276 CHEMICAL SENSES 



stimulation in the turtle (Tucker 1967). The finding that the olfactory cells 

 of Rhinobatus lack cilia is, therefore, of particular interest, especially if this 

 is common in elasmobranchs, a group that in general has high olfactory 

 sensitivity (Hopkins 1926). Furthermore, there are indications that the 

 whole dendritic extremity of the receptor cell may be engaged in active 

 movement (Vinnikov and Titova 1957, Bronshtein 1963). 



Recent observations have confirmed the existence of two or three types of 

 receptor cells in the olfactory epitheliums of fishes (Pyatkina 1974: Aci- 

 penser; Breipohl et al. 1973: Carassius auratus), reported earlier by several 

 authors in a number of vertebrates, including fishes (Dogiel 1886, Morrill 

 1898, Jagodowski 1901, Le Gros Clark 1956, Neuhaus 1955, Castello 1956, 

 Vinnikov 1956, Bannister 1965, Graziadei 1966, 1971, Andres 1965, 1966, 

 1968, 1969; Graziadei 1967; Schulte 1972). Although there is no informa- 

 tion on the occurrence of different bipolar cells in the epitheliums of elasmo- 

 branchs, verification of their presence in other vertebrates, including fishes, 

 is important in that the morphological differentiation may indicate func- 

 tional differences underlying the mechanisms of odor perception and dis- 

 crimination. There is the intriguing possibility that different membrane 

 absorption characteristics of the receptor, which could mediate the olfactory 

 stimulus (Wright 1964; Amoore 1952, 1971; Davies 1971; Ottoson 1971; 

 von Herberhold 1969a, 1969b, 1971, 1972), may be associated with mor- 

 phologically recognizable differences of the cells (Breipohl et al. 1973). To 

 what extent these morphological differences represent functionally different 

 cell types or merely reflect various stages of degeneration or regeneration of 

 one type has not been clearly established (Bannister 1965). An additional 

 aspect of this problem will be referred to later, in relation to morphological 

 differences in olfactory cell axons. 



Below the vesicle, the olfactory cell becomes constricted to various de- 

 grees and makes close contact with supporting or sustentacular cells (de 

 Lorenzo 1963, Ottoson 1965) through junctions whose structure has been 

 studied in some detail (Farquhar and Palade 1965, P^eese 1965, Reese and 

 Brightman 1970, Robertson et al. 1963). In two elasmobranchs (Gingly- 

 mostoma cirratum and Rhinobatus lentiginosus), Reese and Brightman con- 

 firmed the existence of tight junctions with the surrounding supporting cells, 

 just below the cellular apices. Such "tight" junctions, unlike "gap" junc- 

 tions, constitute a seal that effectively separates the olfactory surface from 

 the underlying structures by forming continuous belts around the bipolar 

 and supporting cells. 



Thus, substances in the external medium, carried by the water in the olfac- 

 tory sac, may be barred from directly mixing with the constituents of the body 

 fluids, and vice versa, unless they diffuse through the tight junctions. The latter 

 possibility has been investigated histochemically by Reese and Brightman 

 (1970). Circulating protein (molecular weight 42 000) was effectively barred 

 from reaching the olfactory surface by the tight junctions, which alone pre- 

 vented the exchange between subepithelial interstitial fluid and that surface. 



Proximally, the olfactory cell body continues into the axon, which joins 

 those of other receptor cells to form bundles that traverse the basal lamina. 



