Development of serotoninlike immunoreactivity in the embryonic nervous system of the snail Lymnaea stagnalis

In our initial effort to study the ontogeny of the gastropod nervous system, we used histological techniques to examine the post-embryonic development of cells which exhibit serotoninlike immunoreactivity in Lymnaea (Croll and Chiasson, J. Comp. Neurol. 230:122-142, '89). The present study complements that report by examining the embryonic development of these neurons. The first serotoninlike immunoreactive (SLIR) cells to be detected in the embryos are the paired C4 neurons of the cerebral ganglia. These cells are faintly visible at about 37-38% of embryonic development and have already produced axons which traverse the cerebral commissure. By about 2-3% later the axon tips reach the pedal ganglia and appose the next SLIR cells to appear, the EPe1 neurons. Over the next 30% of development four more pairs of cerebral neurons are added adjacent to the C4 neurons and over ten cells are added to each of the pedal ganglia. At about 70% of development SLIR fibers are first detected in the parietal and visceral ganglia forming the abdominal ring. Around this time the somata of the C1 neurons also first appear in the cerebral ganglia together with their prominent axons projecting to the buccal ganglia. The last 30% of development is marked by a massive addition of SLIR cells (up to 60) in each pedal ganglion. The early appearance of the first SLIR cells suggests that they may be among the first nerve cells to differentiate and that they may play central roles in the formation of the CNS. We hypothesize that most of the animal's neural circuitry is laid down during embryogenesis by a stereotypic ontogenetic program with post-embryonic neurogenesis subserving mostly compensatory and modulatory purposes.