Studies on the mechanosensory innervation of muscle using organotypic culture, reinnervation and immunohistochemistry

Abstract

This thesis studies sensory innervation in mammals using an organotypic co-culture of spinal cord-dorsal root ganglion and skeletal muscle of embryonic rat, the histological changes of reinnervated muscle spindles after nerve section and the localisation of the calcium-binding protein calretinin in cat mechanoreceptor organs. The immediate importance of this project concerns the better understanding of how the normal process of development differs from reinnervation following nerve lesion or section. A range of classical and well defined materials and methods as been used in the work described. The thesis Is divided into Ove chapters: Chapter 1 reviews aspects of the mechanosensory organs which have been studied experimentally in relation to their sensory innervation, including proprioceptive muscle spindle development, reinnervation, and finally, the presence of the calcium-binding protein, calretinin in the mechanoreceptor organs. This provides an introduction and background to the work. Chapter 2 describes the organotypic organisation of spinal-cord, dorsal-root ganglia and skeletal muscle co-culture in vitro. Results show that slices of the spinal-cord, dorsal- root ganglia survive well under experimental conditions and can live for several weeks with feeding every 1-3 days. Sensory neurons can develop and grow in a medium without any additional promoting factor. The presence of structurally identifiable synapses indicates that other neurons are also maintained in culture and have functional connections. In the organotypic culture new muscle fibres can form either from the original explant or from the additional explant. In chapter 3 I describe two abnormal endings present in spindles of the tenuissimus of the cat that had been reinnervated following section of the nerve more than one year previously. The reconstruction of the endings of these two spindles supports the hypothesis of modulation of the primary-ending response by the mechanical properties of the intrafusal muscle fibres, rather than by intrinsic properties of the la afferent itself. They further indicate that, in the absence of a la afferent, intrafusal-fibre differentiation can be maintained by a group II afferent. Chapter 4 concerns the localisation of the calcium-binding protein calretinin, which was studied immunohistochemically in the abductor digiti quinti medius muscle of the cat hind limb. The calretinin immunoreactivity was found in some intrafusal fibres, the primary endings and the cqjsule of the muscle spindles and the sensory terminals of tendon organs and Paciniform corpuscles. The present findings contradict a recent hypothesis that calretinin is associated with rapid adaptation, but suggest that calretinin has a specific function in muscle proprioceptors. Finally, Chapter 5 outlines the conclusions of this study and gives some suggestions for continuation of the work in the future.