TOSHIAKI J. HARA
84
The olfactory epithelial system consists of receptor cells, supporting
or sustentacular cells, and basal cells ( Fig. 2). In some species ( AnguiZZu,
MyxocephaZus, etc. ) , large flasklike cells have also been observed; these
are probably mucous gland cells (Holl, 1965). In burbot (Lota Zota),
furthermore, glial cells have been observed to penetrate into the epithelium from the connective tissue surrounding small bundles of receptor
cell axons (Gemne and Doving, 1969). The individual olfactory receptor
cells of fishes are similar to those of other vertebrates in general
appearance although there is a great variation in details even within a
particular olfactory organ. The receptor cell, which is a bipolar primary
neuron, sends a slender cylindrical process or dendrite toward the surface
of the epithelium. The process terminates in a minute swelling (olfactory
knob) which bears a variable number of cilia. Although the receptor cells
are not distributed uniformly in all olfactory folds, average numbers of
4 8 x 104/mm2 are estimated for the receptor cells in several species
(Holl, 196.5). The fine axons arise from the basal pole of the receptor
cells. They pass through the basement membrane, become grouped in
the submucosa, and form the olfactory nerve fasciculi, which run posteriorly to end in the olfactory bulb.
Recent electron microscopic observations have characterized the fine
structural organization of the olfactory receptor cells in teleost fishes.
Trujillo-Cen6z ( 1961 ) first studied the ultrastructure of olfactory neurons
in fully developed embryos of two cyprinodont species Cnesterodon and
Fitzroyia. He found that the olfactory sensory hairs consist of an undetermined number of long cilia which project into the lumen of the olfactory
pit and that the dendrite of the olfactory neuron contains profiles of small
tubules, aligned parallel to its length. Near the basement membrane of
the epithelium, groups of axons, which, as mentioned, arise from the pole
of the receptor cells opposite to the distal process, are encased in the
surface of the sustentacular cells. Although Trujillo-Cen6z failed to
demonstrate in the sensory hairs of receptor cells any common structural
pattern which could be related to the chemoreceptor mechanism, he considered these structures as devices serving to enlarge the “active surface”
of the cell increasing in this way the effectiveness of the whole receptive
system.
In earlier reports, Jagodowski ( 1901) working with pike, Esox, and
Hopkins (1926) studying Stenesthes claimed that there is a single long
apical process on each olfactory receptor instead of the usual group of
cilia. To resolve this question, Bannister (1965) compared olfactory
receptor cells in two teleosts, a minnow, Phoxinus, and the three-spined
stickleback, Gasterosteus. Phoxinus, like all the Cyprinidae, has a welldeveloped olfactory organ. Gasterosteus by contrast is a microsmatic
84
The olfactory epithelial system consists of receptor cells, supporting
or sustentacular cells, and basal cells ( Fig. 2). In some species ( AnguiZZu,
MyxocephaZus, etc. ) , large flasklike cells have also been observed; these
are probably mucous gland cells (Holl, 1965). In burbot (Lota Zota),
furthermore, glial cells have been observed to penetrate into the epithelium from the connective tissue surrounding small bundles of receptor
cell axons (Gemne and Doving, 1969). The individual olfactory receptor
cells of fishes are similar to those of other vertebrates in general
appearance although there is a great variation in details even within a
particular olfactory organ. The receptor cell, which is a bipolar primary
neuron, sends a slender cylindrical process or dendrite toward the surface
of the epithelium. The process terminates in a minute swelling (olfactory
knob) which bears a variable number of cilia. Although the receptor cells
are not distributed uniformly in all olfactory folds, average numbers of
4 8 x 104/mm2 are estimated for the receptor cells in several species
(Holl, 196.5). The fine axons arise from the basal pole of the receptor
cells. They pass through the basement membrane, become grouped in
the submucosa, and form the olfactory nerve fasciculi, which run posteriorly to end in the olfactory bulb.
Recent electron microscopic observations have characterized the fine
structural organization of the olfactory receptor cells in teleost fishes.
Trujillo-Cen6z ( 1961 ) first studied the ultrastructure of olfactory neurons
in fully developed embryos of two cyprinodont species Cnesterodon and
Fitzroyia. He found that the olfactory sensory hairs consist of an undetermined number of long cilia which project into the lumen of the olfactory
pit and that the dendrite of the olfactory neuron contains profiles of small
tubules, aligned parallel to its length. Near the basement membrane of
the epithelium, groups of axons, which, as mentioned, arise from the pole
of the receptor cells opposite to the distal process, are encased in the
surface of the sustentacular cells. Although Trujillo-Cen6z failed to
demonstrate in the sensory hairs of receptor cells any common structural
pattern which could be related to the chemoreceptor mechanism, he considered these structures as devices serving to enlarge the “active surface”
of the cell increasing in this way the effectiveness of the whole receptive
system.
In earlier reports, Jagodowski ( 1901) working with pike, Esox, and
Hopkins (1926) studying Stenesthes claimed that there is a single long
apical process on each olfactory receptor instead of the usual group of
cilia. To resolve this question, Bannister (1965) compared olfactory
receptor cells in two teleosts, a minnow, Phoxinus, and the three-spined
stickleback, Gasterosteus. Phoxinus, like all the Cyprinidae, has a welldeveloped olfactory organ. Gasterosteus by contrast is a microsmatic