Neurogenesis in the procerebrum of the snail Helix aspersa: a quantitative analysis.

The procerebrum, a specialized structure for olfaction in terrestrial pulmonate molluscs, contains 20,000 to 50,000 small, uniformly sized neurons that increase in number with age. Here I show the likely source of neurons added to the procerebrum of Helix aspersa and that the rate of neuron addition depends on snail weight. After hatching, during the initial exponential growth phase, H. aspersa adds neurons to the procerebral apex by mitosis and from a cerebral tube. In the logistic growth phase beginning 30-40 days post-hatch, neurons also seem to be added to the procerebrum from the peritentacular and olfactory nerves, causing the rate of neuron addition to approximately double; but as in the earlier exponential growth phase, this rate remains a function of snail weight. This neuron addition throughout the life of the snail can be predicted by snail weight. In the two growth phases, the number of neurons in the procerebrum is given by logarithmic functions of snail weight. The results here for H. aspersa provide the basis for experiments to determine the peripheral origin and destination of neuronal precursors that are added to the procerebrum and to determine how neuron addition affects the function of the procerebrum.