14 A. J. D. DE LORENZO 



apparent thickening at their terminations. It has been a consistent observa- 

 tion that the large fibers also appear as " free endings " and thus may 

 represent yet another type of sensory apparatus or cell type. 



Several interesting questions immediately arise. Whenever two kinds of 

 endings are seen in receptor systems one must consider the possibility of 

 efferent endings. Unfortunately the electron microscope does not differ- 

 entiate them for the neuroanatomist. Indeed it would seem most unwise 

 to ascribe functional roles to any structural organelles without biophysical 

 and chemical correlates. It must remain for the physiologist to demonstrate 

 the role, if any, of the various types of endings seen by the electron micro- 

 scopist. Another interesting observation is that of the so-called " synaptic 

 vesicles " seen in the endings. If we assume, for the time being at least, 

 that the gustatory system is chiefly sensory in nature (and the experiments 

 of Guth (1958) support such an assumption), then the vesicles are on the 

 post-synaptic side of the synapse. I have also reported " synaptic vesicles " 

 in two distinct synapses that have been shown to be electrical in nature 

 (1959, 1960b). If they contain chemical transmitters, they are most 

 awkwardly disposed. If on the other hand, they reside in effector endings, 

 then all is quite reasonable. Only future studies will clarify this dilemma. 



Another type of receptor cell attachment is demonstrated in Fig. 16. 

 These are the desmosomes which we also encountered in the olfactory 

 receptor epithelium (Figs. 3 and 4). The inset in Fig. 16 demonstrates the 

 ultrastructural organization typical of desmosomes also seen in other types 

 of epithelium (Fawcett, 1958). 



Our observations on the ultrastructural organization of the gustatory 

 receptors, nerve fibers and their synaptic junctions are summarized in 

 Text-fig. III. 



HISTOPHYSIOLOGY OF GUSTATORY SYNAPTIC JUNCTIONS 



In our early descriptions of the rabbit taste buds, we were somewhat 

 surprised to find that we could not clearly label each cell in the bud either 

 sustentacular or sensory on the basis of cytological appearances. This 

 seemed too simplified a classification, especially when we later observed 

 many cells in the bud which demonstrated pathological changes (de 

 Lorenzo, 1960). The work of Beidler et al. (1960) confirmed our suspicions 

 that a cell turnover was evident. Since then, we have repeated Beidlef s 

 rat experiments in the rabbit. Table 1 represents the data derived by 

 injecting colchicine and counting dividing cells in the taste bud germinal 

 epithehum as described by Beidler (personal communication). Since 

 colchicine is a most toxic substance these results must be carefully inter- 

 preted. However, they do suggest a rather rapid turnover in the foliate 

 papillae. However, the metaphase cells were rarely found within the bud 

 but usually around the margins (see Figs. 17 and 18). To determine whether 



