KEMPF, S.C.: A confocal examination of ampullary neurons in the apical sensory organ of molluscan larvae

The apical sensory organ (ASO) is present in the larvae of many invertebrate species with possible homologues also occurring in lower chordates. In mollusks, the ASO consists of both sensory neurons and non-sensory neurons that are organized as a ganglion below the centrally located ciliary tufts of the pretrochal epithelium. Particularly characteristic of this structure are a group of serotonergic neurons that are associated with sensory cells called ampullary neurons. The ampullary neurons are unusual in that their ciliated sensory dendrite is inverted, consisting of a tight bundle of multiple modified cilia lying within a deep invagination. This invagination opens to the pretrochal surface through a small pore. In previous research, identification of the ampullary neurons has required untrastructural examination of the ASO with transmission electron microscopy (TEM). The present investigation has successfully used anti-tubulin antibodies to clearly reveal the ciliary bundles of these cells in larval whole mounts examined with a BioRad 1024 confocal laser-scanning microscope. This allows a 3-dimentional examination of the organization of this group of neurons relative to surrounding cells and structures of the ASO that is not easily accomplished with TEM. Of particular interest is the apparent long length of the ampullary pores in at least some species and their intimate association with the bases of the ciliary bundles of the ciliary tuft cells. This suggests possible interactions between these two cell types that may relate to recognition of the chemical inducing cue for larval settlement and metamorphosis.