Many hypotheses have been developed to account for the process of olfaction (for reviews see MOULTON and BEIDLER, 1967; DAVIES, 1971 and POYNDER, 1974), but at the present time none of them has been verified. The olfactory organ demonstrates very interesting receptor properties. It interacts with a great variety of compounds, called odorants (STAHL, 1973).
Appropriate biochemical and biophysical methods including the separation of sensory surfaces should be able to provide important evidence about receptive mechanisms and attempts to use such techniques have begun in recent years (e.g. KONORRING,CH and NORRING, 1969; ASH and SKOGEN, 1970; KOYAMA et al., 1971; KOROLEV and FROLOV, 1973; MENCO et al., 1974; MARGOLIS, 1975). Techniques for separating receptor structures have also been suggested by OTTOSON (1970), but sofar none of these methods has yielded a pure fraction of receptor endings (see DODD, 1974). The present account will deal mainly with the anatomy of the bovine olfactory epithelium emphasizing prospects for isolating receptor moieties. The adjacent nasal respiratory epithelium has also been investigated since, like the olfactory bipolar nervous cells such respiratory cells bear cilia. In the latter case, however, they are motile, rather than sensory, thus enabling a determination of features which are specific for the olfactory cilia and a comparison between two adjacent epithelium types allows such specific characterizations (LUCAS and DOUGLAS, 1934; SLEIGH, 1974).
The cow has been used as an experimental animal, since the size of its olfactory organ and the availability of samples seemed most convenient for this work. Some of the studies presented here were carried out on sheep. Indications for behaviour towards odorants in cows have been reviewed by ERNST and PUSHKARSKII (1975).
In Chapter 1, the distal processes of both epithelium types are described, using different microscopical methods, including light microscopy, scanning electron microscopy, thin section transmission electron microscopy and thick section high- voltage transmission electron microscopy. This latter method was used in the hope that it would permit olfactory cilia to be followed over their whole lengths. In this chapter some attention will also be devoted to macroscopic observations.
Chapter 2 deals with the results of freeze-etch and electron spin resonance studies on both epithelium types. Both techniques allow predictions of some of the molecular properties of the receptive area through investigation of intact tissue in vitro.
Chapter 3 deals with a quantitative analysis of the morphological data. Statistical methods are used where it is possible. Several features of the olfactory nerve endings are compared for different nasal areas and for adult as opposed to juvenile animals. Special emphasis is placed on the ciliary processes. Olfactory and respiratory cilia are also compared with each other. Furthermore, estimates for possible receptor concentrations based chiefly on freezeetch results are presented.
Such quantitative information is important for biochemical work, since it indicates whether one can consider nerve ending preparations as homogeneous or if one has to take into account that biochemical preparations may contain morphologically different nerve ending types. Furthermore, some ideas about receptor quantities which might be isolated can be obtained.
Finally, Chapter 4 deals with attempts to isolate peripheral receptor membranes. The conventional criteria used by others for assessing the purity of the fractions have been shown to be inadequate (DODD, 1974); this prompted us to initiate anatomical studies on the bovine olfactory mucosa, so as to provide ourselves with a more adequate basis for future biochemical studies.