Nerve growth factor (NGF) supports tooth morphogenesis in mouse first branchial arch explants

Posterior midbrain and anteorior hindbrain neuroectoderm transdifferentiate into cranial neural crest cells (CNCC), emigrate from the neural folds, an d become crest-derived ectomesenchyme within the mandibular and maxillary p rocesses. To investigate the growth factor requirement specific for the ini tiation of tooth morphogenesis, we designed studies to test whether nerve g rowth factor (NGF) can support odontogenesis in a first branchial arch (FBA ) explant culture system. FBA explants containing neural-fold tissues befor e CNCC emigration and the anlagen of the FBA were microdissected from embry onic day 8 (E8) mouse embryos, and cultured for 8 days in medium supplement ed with 10% fetal calf serum only, or serum-containing medium further suppl emented with either NGF or epidermal growth factor (EGF) at three different concentrations: 50, 100, or 200 ng/ml. Morphological, morphometric, and to tal protein analyses indicated that growth and development in all groups we re comparable. Meckel's cartilage and tongue formation were also observed i n all groups. However, odontogenesis was only detected in explants cultured in the presence of exogenous NGF. NGF-supplemented cultures were permissiv e for bud stage (50 ng/ml) as well as cap stage of tooth morphogenesis (100 and 200 ng/ml), Morphometric analyses of the volume of tooth organs showed a significant dose-dependent increase in tooth volume as the concentration of NGF increased. Whole-mount in situ hybridization and semiquantitative r everse transcription-polymerase chain reaction for Pax9, a molecular marker of dental mesenchyme, further supported and confirmed the morphological da ta of the specificity and dose dependency of NGF on odontogenesis. We concl ude that (1) E8 FBA explants contain premigratory CNCC that are capable of emigration, proliferation, and differentiation in vitro; (2) serum-suppleme nted medium is permissive for CNCC differentiation into tongue myoblasts an d chondrocytes in FBA explants; and (3) NGF controls CNCC cell fate specifi cation and differentiation into tooth organs. Dev Dyn 1999;216:299-310. Pub lished 1999 Wiley-Liss, Inc.dagger