Spatial distribution of fiber types in skeletal muscle: test for a random distribution

New tools for the investigation of muscle fiber-type spatial distributions across histological sections

BACKGROUND

The functional and metabolic properties of skeletal muscles are partly a function of the spatial arrangement of fibers across the muscle belly. Many muscles feature a non-uniform spatial pattern of fiber types, and alterations to the arrangement can reflect age or disease and correlate with changes in muscle mass and strength. Despite the significance of this event, descriptions of spatial fiber-type distributions across a muscle section are mainly provided qualitatively, by eye. Whilst several quantitative methods have been proposed, difficulties in implementation have meant that robust statistical analysis of fiber type distributions has not yielded new insight into the biological processes that drive the age- or disease-related changes in fiber type distributions.

METHODS

We review currently available approaches for analysis of data reporting fast/slow fiber type distributions on muscle sections before proposing a new method based on a generalized additive model. We compare current approaches with our new method by analysis of sections of three mouse soleus muscles that exhibit visibly different spatial fiber patterns, and we also apply our model to a dataset representing the fiber type proportions and distributions of the mouse tibialis anterior.

RESULTS

We highlight how current methods can lead to differing interpretations when applied to the same dataset and demonstrate how our new method is the first to permit location-based estimation of fiber-type probabilities, in turn enabling useful graphical representation.

CONCLUSIONS

We present an open-access online application that implements current methods as well as our new method and which aids the interpretation of a variety of statistical tools for the spatial analysis of muscle fiber distributions.

Sprouting capacity of lumbar motoneurons in normal and hemisected spinal cords of the rat

Nerve sprouting to reinnervate partially denervated muscles is important in several disease and injury states. To examine the effectiveness of sprouting of active and inactive motor units (MUs) and the basis for a limit to sprouting, one of three rat lumbar spinal roots was cut under normal conditions and when the spinal cord was hemisected at T12. Muscle and MU isometric contractile forces were recorded and muscle fibres in glycogen-depleted single muscle units enumerated 23 to 380 days after surgery. Enlargement of intact MUs by sprouting was effective in compensating for up to 80% loss of innervation. For injuries that removed >70-80% of the intact MUs, muscle contractile force and weight dropped sharply. For partial denervation of <70%, all MUs increased contractile force by the same factor in both normally active muscles and muscles whose activity was reduced by T12 hemisection. Direct measurements of MU size by counting glycogen-depleted muscle fibres in physiologically and histochemically defined muscle units, provided direct evidence for a limit in MU size, whether or not the activity of the muscles was reduced by spinal cord hemisection. Analysis of spatial distribution of muscle fibres within the outer boundaries of the muscle unit demonstrated a progressive increase in fibres within the territory to the limit of sprouting when most of the muscle unit fibres were adjacent to each other. We conclude that the upper limit of MU enlargement may be explained by the reinnervation of denervated muscle fibres by axon sprouts within the spatial territory of the muscle unit, formerly distributed in a mosaic pattern.

Spatial distribution of fiber types within skeletal muscle fascicles from Standardbred horses

Skeletal muscle fascicles from superficial and deep portions of semitendinosus (ST) and gluteus medius (GM) muscles from Standardbred trotters were analyzed with regard to muscle fiber type proportion (types I, IIA, and IIB) and spatial distribution. Muscle fibers within a fascicle were divided into four layers (L(1-4)) from the fascicle periphery toward the center. The observed proportions of fiber types among layers were found to be statistically significantly different from a random distribution of fiber types. Type IIB fibers predominated in the peripheral layer, type I fibers prevailed in the layer underneath, and proportions close to the mean of the whole fascicles were observed in the central layer. This pattern of spatial distribution of fiber types within the layers of the fascicles was observed at all four muscle sampling sites. The functional significance of the common pattern is unknown, but possible functional roles are discussed.

Spatial patterns of atrophied muscle fibers during exercised recovery

The effect of run training during the recovery period on the spatial distributions of fiber type was examined in atrophic soleus muscle of adult rats following 28 days of hindlimb suspension. During recovery, clusters of damaged and type IIC fibers were observed, which were more pronounced in the exercised animals than in both exercised and nonexercised control groups. The results indicate that exercise during recovery following suspension-induced hindlimb muscle atrophy produces changes in the soleus fiber-type cross-sectional area, both absolute and relative. These changes were not seen in the sedentary recovery group or in control rats exposed to the same exercise regimen. The author concludes that this treatment, unlike neurogenic pathologies, does not cause any remodeling during recovery, in the sense of changed adjacency relations among fiber types.

A fractal characterization of the type II fiber distribution in the extensor digitorum longus and soleus muscles of the adult rat

A method is proposed for the quantitative characterization of fiber type spatial distribution by means of the (correlation) fractal dimension. The method is applied to type II fiber distributions of the soleus and extensor digitorum longus muscles of the adult rat. The results obtained suggest that these distributions have fractal properties with a strong tendency for spreading, more pronounced in soleus muscle. The density of muscle fibers or the age do not seem to alter the features of the distribution. Computer-generated random patterns have virtually the same fractal dimension as the extensor digitorum longus distributions but fail to approximate those of the soleus muscle. This fractal method could find application as an alternative in the quantitative assessment of the fiber type grouping.

