Programmed cell death in the apical ganglion during larval metamorphosis of the marine mollusc Ilyanassa obsoleta.

The apical ganglion (AG) of larval caenogastropods, such as Ilyanassa obsoleta, houses a sensory organ, contains five serotonergic neurons, innervates the muscular and ciliary components of the velum, and sends neurites into a neuropil that lies atop the cerebral commissure. During metamorphosis, the AG is lost. This loss had been postulated to occur through some form of programmed cell death (PCD), but it is possible for cells within the AG to be respecified or to migrate into adjacent ganglia. Evidence from histological sections is supported by results from a terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay, which indicate that cells of the AG degenerate by PCD. PCD occurs after metamorphic induction by serotonin or by inhibition of nitric oxide synthase (NOS) activity. Cellular degeneration and nuclear condensation and loss were observed within 12 h of metamorphic induction by NOS inhibition and occur before loss of the velar lobes, the ciliated tissue used for larval swimming and feeding. Velar disintegration happens more rapidly after metamorphic induction by serotonin than by 7-nitroindazole, a NOS inhibitor. Loss of the AG was complete by 72 h after induction. Spontaneous loss of the AG in older competent larvae may arise from a natural decrease in endogenous NOS activity, giving rise to the tendency of aging larvae to display spontaneous metamorphosis in culture.