The effects of sialoadenectomy and exogenous EGF on taste bud morphology and maintenance

Taste buds on the dorsal tongue surface are continually bathed in saliva ri ch in epidermal growth factor (EGF). In the following experiment, taste bud number and morphology were monitored following submandibular and sublingua l salivary gland removal (sialoadenectomy), to determine if EGF plays a rol e in the maintenance and formation of taste buds. Adult male rats were divi ded into four groups: sialoadenectomized (SX, n = 4); sialoadenectomized wi th EGF replacement (SX + EGF, n = 5); sham-operated (SH, n = 4); and sham-o perated with exogenous EGF (SH + EGF, n = 5). After a 3 week recovery, SX EGF and SH + EGF animals were given 50 mu g/day EGF in their drinking wate r for 14 days. At day 14, saliva was collected, the animals were killed and the presence of EGF determined by radioligand-binding assay. Tongues were removed and histologically examined for the presence and morphology of tast e buds on fungiform and circumvallate papillae, or immunostained for the pr esence of EGF, TGF alpha (transforming growth factor alpha) and EGFR (EGF r eceptor). The removal of submandibular and sublingual salivary glands resul ted in the loss of fungiform taste buds and normal fungiform papillae morph ology. These effects were reversed by EGF supplementation, indicating a rol e for EGF in fungiform taste bud maintenance. In addition, supplementation of EGF to sham-operated animals increased the size of fungiform taste buds. In contrast, removal of salivary glands had no effect on the size, numbers , or morphology of circumvallate taste buds, suggesting that the formation and maintenance of taste buds in fungiform and circumvallate papillae may i nvolve different and distinct processes. EGF, TGF alpha and EGFR were local ized to distinct layers of the dorsal epithelium and to within both fungifo rm and circumvallate taste buds. Their expression within the epithelium or taste buds was not altered with sialoadenectomy, indicating that the action s of endogenous EGF and TGF alpha are distinct and not regulated by exogeno us EGF and TGF alpha supplied in saliva.