitorum and hallucis longus (FD), gastrocnemius medialis (GM), peroneus longus (PE) and tibialis anterior (TA). In all the five muscles, the relative density of the 'slow' type I fibres showed a striking and similar decrease from proximal toward more distal levels. In addition, the type I fibres were concentrated within smaller and more eccentrically placed regions at distal than at more proximal levels. As a background for the further analysis of these lengthwise aspects of type I fibre regionalization, architectural features of the muscles were determined. Pinnation angles and the position of major tendons and tendon sheets were assessed in fresh specimens. Muscle fibre lengths were measured for single fibres dissected from different regions of macerated muscles. In all cases, fibre length was much shorter than muscle length (mean fraction ranging from 21 to 55%), implying that the proximo-distal changes in histochemical fibre properties were indeed explainable as being due to gradual lengthwise changes in fibre type populations. The similarity of these lengthwise changes across the muscles was in contrast to their differences in other aspects of functional organization, such as the average density of type I fibres and architectural features determining their relative capacities for shortening and force generation. The possible functional role of the proximal accumulation of type I fibres was discussed in relation to circulation and thermal balance; besides, the proximo-distal regionalization of type I fibres might (partly) reflect processes that had been associated with early stages of muscle differentiation. Furthermore, the results underline that, when determining the fibre type composition of rat hindlimb muscles, identifying the proximo-distal level of sampling is a matter of great importance.

Complex three-dimensional patterns of myosin isoform expression: differences between and within specific extraocular muscles

Because complex structural differences in adult extraocular muscles may have physiological and pathophysiological significance, the three-dimensional pattern of myosin heavy chain (MHC) isoform expression within the orbital and global layers of the muscle bellies compared with the distal tendon ends was quantitatively assessed. Three of the six extraocular muscles of adult rabbits were examined for immunohistologic expression of all fast, fast IIA/X, slow, neonatal and developmental MHC isoforms. The percentages of myofibers positive for each of these 5 myosin isoforms were determined in the orbital and global layers. There were relatively similar patterns of fast and slow MHC expression in the orbital and global layers of each of the three muscles examined. There were high levels of developmental MHC in the orbital layers, but significantly fewer developmental MHC positive myofibers in the global layer. The most variable expression was found with the neonatal MHC. There were significant differences between the longitudinal expression of the various isoforms in the middle of each muscle compared with the tendon end. In the orbital layer of all three muscles examined, the large numbers of fibers positive for fast MHC in the middle of the muscle dramatically decreased at the tendon end, with a concomitant increase in expression of slow myosin. There was a greater number of developmental MHC-positive myofibers at the tendon end than in the middle of the muscle in all three muscles examined. In the global layer, the IIA/X-positive myofibers comprised only half of the total number of fast-positive myofibers whereas in the orbital layer they comprised all or almost all of the fast positive myofibers. The configuration of the extraocular muscles is more complex than might be indicated by previous studies. The lateral rectus muscle had the most individual pattern of MHC expression when compared with the inferior rectus and inferior oblique muscles. Together with dramatic cross-sectional MHC fiber type differences between the orbital and global layers of the muscles, there are pronounced longitudinal differences in the proportions of myofibers expressing these five MHC isoforms in the middle region of the muscles and those in the distal tendon ends. This longitudinal progression appears to occur both within single myofibers, as well as within the series of myofibers that comprise the length of the muscle. We also confirm that the number of myofibers is reduced at the tendonous end while the cross-sectional area of each of the remaining myofibers is proportionally increased with regard to those in the muscle belly. Future studies may yet require two additional schemes for anatomic classification of the named extraocular muscles. One will be based on immunohistochemical features of their constituent myofibers as a supplement to classifications based on their electron microscopic appearance, innervation patterns or relative position with regard to the globe and orbit. Another will be based on the proportional length and longitudinal position of individual myofibers within an individual extraocular muscle.

Regional variations of muscle fibre characteristic in m. semitendinosus of growing cattle

The objectives of the present study were to elaborate an intra-muscular profile of metabolic enzyme equipment, contractile and morphometric features along the longitudinal axis of m. semitendinosus at various ages throughout the growth phase. Thirty-seven male Montbéliard cattle, about half of them castrated, were representatively allocated to various slaughter dates, scheduled at 4, 8, 12 and 16 months of age. Samples were collected from proximal, medial and distal locations of m. semitendinosus. Isocitrate dehydrogenase (aerobic metabolism) and lactate dehydrogenase (anaerobic metabolism) were measured spectrophotometrically. Contractile muscle type was classified by quantification of myosin heavy chain I isoform proportion using the sensitive enzyme-linked immunosorbent assay. Mean muscle fibre area was obtained on histologically-stained cross-sections utilizing an image analysis system. Our results indicate the existence of a regular intra-muscular pattern of muscle fibre traits along the length of m. semitendinosus, with decreasing glycolytic activities and concomitantly an increase in oxidative capacity towards the distal extremity. The metabolic characteristics were in good agreement with decreasing cross-sectional muscle fibre areas and the slow myosin heavy chain I isoform proportion becoming gradually more abundant from proximal to distal regions of the muscle. Moreover, the observed gradient was found to be closely related to age and diminished with advanced physiological maturity. At the final slaughter age (16 months) no differences among the distinct portions were detected, m. semitendinosus was longitudinally homogeneous in all the characteristics studied.

Are region-specific changes in fibre types attributable to nonuniform muscle hypertrophy by overloading?

Muscle fibre composition was compared among the proximal (25%), middle (50%) and distal (75%) regions of the muscle length to investigate whether compensatory overload by removal of synergists induces region-specific changes of fibre types in rat soleus and plantaris muscles. In addition, we evaluated fibre cross-sectional area in each region to examine whether fibre recruitment pattern against functional overload is nonuniform in different regions. Increases in muscle mass and fibre area confirmed a significant hypertrophic response in the overloaded soleus and plantaris muscles. Overloading increased the percentage of type I fibres in both muscles and that of type IIA fibres in the plantaris muscle, with the greater changes being found in the middle and distal regions. The percentage of type I fibres in the proximal region was higher than that of the other regions in the control soleus muscle. In the control plantaris muscle, the percentage of type I and IIA fibres in the middle region were higher than that of the proximal and distal regions. With regard to fibre size, type IIB fibre area of the middle and distal regions in the plantaris increased by 51% and 57%, respectively, with the greater changes than that of the proximal region (37%) after overloading. These findings suggest that compensatory overload promoted transformation of type II fibres into type I fibres in rat soleus and plantaris muscles, with the greater changes being found in the middle and distal regions of the plantaris muscle.