A morphological study on the amount and origin of axons in the pleuroabdominal connectives of Aplysia fasciata

The pleuro-abdominal connectives (PACs) in Aplysia fasciata (Poiret) observed at EM showed irregular compartments in which different calibre fibres were intermingled with abundant glia laminae. The estimated total numbers of axons were 1388 in the left and 1832 in the right PAC, with similar size distribution. Co2+ backfills were applied to the severed PACs to highlight the populations of central neurones projecting into these nerves from either side. After left abdominal PAC stump backfills, an average of 51.5 ipsilateral and 9.2 contralateral neurones were stained; correspondingly, right PAC backfills yielded 32.5 and 13.1 neurones. Aided by a preliminary examination of the abdominal ganglion of A. fasciata, a comparison with A. californica was carried out and correspondences of neurones and clusters were proposed. In all, the stained abdominal neurones outnumbered the ones described as projecting into the PACs in A. californica, and a part appeared as newly identified. Backfills applied to pleural left or right PAC stump stained less numerous neurones, all ipsilateral, in the pleural ganglion (average 30.6 and 16.5, respectively) and pedal ganglion (3.3 and 3.9). Many stained fibres could not be traced to a soma; although often crossing the ganglia and entering a peripheral nerve, they appeared to assure long-distance connections with the periphery.