Regional differences in organization of the extracellular matrix and cytoskeleton at the equator of chicken intrafusal muscle fibres

Generating intrafusal skeletal muscle fibres in vitro: Current state of the art and future challenges

Intrafusal fibres are a specialised cell population in skeletal muscle, found within the muscle spindle. These fibres have a mechano-sensory capacity, forming part of the monosynaptic stretch-reflex arc, a key component responsible for proprioceptive function. Impairment of proprioception and associated dysfunction of the muscle spindle is linked with many neuromuscular diseases. Research to-date has largely been undertaken in vivo or using ex vivo preparations. These studies have provided a foundation for our understanding of muscle spindle physiology, however, the cellular and molecular mechanisms which underpin physiological changes are yet to be fully elucidated. Therefrom, the use of in vitro models has been proposed, whereby intrafusal fibres can be generated de novo. Although there has been progress, it is predominantly a developing and evolving area of research. This narrative review presents the current state of art in this area and proposes the direction of future work, with the aim of providing novel pre-clinical and clinical applications.

Pax7 is expressed in the capsules surrounding adult mouse neuromuscular spindles

The multigene Pax family of transcription factors plays an important role in the development of the central nervous system as well as in organ morphogenesis. Expression of one of the members of the family, Pax7, has been described in embryonic muscle and in both embryonic and adult brain. We recently detected Pax7 transcripts in RNA isolated from adult mouse skeletal muscle and brain and here use in situ hybridisation to localise the expression within these tissues. Pax7 expression was observed in neural cells of the brain and in cells of neural crest origin in the inner and outer capsules of neuromuscular spindles. The results suggest that Pax7 may be implicated in the formation and maintenance of neuromuscular contacts within the muscle spindle throughout life.Key words: Pax7 expression, skeletal muscle, neuromuscular spindle, basal lamina, Schwann cells.

Extracellular matrix and transmembrane linkages at the termination of intrafusal fibers and the outer capsule in chicken muscle spindles

Attachments of intrafusal fibers and of the outer spindle capsule at the far polar region were examined by immunohistochemistry in serially sectioned chicken leg muscles. Patterns of distribution of connective tissues and intracellular filaments suggest that, in this segment of the muscle spindle, intrafusal fibers bind laterally with the capsule. Contrary to extrafusal fibers at myotendinous junctions, folded plasmalemmas at the ends of intrafusal fibers were rare. Thus, there was little end-to-end interlocking between intrafusal fibers and the extracellular matrix. The tapered contours of terminating intrafusal fibers resembled those of extrafusal fibers which end in fascicles without tendinous connections. At points where the distal portions of intrafusal fibers closely adjoined and overlapped extrafusal fibers, alpha-actinin, vinculin, filamin, talin, beta 1 integrin, spectrin, and dystrophin occurred with moderate to great frequency. It is generally accepted that these compounds are links in molecular chains that extend from the intracellular space across cell membranes to the extracellular matrix. Their location along substantial lengths of extrafusal fibers, distal capsule, and terminating intrafusal fibers suggests the presence of numerous transverse connections between elements of the terminal portion of the spindle and nonspindle tissues. Hence, it is likely that forces monitored by chicken spindles in muscles undergoing length changes are transferred from extrafusal fibers and extracellular matrix to the receptors in large part via lateral shear instead of by longitudinal tension.

Immunocytochemical localization of vinculin in muscle and nerve

A complete survey of the immunofluorescence distribution of the cytoskeletal protein vinculin in the normal skeletal muscle and peripheral nerve of humans and of different rodent species was performed. Our results enable us to localize vinculin in different types of adhesion plaques such as sarcolemmal costameres and neuromuscular and myotendinous junctions, but also in a fine intermyofibrillar lattice, possibly associated with intermediate filaments and/or with the triads. Moreover, we describe the presence of vinculin in junctional domains of several, previously unrecognized, specialized cells such as: the outer sheath of the muscle spindle capsule, the multilayered flat cells of the perineurium, the smooth muscle cells of epineurial blood vessels, and the endothelial cells in the endoneurium. These data call for a major role of vinculin in mechanisms of adhesion between cells, between cell and substrate and between intermyofibrillar components in the neuromuscular system. Such knowledge provides an anatomical background for studies on the possible pathological effects induced by an impairment in vinculin function.

Tenascin is present in human muscle spindles and neuromuscular junctions

We used immunocytochemistry to investigate the presence of tenascin, an extracellular matrix glycoprotein with very restricted tissue distribution, in human skeletal muscle. Tenascin was found in a short segment of the muscle spindle fibres, in the equatorial region where the sensory endings are found, and in the outer layers of the spindle capsule. Tenascin was also found in the neuromuscular junctions of the extrafusal fibres. The close spatial relationship between tenascin and both sensory and motor nerve endings shown here suggests that this glycoprotein is of functional importance in adult nerve-muscle contacts in human skeletal muscle.

Basal lamina development in chicken muscle spindles

The development of basal laminas was examined in immunohistochemical sections of chicken leg muscle spindles from embryonic day (E) 13 to 8 weeks postnatal. Fragments of basal laminas as seen with immunostaining for isoforms of laminin were already observed in E6 muscles. When clusters of intrafusal myotubes were first recognized at E13-14, they were surrounded by basal laminas which were incomplete both in terms of coverage and molecular composition. More mature basal lamina tubes individually enclosed young myofibers at E18. After afferents made contact with myotubes, synaptic portions of basal laminas at myosensory junctions reacted strongly with antibodies against s-laminin and chondroitin sulfate proteoglycan, while extrasynaptic portions were negative or reacted only weakly. At synaptic basal laminas of neuromuscular junctions heparin sulfate proteoglycan and s-laminin became prominent after E16. Contrary to the early presence of basal lamina proteins around intrafusal fibers, initial deposition of basal lamina proteins in the outer spindle capsule was not recognized until E17-18, and significant amounts were not detected until postnatal week 1. Unlike intrafusal basal laminas, capsular basal laminas developed no distinct specialized regions; however, molecular compositions of intrafusal and capsular basal laminas were similar.