Spatial distribution of muscle fibers

The analysis of Gates and Betz (1993 Anat. Rec., 236:381-389) of the spatial distribution of two different populations of muscle fibers in the lumbrical muscle of the rat is discussed. The conclusion that muscle fibers belonging to a single motor unit are distributed at random in most muscles appears to need reconsideration. A new technique is applied to some examples to show that the amount of clustering of fiber types in a muscle cross section can be quantified.

Spatial patterns of fiber types in atrophied skeletal muscle

This study examined the spatial distributions of different fiber types in the soleus muscle of control rats and in rats subjected to hindlimb unloading for 28 days. The frequencies with which muscle fibers of one type were adjacent to each other and to fibers of other types were tabulated and compared to expectations generated from Monte Carlo simulations. In the normal rat, there is a tendency for Type I fibers to avoid adjacency with each other, a tendency that persisted in the hindlimb-suspended group, despite the substantial shrinkage in size of Type I fibers. We conclude that this treatment, unlike neurogenic pathologies, does not cause any remodeling of the adjacency relations of fibers.

Automated cytometry of fibre size and spatial distribution in the superficial masseter muscle of the rat at three ages

An image cytometry program was applied to sections of the superficial masseter muscle of female and male 21-, 42- and 105-day-old rats. Lesser diameter and spatial distribution of more than 6000 muscle fibres were automatically measured in digital images from muscle sections stained for myofibrillar ATPase activity. In this muscle, only type 2A, 2B and 2C fibres were observed. At the three ages and in both sexes, 2A fibres were found to have the largest diameter and were the most frequent (> 54%). In the 21-day-old animals, females presented larger diameters than did males; in contrast, in the 105-day-old animals, the three fibre types were larger in males than in females. At all ages and in both sexes, type 2A occupied 32 to 80% more cross-sectional area than type 2B. Most images (98%) showed a random spatial distribution of their fibre types. All fibre types grew in diameter with age. The coefficient of variation of the diameter was age-independent and remained under 23%. The finding of an age-independent variable could have a practical application: an increase of the coefficient of variation (> 25%) can be considered as pathological, even without a perfect age-matched control.

A quantitative histochemical study of the spatial distribution of intrafascicular fibre types in the porcine masseter and soleus muscles

The distribution of intrafascicular type I and II muscle fibres together with proportions of edge- and centrally located type I and II fibres within whole fascicles were analysed by myosin ATP-ase histochemistry in 241 porcine masseter fascicles (six masseter muscles) and compared with result from 63 pig soleus fascicles (five soleus muscles). All fascicles were from 11 domestic pigs (1 yr old, 70-90 kg body weight, all female). The proportions of type I fibres (slow) and type II fibres (fast) on the edge of fascicles differed significantly from the proportions centrally. All the soleus fascicles had higher proportions of centrally located type I fibres. Only seven out of 241 (3%) masseter fascicles diverged in this respect and showed reversed intrafascicular fibre-type proportions with more edge-located type I fibres. Analysis of the fascicular distribution of type I and II fibres revealed that the porcine masseter had type II fibres as the predominant type. Between 68-87% of the total fibres were type II (p < 0.001). The intrafascicular content of type I fibres increased towards the deep part of the masseter. In four of five soleus muscles the type II fibre population was dominant (p < 0.01). However, one soleus revealed equal proportions of type I and II fibres.

Spatial arrangement and metabolic capacity of fiber types in self-reinnervated cat muscle

The recovery potential of skeletal muscle was explored by examining cat muscle between 10 and 33 mo after complete transection and immediate surgical reunion of its own nerve. Biochemical analysis of single muscle fibers showed that the activities of key enzymes in energy metabolism (malate and lactate dehydrogenase and adenylokinase) were similar to normal for their respective fiber types, suggesting that incomplete recovery of the ability to sustain submaximal contraction in reinnervated muscles (T.C. Cope, C.B. Webb, and B.R. Botterman. J. Neurophysiol. 65: 648-656, 1991) is explained in some other way. Two independent statistical procedures for assessing the randomness of adjacencies of histochemically identified fiber types showed type grouping in some areas, but there were also many regions with randomly distributed fiber types. These findings demonstrate the potential for substantial recovery of both energy metabolism and dispersion of fiber types after self-reinnervation.

An augmented computer model of motor unit reorganization in neurogenic diseases of skeletal muscle

A computer model of denervation and complete reinnervation in skeletal muscle was originally developed for the purpose of furthering an understanding of the underlying mechanisms of motor unit reorganization in neurogenic diseases. We now describe its successor, a computer model for investigating different rates of denervation and reinnervation, as well as incomplete reinnervation. The new model introduces the concept of permanent denervation and features enhanced interactive control over the distribution of motor unit centers and additional measures of dispersion and co-dispersion of muscle fibers. The use of this model for investigating pathophysiologically significant issues in denervating diseases is illustrated with five different sets of parameters. These simulate some of the processes that may be operational in chronic spinal muscular atrophy, amyotrophic lateral sclerosis, and progressive postpolio muscular dystrophy. The enhanced model will allow in-depth analysis of the influence of hypothesized pathophysiological processes on clinical, electrophysiological and pathological outcomes in human disease.

