Abstract
The initial outgrowth of neurites from chick sympathetic neurons grown in vitro was investigated by time-lapse microscopy with laser-scanning and conventional light microscopes. Video-enhanced contrast, differential interference contrast optics (VECDIC) were used to monitor movements of neuronal cytoplasm, as well as the movements of small beads attached to the surface membrane, and interference reflection microscopy (IRM) was used to determine the concomitant pattern of attachment to the growth substrate (polyornithine or laminin). Related changes in the distributions of actin filaments, microtubules, and neurofilaments were determined by fluorescence labeling methods. Neurite formation on both substrates entailed invasion of the actin cores of filopodia by cytoplasm containing microtubules and neurofilaments. Small beads attached to the surface membrane surrounding the cytoplasm moved outward simultaneously with the cytoplasm. Cytoplasm invaded filopodia of neurons plated on laminin soon after attachment to the substrate or, for neurons generated in vitro, within as little as 3 min after cytokinesis. However, cytoplasm invaded filopodia of neurons grown on polyornithine only when they contacted a three-dimensional object such as another cell or a large, polyornithine-coated polystyrene bead. The observation that adhesion of filopodia to polyornithine-coated beads can initiate neurite formation is inconsistent with the commonly held view that neurite formation requires adhesion mediated by specific cell adhesion molecules. Simultaneous IRM and DIC imaging showed that cytoplasm invaded filopodia when only their tips were closely apposed to a substrate but not when they were closely apposed to a substrate along their entire lengths. These findings help to elucidate the mechanisms by which interactions between the cytoskeleton and the growth substrate initiate and produce the neuronal movements that lead to the formation of neurites.