ULTRASTRUCTURE AND HISTOPH YSIOLOG Y OF MEMBRANES 11 



details of the most interesting taste pores are not resolvable. Again we 

 must turn to electron microscopy. 



Receptor Cells 



Since our early description of taste buds in the rabbit, great progress 

 has been made in preparatory techniques for electron microscopy. I 

 think it worthwhile, therefore, to briefly present some of our recent 

 observations on taste receptors before going on to the innervation data. 



The cells within the taste buds were classified as sustentacular and 

 gustatory by the early histologists, primarily on the basis of their sizes, 

 shapes and staining characteristics. Although most histologists find this 

 classification useful, the electron microscopic studies suggest this is too 

 simplified a view. From these studies it appears that many of the cells 

 are transitional, i.e. represent diff'erent stages of the same basic cell type. 

 In addition various types of degenerating cells have been seen in the taste 

 bud by electron microscopy (de Lorenzo, 1960). 



Figure 10 is a relatively low magnification micrograph demonstrating 

 the typical morphological appearance of the apical portion of a taste bud. 

 About six receptor cells (Tr) are seen near the region of the taste pore. 

 The apical tips of the receptors demonstrate cell surface specializations— 

 microvilli (Mi). The microvilli are extensions of the plasma membrane 

 and are bounded by the typical unit membrane complex (see above). 

 The microvilli are from 0. 1// to 0.2// wide and two or more microns long and 

 were first described by de Lorenzo (1958). It is worth emphasizing that 

 there are no " taste hairs " seen and therefore these " structures " seen in 

 light microscopy must be artifacts. The cytoplasm of the receptor is 

 characterized by large clusters of vesicles and fibrillar material particularly 

 in the regions near the microvilli. Between the cells, in the region of the 

 taste pore, are large amounts of a dense osmiphilic material that may 

 correspond to the mucoid substance described by Ranvier. The apical 

 cytoplasm of the receptors contains, in addition, accumulations of mito- 

 chondria and large numbers of dense granules (Fig. 11), These granules 

 are enclosed in a membrane (Fig. 11) and resemble neurosecretory and 

 " catechol-amine granules " seen in other tissues. The plasma membranes 

 of the receptor cell show other kinds of specializations at their lateral 

 surfaces. The cells always interdigitate with each other, particularly at 

 their apical ends (see Figs. 10 and 11). Figure 11 shows yet another cell 

 surface specialization at the arrows — the desmosome which will be described 

 in more detail later. Note, however, that in this case the cell surfaces 

 involve the sensory receptor cells alone. Endoplasmic reticulum, RNP 

 granules, mitochondria and Golgi apparatus complete the cytoplasmic 

 components of the gustatory receptors. 



