The pathology of the human muscle spindle. A light microscopic, biometric and histochemical study

Muscle spindles of the rat sternomastoid muscle

The sternomastoid (SM) muscle in rodents presents a peculiar distribution of fiber types with a steep gradient from the ventral, superficial, white portion to the dorsal, deep, red region, where muscle spindles are restricted. Cross section of the medial longitudinal third of the rat SM contains around 10,000 muscle fibers with a mean diameter of 51.28±12.62 (μm +/- SD). Transverse sections stained by Succinate Dehydrogenase (SDH) reaction clearly presents two distinct regions: the dorsal deep red portion encompassing a 40% cross section area contains a high percentage of packed SDH-positive muscle fibers, and the ventral superficial region which contains mainly SDH-negative muscle fibers. Indeed, the ventral superficial region of the rat SM muscle contains mainly fast 2B muscle fibers. These acidic ATPase pH 4.3-negative and SDH-negative 2B muscle fibers are the largest of the SM muscle, while the acidic ATPase pH 4.3-positive and SDH-positive Type 1 muscle fibers are the smallest. Here we show that in thin transverse cryosections only 2 or 3 muscle spindle are observed in the central part of the dorsal deep red portion of the SM muscle. Azan Mallory stained sections allow at the same time to count the spindles and to evaluate aging fibrosis of the skeletal muscle tissue. Though restricted in the muscle red region, SM spindles are embedded in perimysium, whose changes may influence their reflex activity. Our findings confirm that any comparisons of changes in number and percentage of muscle spindles and muscle fibers of the rat SM muscle will require morphometry of the whole muscle cross-section. Muscle biopsies of SM muscle from large mammals will only provide partial data on the size of the different types of muscle fibers biased by sampling. Nonetheless, histology of muscle tissue continue to provide practical and low-cost quantitative data to follow-up translational studies in rodents and beyond.

Muscle spindles exhibit core lesions and extensive degeneration of intrafusal fibers in the Ryr1(I4895T/wt) mouse model of core myopathy

Muscle spindles from the hind limb muscles of adult Ryr1(I4895T/wt) (IT/+) mice exhibit severe structural abnormalities. Up to 85% of the spindles are separated from skeletal muscle fascicles by a thick layer of connective tissue. Many intrafusal fibers exhibit degeneration, with Z-line streaming, compaction and collapse of myofibrillar bundles, mitochondrial clumping, nuclear shrinkage and pyknosis. The lesions resemble cores observed in the extrafusal myofibers of this animal model and of core myopathy patients. Spindle abnormalities precede those in extrafusal fibers, indicating that they are a primary pathological feature in this murine Ryr1-related core myopathy. Muscle spindle involvement, if confirmed for human core myopathy patients, would provide an explanation for an array of devastating clinical features characteristic of these diseases and provide novel insights into the pathology of RYR1-related myopathies.

Muscle spindle distribution, morphology, and density in longus colli and multifidus muscles of the cervical spine

STUDY DESIGN

Tissue blocks comprising muscle and bone from C5 to C7 segments were harvested at autopsy from 16 individuals ranging in age from 4 to 77 years. The prevertebral longus colli and postvertebral multifidus muscle pairs from one side in each individual were randomly selected for this study of muscle spindles.

OBJECTIVES

To determine muscle spindle distribution, morphology, and density for the longus colli and multifidus in caudal segments of the human cervical spine, and to assess whether changes are evident from infancy to old age.

SUMMARY OF BACKGROUND DATA

Age-related changes to the osteoligamentous framework of the cervical spine have been well documented. Postural modification accompanies these structural alterations, but there have been limited attempts to document whether muscle sustains a comparable level of morphologic alteration. Previous studies have examined muscle spindles in the neck muscles of various animal models and in a variety of isolated human muscles. However, most of these studies incurred bias through sampling and methodologic assumptions.

METHODS

The longus colli and multifidus were resected between C5 and C7, and between left and right pairs selected randomly for spindle analysis. These vertebral segments were selected deliberately because they form the apex of the cervical lordosis and the site at which the greatest age-related modification occurs. The tissue was processed in paraffin, sectioned, and then stained by Masson's trichrome. Spindle characteristics were examined using light microscopy and analyzed by unbiased stereologic methods. A one-sample paired t test was used to ascertain whether the differences in spindle density between the two muscles were statistically significant.

