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striatum ‘plays a role’ in response gating ofprey-catching via pretectum, I hope thereader will not get the impression that thisis the only connection mediating striato-tectal influences (we have found a directconnection in salamanders8). The impor-tant point – based on neuroanatomical,neurophysiological and behavioral data – isthat prey-catching in anurans can takeadvantage of a modulatory striato–pre-tecto–tectal channel in order to translate

perception into action, a channel which, inmammals, obviously does not exist.

Jörg-Peter EwertNeurobiologie, Fachbereich

Biologie/Chemie, Universität Kassel,34132 Kassel, Bundesrepublik,

Germany.

References1 Gonzalez, A. and Smeets, W.J.A.J.

(1991) J. Comp. Neurol. 303, 457–477

2 Wilczynski, W. and Northcutt, R.G.(1983) J. Comp. Neurol. 214, 333–343

3 Marín, O. et al. (1997) J. Comp. Neurol.380, 23–50

4 Marín, O. et al. (1997) J. Comp. Neurol.378, 50–69

5 Glagow, M. and Ewert, J-P. (1996)Neurosci. Lett. 220, 215–218

6 Glagow, M. and Ewert, J-P. (1997) J. Comp. Physiol. 180, 1–9

7 Glagow, M. and Ewert, J-P. (1997) J. Comp. Physiol. 180, 11–18

8 Finkenstädt, T. et al. (1983) Cell TissueRes. 234, 39–55

L E T T E R S T O T H E E D I T O R

Some decades ago, Stone and associatesreported that newt taste buds would de-velop in the absence of gustatory innerv-ation1 and could subsequently survive inthe absence of any innervation2. Morerecently Northcutt and Barlow3 examinedaxolotl taste-bud development and, withembryonic transplantation and dye injec-tion, confirmed and extended these classicobservations. Rather than migrating in fromeither the neural crest or placodes, axolotltaste-cell precursors originate in local epi-thelial tissue where their progeny can differ-entiate into taste buds even in the absenceof innervation. Does the well-establishednerve-independence of salamander tastebuds also apply to mammalian taste buds,as Northcutt and Barlow now propose?

More than 90% of rat or mouse vallatetaste buds develop postnatally, providedthat the vallate papilla remains innerv-ated4–6. If, instead, the IXth nerve is inter-rupted in the newborn rat tongue (P0–P3),it both prevents the development of mostvallate taste buds7,8 and also permanentlyeliminates the competence of the re-innervated gustatory epithelium to supporttaste bud development9,10. I here summa-rize recent evidence establishing that prenatal taste bud development is alsonerve-dependent in mammals.

The embryonic absence of brain-derived neurotrophic factor (BDNF) inbdnf-null mutant mice caused a loss of gus-tatory innervation associated with theseverely impaired development of tastepapillae and taste buds11–13. Specifically, thesparseness of gustatory innervation washighly correlated with a smaller gustatoryepithelium (r 5 10.94) and fewer tastebuds (r 5 10.96) (Ref. 13). The rescue ofsmall numbers of BDNF-deprived tasteneurons by local epithelial factors, such asNT-3, accounts for the residual presenceof a few vallate taste buds5,13. Using a quitedifferent method to destroy developingtongue sensory neurons, Morris-Wiman et al. almost completely prevented thedevelopment of fungiform papillae and

their taste buds in normal mouse em-bryos injected with the neurotoxin, beta-bungarotoxin14.

By eliminating most of the gustatoryinnervation, null mutations of bdnf or trkBalso eliminated many fungiform papillaeand taste buds12,13,15,16. Those fungiformtaste buds that remained were likely tohave obtained support from the richplexus of trigeminal nerve fibers present ineach fungiform papilla. Trigeminal axonscan provide modest trophic support ofadult taste buds17. In contrast to the partialpersistence of scattered fungiform tastebuds and papillae after nerve transection inadult rats, chorda-lingual nerve transectionin neonates caused every fungiform papillato lose its taste bud and revert to a filiformor filiform-like spine18. This observationdecisively establishes that, like vallate tastebuds, fungiform taste buds are also whollynerve-dependent at birth.

The unanimity of recent studies on theeffects of sensory denervation on fetal andnewborn rodent tongues has reconfirmedthe canonical view that mammalian taste-bud development is nerve-dependent.More precisely, mammalian taste axonsappear to contribute sequentially to papillamorphogenesis, gustatory competence,and taste-bud formation. Comparativebiologists may note with satisfaction thatthe singularity of taste-bud development insalamanders presents a splendid opportu-nity to examine epithelial cell interactionsuncomplicated by the reality of the nervedependence of taste bud development inmammals.

Bruce OakleyDept of Biology, University of

Michigan, Ann Arbor, MI 48l09, USA.

References1 Stone, L.S. (1940) J. Exp. Zool. 83,

481–5062 Wright, M.R. (1964) J. Exp. Zool. 156,

377–3903 Northcutt, R.G. and Barlow, L.A.

(1998) Trends Neurosci. 21, 38–42

4 Hosley, M.A. and Oakley, B. (1987)Anat. Rec. 218, 216–222

5 Cooper, D. and Oakley, B. (1998) Dev.Brain Res. 105, 79–84

6 Oakley, B. et al. (1991) Dev. Brain Res.58, 215–221

7 Hosley, M.A. et al. (1987) J. Comp.Neurol. 260, 224–232

8 Hosley, M.A. et al. (1987) J. Neurosci. 7,2075–2080

9 Oakley, B. (1993) Dev. Brain Res. 72,259–264

10 Oakley, B. (1993) in Mechanisms of TasteTransduction (Simon, S.A. and Roper, S.D.,eds), pp. 105–125, CRC Press

11 Zhang, C. et al. (1997) NeuroReport 8,1013–1017

12 Nosrat, C. et al. (1997) Development124, 1335–1342

13 Oakley, B. et al. (1998) Dev. Brain Res.105, 85–96

14 Morris-Wiman, J. et al. in XII Int. Symp.Olf. and Taste, Ann. New York Acad. Sci.(in press)

15 Fritzsch, B. et al. (1997) Int. J. Dev.Neurosci. 15, 563–576

16 Oakley, B. in XII Int. Symp. Olf. andTaste, Ann. New York Acad. Sci. (in press)

17 Oakley, B. et al. (1990) Neuroscience 36,831–838

18 Nagato, T. et al. (1995) Acta Anat. 153,301–309

There is no longer much doubt that in ver-tebrates a regional specification of endo-derm does occur during gastrulation1, oreven before2, and, as Northcutt andBarlow3 have shown, a population of taste-bud progenitor cells might be establishedat that time. However, comparative ana-tomical observations on fish taste budscaution us not to be too hasty in extend-ing the scenario, proposed initially for am-phibians, to vertebrates generally. Firstly,taste buds in fishes are not confined toregions of the pharyngeal endoderm thatinvolute during gastrulation, but (indistin-guishable in ultrastructure from those ofthe mouth cavity4), can occur on barbelsand even the tips of pectoral-fin rays aswell. Secondly, the number of innervatedtaste buds could depend on specific signalsarriving from taste buds or their pro-genitors in amphibia. However, at least infishes, early and repeated taste-bud stimu-lation (i.e. use), might be important, notonly in maintaining taste-bud innervation,but could also lead to a multiplication ofinnervated taste-bud sites in the growing

Vertebrate taste-bud development:are salamanders the model?