Determination of the early age of onset of equine recurrent laryngeal neuropathy. 1. Muscle pathology

The age of onset of equine recurrent laryngeal neuropathy has not been ascertained, although the clinical condition of left laryngeal hemiplegia ("roaring") has been recognized for centuries. The purpose of this study was to evaluate the laryngeal muscles of draft horse foals for the presence of fiber-type grouping, indicating denervation and reinnervation, and to determine if histological evidence of recurrent laryngeal neuropathy was present. Abductor and adductor laryngeal muscles from the left and right sides were collected immediately after euthanasia from male draft horse foals, six less than 2 weeks and four 6 months of age, and stained for myosin ATPase. A morphometric test was used to objectively evaluate several areas from each muscle for fiber-type grouping. Extensive fiber-type grouping which was characteristic of recurrent laryngeal neuropathy was found in one of the young foals and all of the older foals. Four of the young foals had some areas of fiber-type grouping suggestive of mild, early changes associated with recurrent laryngeal neuropathy. One of the young foals had no fiber-type grouping present in any of the laryngeal muscles evaluated. These findings suggest an early age of onset of recurrent laryngeal neuropathy.

Modeling fiber type grouping by a binary Markov random field

A new approach to the quantification of fiber type grouping is presented, in which the distribution of histochemical type in a muscle cross section is regarded as a realization of a binary Markov random field (BMRF). Methods for the estimation of the parameters of this model are discussed. The first order BMRF, which is used in this article, contains 2 parameters: alpha and beta. The parameter beta is of prime importance, as it is an interaction parameter which governs the degree of type grouping. The value of this parameter is estimated for 9 muscle biopsies. The interpretation of the results is discussed.

Spatial distribution of motor unit fibres in fast- and slow-twitch rat muscles with special reference to age

The spatial arrangement and morphometrical properties of the muscle fibres within single motor units (motor unit fibres) were studied in fast-twitch units of the tibialis anterior (TA) and in slow-twitch units of the soleus, using a computer-assisted model. The motor unit fibres were identified by the glycogen-depletion technique and the position of each fibre was defined by (x, y)-coordinates. The distance between each fibre and the nearest motor unit fibre (nearest-neighbour distance), and the distance between each fibre and each of the other fibres in the unit (interfibre distance), was calculated and plotted. Comparisons were made between young adult (3-6 months) and old (20-25 months) rats. In old animals, the motor units of TA and the soleus were larger (P less than 0.05 and P less than 0.01), contained an increased number of muscle fibres (P less than 0.01) and covered a larger portion of the muscle cross-section (P less than 0.01 and P less than 0.1). These changes indicate the presence of an age-related denervation-reinnervation process in both types of muscles. In the young adult group, the fast-twitch motor unit fibres of TA were non-randomly arranged (P less than 0.05-0.01) whereas the fibre arrangement within the slow-twitch motor units of the soleus was not significantly different from random. In old animals, the fibre arrangement was non-random in both fast- and slow-twitch motor units. In TA, the distribution of nearest-neighbour distances showed an increased (P less than 0.05) proportion of short distances in old age, whereas the distribution of interfibre distances was unchanged. In the soleus, the distribution of interfibre distances showed an age-related displacement to the left at short distances (P less than 0.05) and to the right at long distances (P less than 0.01), but the distribution of nearest-neighbour distances was not significantly altered. It is concluded that motor unit fibres are non-randomly arranged in the fast-twitch motor units studied and that a non-random rearrangement of motor unit fibres takes place in both fast- and slow-twitch units during the ageing process. This age-related rearrangement is secondary to a denervation-reinnervation process and it appears as if different types of reinnervation predominate in fast- and slow-twitch units.

Arrangement of fiber types within fascicles of human vastus lateralis muscle

A total of 106 fascicles at 6 predetermined areas of the vastus lateralis muscle from 9 healthy men, aged 18 to 40 years, were analyzed. Fibers in a fascicle were divided into layers according to their relation to the perimysium. In each layer the proportions of type 1 and 2, subdivided into 2a, 2b, and 2c fibers were determined and normalized by the fiber type proportion in the whole fascicle. A consistent arrangement of fiber types within the fascicles was obtained, regardless of subject, sampling site, fiber type proportion, and fascicle size. A high proportion of 2b fibers on the border, a prevalence of type 1 fibers in the layer beneath, and a rather uniform distribution of 2a fibers in all layers are the main characteristics of the distribution of fibers in a fascicle. Developmental processes in fiber type differentiation most probably constitute the basis for fiber type arrangement, which can further be influenced by local factors.