Morphology of neurosecretory cells in basommatophoran snails homologous with egg-laying and growth hormone-producing cells of Lymnaea stagnalis

Abstract

In a light and electron microscope study, neurosecretory cells morphologically homologous with the egg-laying hormone-producing caudodorsal cells (CDC) and growth hormone-producing dorsal cells (DC, light green cells) of the freshwater basommatophoran snail Lymnaea stagnalis have been found in five genera (seven species) of Basommatophora, viz. in Lymnaea palustris and Lymnaea ovata, in Planorbis planorbis and Planorbis vortex, in Planorbarius corneus, in Bulinus truncatus, and in Biomphalaria glabrata. It is concluded that the functions of these cells are homologous as well. The homologies of the respective neuron types regard their locations in the cerebral ganglia, their clustering in groups, location of their neurohemal area (CDC: cerebral commissure; DC: median lip nerves), and ultrastructural characteristics (e.g., abundance of rough endoplasmic reticulum, well-developed Golgi apparatus, presence of two types of neuron-specific secretory granules, and release of granule contents by exocytosis into the hemolymph). In addition, CDC show large electron-dense granules and DC reveal infoldings of the plasma membrane at the abaxonal side of the soma as well as synaptic input. On the other hand, each neuron type shows species-specific characteristics, particularly with regard to the number of cells and the structure of the neurohemal area. Furthermore, the CDC show marked differences between genera in the morphology (especially the mean diameter) of type 2 and, particularly, type 1 secretory granules. The morphology of the two types of secretory granules in the DC differs strongly between species. The possible relation between the morphology and the chemical contents of secretory granules has been discussed.