Molecular and tissue interactions governing induction of cranial ectodermal placodes

Abstract

Whereas neural crest cells are the source of the peripheral nervous system in the trunk of vertebrates, the "ectodermal placodes," together with neural crest, form the peripheral nervous system of the head. Cranial ectodermal placodes are thickenings in the ectoderm that subsequently ingress or invaginate to make important contributions to cranial ganglia, including epibranchial and trigeminal ganglia, and sensory structures, the ear, nose, lens, and adenohypophysis. Recent studies have uncovered a number of molecular signals mediating induction and differentiation of placodal cells. Here, we described recent advances in understanding the tissue interactions and signals underlying induction and neurogenesis of placodes, with emphasis on the trigeminal and epibranchial. Important roles of Fibroblast Growth Factors, Platelet Derived Growth Factors, Sonic Hedgehog, TGFbeta superfamily members, and Wnts are discussed.

Figure 1

Locations of placodal regions in chicken embryos. For St. 5–10, merged fate maps are shown on the left hand side of the embryo as colored dots, with general placodal regions on right. A. At St. 5, fate mapping shows a broad overlap of presumptive otic and epibranchial precursors. B. At St. 7, the olfactory and lens precursor domains extensively overlap as well as trigeminal, epibranchial and otic precursor domains. C. At St. 8, the general trigeminal placode domain has been expanded rostrally to include placodal cells that give rise to trigeminal nerve cell bodies. D. Placodal domains continue to separate over time (St. 10). E. Placodes are found in distinct morphologically identifiable regions by St. 13–14. Adenohypophseal placode is surrounded by a dashed line to indicate the fate mapping was done at a lower resolution and not done at all time points. AD=adenohypophysis (red), E=epibranchial (blue), Lens (yellow), OLF=olfactory (orange), O=otic (purple), TGP=trigeminal placode (green), TG=trigeminal ganglion, MB=midbrain, HB=hindbrain, NP=neural plate, PS=primitive streak, NC=neural crest.