Ultrastructural studies on membrane, cytoskeletal, mucous, and protective compartments in olfaction

There is a great variety in the morphological appearance of olfactory structures across the metazoan animal kingdom. Despite this variety the receptive structures themselves have a strikingly similar architecture, namely some type of elongated cellular extension that is spanned by a membrane and surrounded by mucus. These cellular extensions can either be modified primary or secondary cilia, or microvilli. There are more similarities between membranes of these extensions than between the cytoskeletal elements immediately underneath the membranes. One might infer that the cytoskeletal elements of the cellular extensions merely serve as a scaffold for the membranes, whereas the similarity in membrane ultrastructure provides morphological evidence supporting the concept that these membranes are responsible for the initial olfactory transduction process. The transduced message is transported to the brain, where it is decoded to initiate the cascade of events resulting in the organisms' appropriate behavioral response to the initial odorous stimulus. The varying appearance of olfactory structures across the animal kingdom is probably produced by evolutionary pressure to adapt the olfactory system to the animal's environment. This review deals with the ultrastructural aspects of these facets of olfaction.