Observations on morphology and function of cutaneous and subcutaneous sensory mechanoreceptors : an experimental study in mammals

Sammanfattning: Tactile sensory discrimination is dependent upon an intact system of peripheral mechanoreceptors, their nerve fibre afferents with cell bodies in the dorsal root ganglion and the dorsal column-medial lemniscal system which relays tactile information from the skin to the cortex. Injury to any part of the afferent pathways leads to compromised mechanoreceptive perception. The present study was initiated to extend our knowledge on the morphology and function of peripheral receptors subserving mechanoreception in mammals during normal conditions and after nerve injury. Human Pacinian corpuscles (PC) were clustered close to digital nerves and vessels. Differences in corpuscular size did not reflect morphological variations. PCs in distal phalanges were smaller than in proximal parts of the hand. Nerve afferents of PCs, recognised in close neighbourhood to SA 11 units, were segregated in an orderly fashion in the median nerve at elbow level. PCs but non Meissner corpuscles (MC) and Merkel cells were immuno-reactive to the glucose transporter 1 (Glut-1). Immature and denervated PCs had a strong Glut-1 immuno-reaction. Perineurial cells of peripheral nerves down-regulated Glut-1 during Wallerian degeneration. TGF-ß isoforms appeared in normal and denervated mechanoreceptors. TGF-ß was maintained in sensory neurons, satellite cells of dorsal root ganglia and in peripheral nerves following injury. TGF-ß signalling seems to involve autocrine and paracrine mechanisms. Lamellar cells of the PC expressed the low affinity p75 receptor for neurotrophins and the high affinity trk B receptor in young and adult animals. The increased expression of P75 and trk B in denervated immature and adult PCs supports the notion that anterogradely transported BDNF might promote development and regeneration in PCs. Kitten PCs exhibited a growth inhibition following nerve injury. Conclusion: The data agree with the idea of a general orderly organisation of the somatosensory system in the periphery. Mechanoreceptors differ from each other in terms of their expression of growth factors, cytokines, neurotrophins and their respective receptors. The Pacinian corpuscle can be considered to have the most resemblance to peripheral nerves.