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The life span of taste bud cells in rat circumvallate papillae was measured by autoradiography after labeling them with a pulse of [3H]thymidine. Specimens of circumvallate papillae were taken daily 1·5‐18·5 days after the isotope was administered; thereafter, specimens were taken on alternate days until 25·5 days. For each time interval, the number of labeled cell nuclei was counted in 200‐450 taste buds and plotted as the ratio of labeled cells/taste bud v. time after injection of [3H]TdR. In all, 6958 taste buds were counted. The total number of labeled cells (dark plus light) per taste bud reached peaks at 6·5, 13·5 and 20·5 days. The curve for the number of labeled dark cells/bud had essentially the same shape as that for total cells. The number of labeled light cells/bud reached a modest peak at 6·5 days and slowly declined to a plateau for the remainder of the experiment. The data show that an average of 2 days elapsed after injection before labeled dark cells entered the bud and they spent an average of 7 days in the non‐proliferating taste bud compartment; thus, the life span of the dark cell was 9 days. The life span of the light cell was difficult to estimate quantitatively, but this cell type was labeled at a much slower rate than dark cells and is assumed to have a significantly longer tenure in the taste bud.

Vertebrate olfactory sensory neurons are replaced continuously throughout life. We studied the effect of age on proliferation in olfactory epithelium in postnatal rats ranging in age from birth (P1) until P333. Using BrdU to label dividing cells, we determined the proliferation density of basal cells, i.e., the number of labeled nuclei/unit length (240 m) of olfactory epithelium in coronal sections from six different anterior-posterior levels from each animal. A total length of Ͼ1 m of olfactory epithelium was counted in each age group. We observed a dramatic decrease of proliferation density from P1 through P333. At P1, proliferation density is 151 cells/mm; it decreases to approximately half at P21 (70 cells/mm), and half again at P40 (37 cells/mm). At P333 the proliferation density was only 8/mm, ϳ5% of that seen at P1. The changes were clearly related to age and not to body weight, because the values were essentially identical for males and females of the same age but of different body weight. Proliferating cells appear in patches that, after P40, become more separated from one another and contain fewer cells. In 6-and 11-month-old rats, 30 and 45% of all units contained no labeled cells. We confirmed the data of others that the olfactory surface area continuously increases with age; we showed that there is a reciprocal relationship between proliferation density and surface area. The proliferating cells provide neurons to sustain growth as well as to replace dying cells.

Early development of fungiform papillae on the fetal rat tongue was examined: (1) to determine whether morphogenesis of the taste bud-bearing fungiform papillae is induced by nerve and (2) to study the growth pattern of the two sensory nerves that innervate the papilla. The papillae first appear on the 15th day of gestation (E15; E1 is the day when the dam is sperm positive) in rows parallel to the midline sulcus. There appears to be a medial-lateral and an anterior-posterior gradient in the sequence of papilla differentiation. The epithelium of the early papilla resembles a multilayered placode topped by a flattened surface periderm. Close examination of the peridermal cells at the apex of the papillae reveals that the cells have fewer surface microvilli and their cytoplasm is more electron opaque than that of similar cells in interpapillary regions. The basal cells in the placode-like epithelium differ from those in interpapillary regions in that they are postmitotic and have more mitochondria. At later stages, the papilla acquires a mesenchymal core and nerves grow into the core. Results from organ culture experiments of tongue fragments taken from E14 fetuses indicate that morphogenesis of fungiform papillae is initiated in the absence of sensory nerve influence, but the nerve exerts a trophic effect on their maintenance. The two sensory nerves of the tongue, the chorda tympani and the lingual branch of the trigeminal nerve, enter the tongue mesenchyme at E14 and grow toward the epithelium. By E15 the chorda tympani branches have reached the developing fungiform papillae, by E16 many have entered the papilla, and by E17 they have penetrated the epithelium at the papilla apex. Their fibers are associated exclusively with the cells at the papilla apex, where the taste bud will develop. The trigeminal nerve ramifies beneath the surface of the entire epithelium by E15. Later, it, too, sends branches into fungiform papillae; these ascend along the trunk of the chorda tympani and at E17 terminate in the connective tissue core around the chorda tympani field. The results are compatible with the notion that the tongue epithelium exerts a general tropic effect on growing axons of both sensory nerves, and the epithelial cells of the fungiform papilla apex exert a similar effect to which only the chorda tympani axons are responsive.

Olfactory marker protein (OMP) is an abundant, phylogenetically conserved, cytoplasmic protein of unknown function expressed almost exclusively in mature olfactory sensory neurons. To address its function, we generated OMP-deficient mice by gene targeting in embryonic stem cells. We report that these OMP-null mice are compromised in their ability to respond to odor stimuli, providing insight to OMP function. The maximal electroolfactogram response of the olfactory neuroepithelium to several odorants was 20-40% smaller in the mutants compared with controls. In addition, the onset and recovery kinetics following isoamyl acetate stimulation are prolonged in the null mice. Furthermore, the ability of the mutants to respond to the second odor pulse of a pair is impaired, over a range of concentrations, compared with controls. These results imply that neural activity directed toward the olfactory bulb is also reduced. The bulbar phenotype observed in the OMP-null mouse is consistent with this hypothesis. Bulbar activity of tyrosine hydroxylase, the rate limiting enzyme of catecholamine biosynthesis, and content of the neuropeptide cholecystokinin are reduced by 65% and 50%, respectively. This similarity to postsynaptic changes in gene expression induced by peripheral olfactory deafferentation or naris blockade confirms that functional neural activity is reduced in both the olfactory neuroepithelium and the olfactory nerve projection to the bulb in the OMP-null mouse. These observations provide strong support for the conclusion that OMP is a novel modulatory component of the odor detection/signal transduction cascade.

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