An ultrastructural study of the inner core of the Pacinian corpuscle

Pacinian corpuscles from the hindfeet and mesentery of cats have been examined by light and electron microscopy. The study focuses on the inner core region which houses the single, non-myelinated terminal of the afferent axon. This region of the axon possesses specialized axon processes which enormously increase the surface area of the axolemma. The axon processes are long, branched, filiform structures, containing exclusively ~6 nm microfilaments, and are reminiscent of filopodia on the tips of growth cones of axons and dendrites. These axon processes emanate from the two poles of the elliptical terminal axon, from several sites in the transitional zone, and from the entire surface of the ultraterminal axon, the bulbous, branched, distal extremity of the nerve fibre. Each branch of an axon process articulates with an inner core cell hemilamella except at the distal end of the ultraterminal where axon processes may also approximate the inner edge of the outer core. The base of each axon process contains an elaborate array of organelles including clear and dense-core vesicles of synaptic vesicle size, all enmeshed in ~6 nm microfilaments. The structure and location of the axon processes appear to be eminently suitable for detecting pressure transients transmitted through the outer core of the corpuscle. It is suggested that the mechano-electrical transduction system has, as its morphological substrate, multiple units, each formed by a branched axon process and its specialized basal region. The existence of sympathetic axon terminals abutting the central axon is disputed but the presence of unusual, dense-centred and elongated vesicles within cellular profiles close to the axon is confirmed.