CYTOMECHANICS OF NEURITE OUTGROWTH FROM CHICK BRAIN NEURONS

Mechanical tension is a direct and immediate stimulus for neurite init iation and elongation from peripheral neurons. We report here that the relationship between tension and neurite outgrowth is equally initima te for embryonic chick forebrain neurons. Culture of forebrain neurons was unusually simple and reliable, and some of these cells undergo ea rly events of axonal-dendritic polarity. Neurite outgrowth can be init iated de novo by experimental application of tension to the cell margi n of forebrain neurons placed into culture 8-12 hours earlier, prior t o spontaneous neurite outgrowth. Experimentally induced neurite elonga tion from these neurons shows the same robust linear relationship betw een elongation rate and magnitude of applied tension as peripheral neu rons, i.e. both show a fluid-like growth response to tension. Although forebrain and sensory neurons manifest a similar distribution of grow th sensitivity to tension (growth rate/unit tension), chick forebrain neurons initiated and elongated neurites at substantially lower net te nsions than peripheral neurons. This is because, unlike peripheral neu rons, there is no minimum threshold tension required for elongation in forebrain neurons; all positive tensions stimulate neurite outgrowth. Consistent with this observation, chick forebrain neurons showed weak retractile behavior in response to slackening compared to sensory neu rons. Neurites that were slackened showed only transient elastic behav ior and never actively produced tension, as do chick sensory neurons a fter slackening. We conclude that tension is an important regulator of both peripheral and central neuronal growth, but that elastic behavio r is much weaker for forebrain neurons than peripheral neurons from th e same developing organism. These data have significance for the under standing of the morphogenetic events of brain development.