Specializations of plasma membranes in Pacinian corpuscles: implications for mechano-electric transduction

Pacinian corpuscles of cat mesentery were studied with freeze-fracture and thin sectioning methods after chemical fixation. Intramembranous particles (IMPs) exhibit differences in both density and pattern of distribution between the axolemma of the smooth short axis (x-axis) region and that of the axonal spine region of the long axis (y-axis) of the axon terminal. The axolemma of the x-axis has IMPs at a density of 2687 +/- 581 per micron2 (mean +/- S.E.M.), and these particles are 9.0 +/- 1.7 nm (mean +/- S.D.) in diameter. In contrast, the axolemma of the y-axis has a higher density of IMPs (3607 +/- 612 per micron2) which are larger (diameter, 10.0 +/- 1.7 nm). The particle distribution is not homogeneous in x-axis membranes as there are small patchy areas devoid of particles scattered throughout the entire surface. The E-face of the axolemma has a low density of IMPs (approximately 200 per micron2 in both x- and y-axes). However, IMPs in the E-face are smaller (approximately 9 nm) in the x-axis than in the y-axis (approximately 10 nm). The inner core lamellar cells have IMPs at a density of 3276 +/- 739 per micron2 and 553 +/- 169 per micron2 in the P- and E-faces, respectively. The particles are about 10 nm in diameter in both faces. Many gap junctions occur between lamellar cells especially near the clefts, suggesting that hemilamellae of each inner core half are kept at the same electrotonic potential. The outer core lamellar cells have IMPs at a density of 2239 +/- 403 per micron2 and 536 +/- 123 per micron2 in their P- and E-faces, respectively. The particles are approximately 10 nm in diameter in both faces. A noteworthy finding is that tight junctions are prominent at cell-to-cell appositions within individual lamellae, especially in the first and second (or sometimes third) innermost lamellae of the outer core. These tight junctions are considered to be a barrier to the leakage of fluid and/or ions between interlamellar spaces as well as between inner and outer cores. An intermediate cell layer is identified between the inner and outer cores. The connective tissue space of this cell layer corresponds to the endoneurium, indicating that intermediate layer cells are comparable to endoneurial fibroblasts.(ABSTRACT TRUNCATED AT 400 WORDS)