DYNAMICS OF TASTE CELLS 135 



a simple one in the taste system. The possibility of other interactions 

 between taste cells in the taste bud is given by the fact that de Lorenzo 

 notes many fine bridges between adjacent taste cells. 



When taste buds were first discovered, the cells within were classified into 

 two groups and thought to be supporting and sensory in function. Later 

 anatomists examined the cells more closely and classified them into many 

 more types. Still others noted signs of degeneration within the taste buds 

 and also mitotic structures which led them to suggest that all the cells within 

 the taste bud were taste cells but in different stages of development (Parker, 

 1922). It was reasoned several years ago that if the taste cells were being 

 continually replaced, this could be demonstrated by injecting the rat with 

 colchicine, a known mitotic inhibitor (Beidler, 1961b). This was done and 

 histological examination showed mitotic division in the germinal epithelial 

 cells of the surface of the tongue as well as around the taste bud, but never 

 inside the taste bud. Thus, mature taste cells do not undergo mitotic 

 division. However, electrophysiological recordings show a marked drop 

 in magnitude of response to chemical stimuli as early as 2-3 hr after col- 

 chicine injection and no response after 8-10 hr when histological investiga- 

 tion shows the taste buds to be degenerating. Does this mean that col- 

 chicine blocks cells surrounding the taste bud from dividing and moving 

 into the taste bud to form taste cells, or is colchicine merely eliciting a very 

 toxic reaction? The chromosomes of newly dividing cells can be tagged by 

 labelling a nucleic acid precursor, thymidine, with an isotope. This was 

 done in the rat and the fungiform papillae examined by autoradiographs to 

 reveal the epithelial cells around the taste bud dividing and some of the 

 daughter cells entering the taste bud (see Fig. 1). Quantitative data reveal 

 an average life span of less than 3-5 days for the rat taste cells. Recently 

 de Lorenzo repeated the above-mentioned experiments using the foliate 

 papillae of the rabbit and confirmed the concept of continuous taste cell 

 replacement (de Lorenzo, personal communication). 



No longer can the researcher consider the taste receptors as merely trans- 

 ducers without adequate consideration of their biological character. The 

 duration of a single electrophysiological experiment can be an appreciable 

 fraction of the life span of the cell. Also the histochemist must consider 

 the fact that the array of biochemicals in the taste bud are different than 

 would be the case if the taste cells were of a more permanent character and 

 showed neither modulation nor degeneration. 



The continual formation of new taste cells may be related to the fact that 

 taste buds degenerate and disappear shortly after the taste nerve is cut. We 

 placed alcohol on the rat chorda tympani in the middle ear, and within 

 4-8 hr the taste buds became smaller and the epithelial cells around the 

 taste buds thickened. After 96 hr only a few taste cells remained, and no 

 taste pore was present. In 7 days there were no signs of taste buds. As 



