Chorda tympani nerve transection at different developmental ages produces differential effects on taste bud volume and papillae morphology in the rat

Abstract

Chorda tympani nerve transection (CTX) results in morphological changes to fungiform papillae and associated taste buds. When transection occurs during neonatal development in the rat, the effects on fungiform taste bud and papillae structure are markedly more severe than observed following a comparable surgery in the adult rat. The present study examined the potential "sensitive period" for morphological modifications to tongue epithelium following CTX. Rats received unilateral transection at 65, 30, 25, 20, 15, 10, or 5 days of age. With each descending age at the time of transection, the effects on the structural integrity of fungiform papillae were more severe. Significant losses in total number of taste buds and filiform-like papillae were observed when transection occurred 5-30 days of age. Significant reduction in the number of taste pores was indicated at every age of transection. Another group of rats received chorda tympani transection at 10, 25, or 65 days of age to determine if the time course of taste bud degeneration differed depending on the age of the rat at the time of transection. Taste bud volumes differed significantly from intact sides of the tongue at 2, 8, and 50 days post-transection after CTX at 65 days of age. Volume measurements did not differ 2 days post-transection after CTX at 10 or 25 days of age, but were significantly reduced at the other time points. Findings demonstrate a transitional period throughout development wherein fungiform papillae are highly dependent upon the chorda tympani for maintenance of morphological integrity.

Figure 1

(A–C) Examples of the epithelial surface of fungiform papillae stained with methylene blue. A: Pore papilla (the arrow indicates the pore); B: No Pore papilla; C: Filiform-like papilla. The scale bar (50 μm) in B applies to each section in this series. (D–F) Ten-micrometer sections of fungiform papillae stained with hematoxylin. D: Fungiform papilla with a taste bud (the taste bud region is indicated with the dashed line); E: “empty” papilla; F: filiform-like papilla. The scale bar in E applies to this series of figures.

Figure 2

Average number (±SEM) of fungiform papillae observed 50 days posttransection following CTX at various developmental ages. Counts were made on the denervated side of the tongue (NeoCTX) and the intact sides of the tongue (Intact). A: The total number of papillae across all categories (B: Pore papillae; C: No Pore papillae; D: Filiform-like papillae).

Figure 3

Volume measurements of taste buds 2, 8, or 50 days after CTX at 10 days of age. Histograms represent the number of taste buds within each volume range for the denervated (CTX) and Intact sides of the tongue. The age of the animals at the time of sacrifice is included in parentheses at each time point. Two animals were included at each time point and 2 mm of tongue tissue per rat is represented. The total number (N) and average volume (M) of taste buds is included at each age.

Figure 4

The percent difference in numbers of taste buds on intact and denervated sides of the tongue following transection at 10, 25, or 65 days of age. Since sides of the tongue normally have relatively equal numbers of taste buds, the percent difference may be interpreted as a “loss” of taste buds on the denervated (CTX) side of the tongue. Percent differences in total numbers are shown 2, 8, or 50 days after CTX.

Figure 5

Histograms representing volume measurements of taste buds at 2, 8, or 50 days after CTX at 25 days of age. The age of the animals at the time of sacrifice is presented in parentheses. Denervated (CTX) and intact sides of the tongue are presented as separate graphs at each time point. Histological analysis of a total of 4 mm of tongue tissue (2 rats per group, 2 mm tissue per rat) is shown along with the total number (N) of taste buds and the average (M) taste bud volume per group.

Figure 6

Histograms of the number of taste buds per volume range on the denervated (CTX) and intact sides of the tongue following CTX at 65 days of age. Animals were sacrificed 2, 8, or 50 days after CT transection (age of animal at time of sacrifice listed within parentheses). The total number (N) of taste buds and the average (M) volume of taste buds is shown. Histological analysis included 2 mm of tongue tissue per rat with 2 rats per age group.