High-voltage electron microscopy and 3-D reconstruction of solitary chemosensory cells and Di-I labeling of primary afferent nerves

Abstract

Solitary chemosensory cells (SCCs) resembling taste bud cells in shape and ultrastructure have been found in fish and amphibians. A quantitative survey in teleosts has revealed that SCCs may be even more abundant than taste bud cells. Due to their scattered distribution, however, this abundant and potentially important vertebrate chemosensory system has eluded closer examination. This situation has changed with the introduction of a suitable research model: the anterior dorsal fin (ADF) of rocklings (Gadidae, Teleostei). This specialized chemosensory organ contains several million SCCs that converge onto fine caliber recurrent facial nerve fibers. The ADF model has allowed us to investigate SCC innervation, fine structure and function.Using high voltage electron microscopy of serial sections and computer-assisted three-dimensional reconstructions, we have obtained significant quantitative data on the spatial structure of SCC assemblages in the ADF epidermis and of synaptic contacts between SCCs and innervating facial nerves. Approximately 15% of all cells present in the ADF epidermis are SCCs, making up 30% of the epidermal volume. Unlike taste buds, the ADF contains only a single sensory cell type and lacks basal cells or specialized “supporting cells.”