Semi-serial electron-micrographic reconstruction of putative transducer sites in Pacinian corpuscles.

The Pacinian corpuscle (PC) is composed of an afferent neurite surrounded by an accessory capsule formed by concentric layers of lamellae. Projecting from the neurite, which is elliptical in cross-section, are "filopodia" or axonal, spike-like extensions. These filopodia are the putative sites of transduction. It has been proposed that two populations of filopodia organized in morphofunctional opposition exist, and that this arrangement is responsible for the bidirectional sensitivity of PCs as seen in receptor potential recordings. In order to explore this possibility, PCs obtained from cat mesentery were processed for electron microscopy, and semiserial reconstructions were made. We evaluated the extensions' (n > 110) locations, inclusions, shapes, and sizes. The filopodia were found to project along the major elliptical axis of the neurite, their density being approximately 2.8 per micron. The filopodia were found to contain filaments, vesicles, and amorphous ground substance, and dense accumulations of mitochondria were found at their bases. Measurements of their size (i.e., length, width, and height) suggest that there are two different types of filopodia. No other obvious relations among filopodial type, location along the neurite, and landmarks for transduction were found. The presence of the two filopodial types may be the basis for the bidirectional sensitivity of the PC.