THE FROG TASTE DISC - A PROTOTYPE OF THE VERTEBRATE GUSTATORY ORGAN

The frog taste disc (TD) is apparently the largest gustatory organ fou nd in vertebrates and seems to differentiate into a specialized variet y of the prototypic scheme of the taste bud. An explanation for this u nusual organization is lacking although it is possible to speculate th e existence of environmental and nutritional requirements. Up to the p resent time, the most common model of the TD was based on two main cel l types (sensory and sustentacular). This model may oversimplify the m orphology of this structure since more numerous cell types have been d escribed. We now propose a new model of the TD, based on comprehensive data on the ultrastructure of the organ obtained in the last 20 years . The main conclusions are the following: (1) the TD is a pluristratif ied epithelium with a general organization similar to that of the olfa ctory and vomeronasal epithelium; (2) it has a skeleton composed of th ree different types of epithelial cells; (3) the chemoreceptorial surf ace is covered by different microenvironments; (4) three different typ es of neuro-epithelial systems are present; the type II is an 'open' s ensory cell with axonal contacts devoid of vesicles; the type III is a n 'open' sensory cell with synaptic-like junctions; the type IV is a ' closed' sensory cell with a 'Merkel-neurite complex'; (5) the nerve fi bers in the basal plexus are mostly cholinergic while the peridiscal n erve fibers are mostly peptidergic. The presence of several cell types in the TD must be considered using these large receptors in electroph ysiological studies or as a source of isolated cells, and their comple xity must induce caution in the interpretation of the data. Text books of histology usually describe the peripheral structures associated wi th taste as very simple: an idea that probably must be revised. A tast e organ is a highly complex structure composed of several sensory syst ems and a comparative approach can aid comprehension of its general or ganization. The study of the 'large taste organs' present in some spec ies of amphibians can provide useful data for knowledge of the gustato ry system of vertebrates.