Role of extracellular matrix in the regeneration of a pacinian corpuscle

The pacinian corpuscle is composed of an inner and an outer core or bulb. The former is formed by tightly packed and multi-layered thin cellular processes (lamellae) of lamellar cells which surround a centrally located axon terminal, and the latter, also called the capsule, is made up of very loosely piled layers of thin cells which encircle the inner core. Lamellar cells of the inner core are considered to be specialized Schwann cells, and the outer core cells are modified perineurial cells. In the present study, the matrix filling the extracellular spaces of the inner core consisted of basal lamina-like amorphous materials, sparce fine collagen fibrils, and the ground substance embedding these structural components. No definite basal laminae were found on the inner core lamellae except on the peripherally located ones which had distinct basal laminae. Outer-core cells were invested along the entire contour by distinct basal laminae. The interspace between the inner and outer cores was a continuation of the nerve endoneurium. The purpose of this investigation was to determine whether the extracellular matrix of the pacinian corpuscle, especially that of the inner core, has the ability to cause corpuscle regeneration, i.e. to make the regenerating axons and Schwann cells differentiate into corpuscular axon terminals and inner core cells, respectively. Pacinian corpuscles in the periosteum at the distal end of the fibula of mice were repeatedly frozen (3-5 times) in situ with forceps cooled with liquid nitrogen. Within 2-3 days, all the cellular constituents of the corpuscle had degraded, while the extracellular matrices of the inner and outer cores apparently remained undamaged. After 5-7 days, regenerating axons and accompanying immature Schwann cells entered these extracellular matrices of the inner cores. A remarkable finding was that these immature Schwann cells were detached from the axon, and sent thin cellular processes around the axon in a characteristic fashion, basically forming the same pattern as lamellae in a normal corpuscle. The regeneration of the inner core was completed by about 40 days after the freezing treatment. In the outer core, perineurial cells proliferated and extended through the basal lamina tubes of the old cells, becoming new outer core cells. These findings indicate that the extracellular matrix of the pacinian corpuscle has a specific property to cause the regeneration of the corpuscle.