Ontogenesis of the snail, Helix aspersa: Embryogenesis timetable and ontogenesis of GABA-like immunoreactive neurons in the central nervous system

Late stages of embryogenesis in the terrestrial snail Helix aspersa L. were studied and a developmental timetable was produced. The distribution of ga mma-aminobutyric acid-like immunoreactive (GABA-ir) elements in the CNS of the snail was studied from embryos to adulthood in wholemounts. In adults, approximately 226 GABA-ir neurons were located in the buccal, cerebral and pedal ganglia. The population of GABA-ir cells included four pairs of bucca l neurons, three neuronal clusters in the pedal ganglia, two clusters and s ix single neurons in the cerebral ganglia. GABA-ir fibers were observed in all ganglia and in some nerves. The first detected pair of GABA-ir cells in the embryos appeared in the buccal ganglia at about 63-64% of embryonic de velopment. Five pairs of GABA-ir cell bodies were observed in the cerebral ganglia at about 64-65% of development. During the following 30% of develop ment three more pairs of GABA-ir neurons were detected in the buccal gangli a and over fifteen cells were detected in each cerebral ganglion. At the st age of 70% of development, the first pair of GABA-ir neurons was found in t he pedal ganglia. In the suboesophageal ganglion complex, GABA-ir fibers we re first detected at about 90% of embryonic development. In the posthatchin g period, the quantity of GABA-ir neurons reached the adult status in four days in the cerebral ganglia, and in three weeks in the pedal ganglia. In j uveniles, transient expression of GABA was found in the pedal ganglia (four th cluster).