Mechanisms of Mechanotransduction in the Pacinian Corpuscle

Touch perception is important in most living organisms and extremely sensitive detection systems have evolved to meet this need. Pacinian corpuscles (PCs) are primary mechanoreceptors. In the human, they are found in the skin (where they act as touch receptors), in the joints, in muscles and in many organs (where they act as motion sensors). The purpose of the work described in this thesis is to investigate how the performance of the PC is achieved, with reference to structure, mechanical properties and possible transduction mechanisms. PCs were obtained from the equine hoof and their distribution and clustering were investigated. Corpuscles were located in the frog area of the hoof (the digital cushion); they were found to be surrounded by adipose tissue and often closely associated with blood vessels. The physiological implications of these observations are discussed. The structure and composition of corpuscles was investigated using confocal microscopy with histological stains for collagen, proteoglycans and lipids. Nonlinear microscopy was also used to investigate the distribution of collagen (by secondharmonic generation), elastin (by intrinsic two-photon fluorescence) and membrane 4 lipids (by coherent Raman imaging). These techniques provided novel insights into the three-dimensional structure of the intact corpuscle, demonstrating: (i) three clearly distinguishable zones – the outer zone, the inner zone, and the core; (ii) blood vessels running through the outer lamellae and the core; (iii) the presence of proteoglycans – less in the outer zone than in the inner zone; (iv) two types of collagen fibres (one type associated with the lamellae and the other forming a complex fibre network through the inter-lamellar spaces); (v) occasional elastic fibres; (vi) a sheath of adipose tissue closely associated with the corpuscle’s outer surface. Mechanical testing by micro indentation, micropipette aspiration and osmotic challenge showed that the outer zone was stiff and able to quickly restore its original shape after distortion. Dynamic mechanical properties were investigated over a range 50 to 400 Hz. Observations of lamellar displacement (amplitude and phase) were consistent with the predictions of the Loewenstein-Skalak model. This model includes 30 lamellae; however, the same overall frequency response could be replicated in a singlelamella model with suitably chosen parameters. The benefits of a lamellar structure for transduction of mechanical signals therefore remain unresolved. 5 The permeability of the corpuscle to water and solutes was investigated using osmotic swelling and fluorescence tracer techniques. Both revealed unexpected complexity in the pathways of uptake to the inner core and demonstrated the presence of an impermeable boundary between the inner and outer zones, whose implications for mechanotransduction and nutrition in the corpuscle remain to be determined.