Freeze-fracture study of the mechanoreceptive digital corpuscles of mice.

The freeze-fracture replication technique was used to study the mechanoreceptive digital corpuscles in toe pads of mice. The axon terminal plasmalemma had intramembranous particles (IMPs) at a density of 2367 +/- 517 microns-2 (mean +/- S.E.M.) in the P-face and 84 +/- 4 microns-2 in the E-face. Particles were 10 +/- 1.8 nm in diameter in the P-face and 10 +/- 1.5 nm (mean +/- S.D.) in the E-face. Particle-rich and particle-free areas were noted in the P-face. The lamellar cell plasmalemma had IMPs at a density of 3359 +/- 224 microns-2 in the P-face and 265 +/- 95 microns-2 in the E-face. Particles were 10 +/- 1.4 nm in diameter in the P-face and 10 +/- 1.6 nm in the E-face. Non-terminal unmyelinated fibres in the connective tissue compartment of toe pads were also examined: the P-faces of the axolemma and Schwann cell plasmalemma had IMPs at a density of 1356 +/- 283 microns-2 and 1514 +/- 514 microns-2, respectively, while the E-face of these membranes had only a few particles. Particles were 9 +/- 1.2 nm and 10 +/- 1.6 nm in diameter in the P-faces of axon and Schwann cell plasmalemmata, respectively. The results show that the IMPs in terminal axolemma and in lamellar cell plasmalemma have a much higher density than those of non-terminal axons or Schwann cells in myelinated and unmyelinated fibres. In addition, IMPs in the terminal axolemma are larger than those in non-terminal axolemma except for the nodal axolemma. It can be said that plasmalemmata of both the axon terminals and lamellar cells of digital corpuscles are specialized in terms of IMPs, suggesting that they have specific physiological properties in mechanoreceptive functions including mechano-electric transduction.