Expression of ErbB3, ErbB4, and neuregulin-1 mRNA during tooth development

The receptor tyrosine kinases ErbB3 and ErbB4, which bind to various variants of neuregulin (NRG), play fundamental roles in neural development and in organs, which form through epithelial-mesenchymal interactions. Here, we demonstrate that NRG-1 and the receptors ErbB3 and ErbB4 are expressed locally during rodent tooth development. However, the mRNA expression patterns of ErbB3 and ErbB4 were distinctly different during odontogenesis. Examinations of teeth in genetically heart-rescued ErbB4-/- mice did not reveal any obvious deviation from the normal phenotype. The results suggest that ErbB3 and ErbB4 may participate in tooth morphogenesis. The specific interactions between NRG isoforms and ErbB receptors during this process remain to be determined.

Chondroitin sulphate-binding molecules may pattern central projections of sensory axons within the cranial mesenchyme of the developing mouse

During mammalian hindbrain development, sensory axons grow along highly stereotyped routes within the cranial mesenchyme to reach their appropriate entry points into the neuroepithelium. Thus, trigeminal ganglion axons always project to rhombomere (r)2, whilst facial/acoustic ganglia axons always project to r4. Axons are never observed to enter the mesenchyme adjacent to r3, raising the possibility that r3 mesenchyme contains an axon growth-inhibitory activity. Conversely, in mice which lack the erbB4 receptor (normally expressed in r3), trigeminal and facial/acoustic ganglia axons misproject into r3 mesenchyme, suggesting that the putative axon barrier is absent. To investigate this hypothesis, we have developed an in vitro model in which dissociated wild-type embryonic trigeminal ganglion neurons are cultured on longitudinal cryosections of embryonic mouse head. We observed that on wild-type embryonic day 10 (E10) cryosections, neurites generally failed to grow into r3 mesenchyme from the adjacent r2 or r4 mesenchyme. This barrier was removed if cryosections were pretreated with chondroitinase or were washed with excess chondroitin 6-sulphate or hypertonic saline. By contrast, when trigeminal neurons were seeded onto cryosections of E10 erbB4 -/- embryo heads their neurites readily entered mutant r3 mesenchyme. Immunohistochemical analysis demonstrated chondroitin-sulphated proteoglycans throughout the cranial mesenchyme in both wild-type and erbB4 -/- embryos. We propose that trigeminal axons are excluded from wild-type r3 mesenchyme by a growth-inhibitory activity which associates with chondroitin-sulphated proteoglycans and that the synthesis of this activity may rely on signals transduced by erbB receptors.