RESULTS

The longus colli has a high density of muscle spindles, which appear clustered and concentrated anterolaterally, away from the vertebral body. The multifidus has a low density of muscle spindles, which are found predominantly as single units concentrated closely to the vertebral lamina. No change in spindle distribution, morphology, and density were observed with age.

CONCLUSIONS

The current study examined spindle characteristics for an intrinsic neck muscle pair whose coactivation contributes to segmental stability of the cervical spine. The distribution and morphology of muscle spindles differ between the longus colli and the multifidus. In addition, these muscles have significant differences in terms of mean spindle density. Spindle characteristics represent one of many factors that govern proprioceptive regulation in skeletal muscle, and in neck muscles, the central connectivity of these receptors remains undefined. Therefore, although there are anatomic differences between the neck flexor and extensor, the functional implications of these differences are not clear. It is also of interest that spindle characteristics remain unchanged in these intrinsic muscles whose underlying segments are subject to age-related osteoligamentous changes.

Myopathy with muscle spindle excess: A new congenital neuromuscular syndrome?

An infant presented with congenital weakness, hypotonia, arthrogryposis, atrial tachycardia, and a left intra-abdominal neuroblastoma. Muscle biopsy revealed marked excess of muscle spindles with atrophy of extrafusal fibers. The patient expired at age 14 months from progressive cardiorespiratory failure. Postmortem examination demonstrated muscle-spindle excess in other muscles, along with hypertrophic obstructive cardiomyopathy and organomegaly. Muscle spindle excess has previously been reported in two patients with Noonan syndrome and progressive hypertrophic cardiomyopathy. Muscle spindle excess with hypertrophic cardiomyopathy, organomegaly, and, possibly, congenital neuroblastoma suggests a syndromic association and may represent an unusual form of congenital myopathy.

The effect of tenotomy and immobilization on muscle spindles and tendon organs of the rat calf muscles. A histochemical and morphometrical study

The morphological and histochemical alterations in the muscle spindles and Golgi tendon organs of tenotomized or immobilized calf muscles of rats (m. soleus, m. gastrocnemius) were studied in 54 animals. The intact contralateral feet served as controls. There was no change in the number of mechanoreceptors in either the tenotomized or the immobilized muscles. The diameter of the intrafusal fibers was increased somewhat after 1 week, but decreased by 14% to 40% in 3 weeks. The periaxial space of muscle spindle had diminished or disappeared by 2 weeks from the beginning of the procedures. The thickness of the outer capsule of the spindles had markedly increased, especially in the polar region. In the Golgi tendon organs the capsule had also thickened significantly and the internal space diminished. The alterations were similar following tenotomy or immobilization, but more marked after tenotomy than after immobilization.

Alterations in muscle spindle morphology in advanced stages of murine muscular dystrophy

Muscle spindles in the soleus of 1-year-old dystrophic mice of the C57BL/6J dy2J/dy2J strain were studied by microscopic and morphometric methods, and comparisons were made with those in age-matched normal tissue. Transverse epon sections were cut through various regions of an individual receptor, and subsequent 90 degrees reorientation enabled longitudinal examination of the same spindle. In dystrophy, alterations were detected in the outer capsule and consisted of a significant increase in its overall thickness in equatorial regions. Perineurial proliferation accompanied histiocyte and collagen infiltration. Within the equator, intrafusal fibers and sensory terminals appeared unaffected by dystrophy. Alterations in the intrafusal fibers were restricted to polar zones where the mean diameters of chain and bag fibers were significantly reduced. Polar chain fibers exhibited a greater degree of atrophy in dystrophy with a 40% diminution in size. Ultrastructural changes in intrafusal fiber polar regions were less pronounced compared to the surrounding dystrophic muscle. Mitochondrial alterations in affected intrafusal fibers included intramatrix inclusions and glycogen deposition. Vacuolization of the sarcoplasmic reticulum and subsarcolemmal tubular aggregates were also observed in polar regions of dystrophic chain fibers. Regional variation in spindle involvement in advanced murine dystrophy provides evidence that the equatorial contents of this receptor are sequestered from the deleterious effects of the disease. Capsular thickening in the equator may be an adaptive response, preventing the intrafusal fibers from undergoing the moderate change and atrophy observed at their polar ends.