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 ste reotyped routes within the cranial mesenchyme to reach their appropriate en try points into the neuroepithelium. Thus, trigeminal ganglion axons always project to rhombomere (r)2, whilst facial/acoustic ganglia axons always pr oject to r4. Axons are never observed to enter the mesenchyme adjacent to r 3, raising the possibility that r3 mesenchyme contains an axon growth-inhib itory 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 wil d-type embryonic day 10 (E10) cryosections, neurites generally failed to gr ow into r3 mesenchyme from the adjacent r2 or r4 mesenchyme. This barrier w as removed if cryosections were pretreated with chondroitinase or were wash ed with excess chondroitin 6-sulphate or hypertonic saline. By contrast, wh en trigeminal neurons were seeded onto cryosections of E10 erbB4 -/- embryo heads their neurites readily entered mutant r3 mesenchyme. Immunohistochem ical analysis demonstrated chondroitin-sulphated proteoglycans throughout t he cranial mesenchyme in both wild-type and erbB4 -/- embryos. We propose t hat trigeminal axons are excluded from wild-type r3 mesenchyme by a growth- inhibitory activity which associates with chondroitin-sulphated proteoglyca ns and that the synthesis of this activity may rely on signals transduced b y erbB receptors. (C) 1999 Academic Press.