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 gamma-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 buccal neurons, three neuronal clusters in the pedal ganglia, two clusters and six 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 development. Five pairs of GABA-ir cell bodies were observed in the cerebral ganglia at about 64–65% of development. During the following 30% of development three more pairs of GABA-ir neurons were detected in the buccal ganglia and over fifteen cells were detected in each cerebral ganglion. At the stage of 70% of development, the first pair of GABA-ir neurons was found in the pedal ganglia. In the suboesophageal ganglion complex, GABA-ir fibers were first detected at about 90% of embryonic development. In the posthatching 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 juveniles, transient expression of GABA was found in the pedal ganglia (fourth cluster).