DYNAMICS OF PROCESS FORMATION DURING DIFFERENTIATION OF TECTAL NEURONS IN EMBRYONIC ZEBRAFISH

Neurons acquire their distinct shapes after passing through many trans itional stages in early development. To reveal the dynamics and spatio temporal sequence of process formation in situ, the growth of neurons in the optic tectum of live zebrafish embryos (54 to >100 h old) was m onitored using time-lapse videorecordings, Neurons were labeled by inj ecting the fluorescent vital dye DiO into the cell-rich layer of the d eveloping tectum in 50- to 70-h-old embryos, In phase 1, tectal neuron s possess an apical ''primary process'' which reaches to the ventral a spect of the tectal neuropil, The primary process produces at its tip short transitory branches, some with growth cones, over a period of ro ughly 6 h, One of the growth cones then elongates rapidly and grows to ward the caudal tectum via a route characteristic of efferent axons, A fter retraction of excess branches and growth cones, branching activit y resumes at the tip of the primary process to form the dendritic tree (phase 2), The dendritic tree develops in the tectal neuropil through emission and retraction of many branches during a period of >20 h (ou r longest continuous time-lapse movie), The tectal territory ''explore d'' in this way is larger than the area finally covered by the tree re sulting from growth and loss of branches, The dynamics observed here d irectly are probably characteristic for dendrite formation in vertebra tes, Moreover, consistent with the sequence of neuronal differentiatio n observed in vitro, the growth of the axon precedes that of the dendr ites, although both emerge from a common primary process in this type of tectal neuron. (C) 1997 John Wiley & Sons, Inc.