Embryogenesis of the central nervous system of the pond snail Lymnaea stagnalis L. An ultrastructural study

The ultrastructural characteristics of the developing CNS of the pond snail , Lymnaea stagnalis, were investigated, with special attention paid to thre e specific stages of embryonic development, representing distinctly differe nt phases of both the body morphogenesis and gangliogenesis. These were the 35% (veliger), the 50-55% (metamorphic), and the 75% (post-metamorphic, ad ult-like) stages of embryonic development. Also, a brief comparison was don e with the CNS of hatchlings (100% of embryonic development). During embryo genesis specialized axo-axonic synapses and elements of the glial system, i ncluding the ganglionic (neural) sheath, were rarely observed, whereas the frequent occurrence of unspecialized axo-somatic contacts could be demonstr ated. Synapse-like axo-axonic connections could be found first in 75% embry os, showing asymmetric vesicle clustering on the "presynaptic" side and inc reased electron density of the apposed membranes. These phenomena may refle ct the dominance of modulatory processes in the CNS during embryogenesis, a nd the absence of the neural sheath may facilitate trophic and/or hormonal influences within the developing ganglia. The gradual increase in the size of ganglia and the diameter of their neuropils was not accompanied by any w idening of the cell body layer or increasing diameter of the nerve cell bod ies until the very end of embryogenesis. With respect to the ultrastructura l organization of the neuropil, and possibly the entire CNS, a determining stage seems to be that of metamorphosis. Two types of neuropil could be obs erved at this time; the metamorphosing neuropil with an irregular organizat ion of wavy axon profiles, and well-structured neuropil with a regular orga nization of axon profiles. Ganglia with irregular or regular neuropil occur red simultaneously in the developing CNS.