In a variety of species, developing retinal axons branch initially more widely in their visual target centers and only gradually restrict their terminal arbors to smaller and defined territories. Retinotectal axons in fish, however, appeared to grow in a directed manner and to arborize only at their retinotopic target sites. To visualize the dynamics of retinal axon growth and arbor formation in fish, time-lapse recordings were made of individual retinal ganglion cell axons in the tectum in live zebrafish embryos. Axons were labeled with the fluorescent carbocyanine dyes Dil or DiO inserted as crystals into defined regions of the retina, viewed with 40x and 100x objectives with an SIT camera, and recorded, with exposure times of 200 msec at 30 or 60 sec intervals, over time periods of up to 13 hr. (1) Growth cones advanced rapidly, but the advance was punctuated by periods of rest. During the rest periods, the growth cones broadened and developed filopodia, but during extension they were more streamlined. (2) Growth cones traveled unerringly into the direction of their retinotopic targets without branching en route. At their target and only there, the axons began to form terminal arborizations, a process that involved the emission and retraction of numerous short side branches. The area that was permanently occupied or touched by transient branches of the terminal arbor--"the exploration field"--was small and almost circular and covered not more than 5.3% of the entire tectal surface area, but represented up to six times the size of the arbor at any one time. These findings are consistent with the idea that retinal axons are guided to their retinotopic target sites by sets of positional markers, with a graded distribution over the axes of the tectum.

The growth dynamics of individual DiO‐labeled retinal axons deprived of normal neural impulse activity by TTXZ was monitored in the tectum of living zebrafish embryos with time‐lapse video microscopy and compared with normal active axons. Growth cones of TTX‐blocked axons advance intermittently with an average velocity similar to normal axons. While exploring their local environment, they are broadened and bear ruffling lamellipodia and filopodia, but become streamlined when advancing. The activity‐deprived axons grow directly towards their retinotopic target sites in the tectum as do their normal counterparts and very rarely extend branches en route. Much like normal axons, TTX‐blocked axons begin to branch and develop their terminal arbors only at their retinotopic target area. They emit and retract numerous short side branches over a period of several hours. Thearea they contact (the “exploration field”) is of similar dimension as that of active axons, covering from 1% to 7.4% of the tectal neuropil surface, but the final arbor, cover an area only one‐half to one‐sixth as large. TTX arbors are as small as arbors of normal active axons and retinotopically correct. Thus, the typical exploratory growth behavior of developing retinal axons in the tectum, the dynamics of terminal arbor formation at retinotopically correct sites, the dimension of the exploration field, and the shaping of the arbors in zebrafish embryos are unaffected by TTX‐induced neural impulse blockade. 1994 John Wiley & Sons, Inc.

Neurons acquire their distinct shapes after passing through many transitional stages in early development. To reveal the dynamics and spatiotemporal sequence of process formation in situ, the growth of neurons in the optic tectum of live zebrafish embryos (54 to >100 h old) was monitored using time‐lapse videorecordings. Neurons were labeled by injecting the fluorescent vital dye DiO into the cell‐rich layer of the developing tectum in 50‐ to 70‐h‐old embryos. In phase 1, tectal neurons possess an apical “primary process” which reaches to the ventral aspect of the tectal neuropil. The primary process produces at its tip short transitory branches, some with growth cones, over a period of roughly 6 h. One of the growth cones then elongates rapidly and grows toward the caudal tectum via a route characteristic of efferent axons. After retraction of excess branches and growth cones, branching activity 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 (our longest continuous time‐lapse movie). The tectal territory “explored” in this way is larger than the area finally covered by the tree resulting from growth and loss of branches. The dynamics observed here directly are probably characteristic for dendrite formation in vertebrates. Moreover, consistent with the sequence of neuronal differentiation observed in vitro, the growth of the axon precedes that of the dendrites, although both emerge from a common primary process in this type of tectal neuron. © 1997 John Wiley & Sons, Inc. J Neurobiol 32: 627–639, 1997

Neurons acquire their distinct shapes after passing through many transitional stages in early development. To reveal the dynamics and spatiotemporal sequence of process formation in situ, the growth of neurons in the optic tectum of live zebrafish embryos (54 to > 100 h old) was monitored using time-lapse videorecordings. Neurons were labeled by injecting the fluorescent vital dye DiO into the cell-rich layer of the developing tectum in 50- to 70-h-old embryos. In phase 1, tectal neurons possess an apical "primary process" which reaches to the ventral aspect of the tectal neuropil. The primary process produces at its tip short transitory branches, some with growth cones, over a period of roughly 6 h. One of the growth cones then elongates rapidly and grows toward the caudal tectum via a route characteristic of efferent axons. After retraction of excess branches and growth cones, branching activity 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 (our longest continuous time-lapse movie). The tectal territory "explored" in this way is larger than the area finally covered by the tree resulting from growth and loss of branches. The dynamics observed here directly are probably characteristic for dendrite formation in vertebrates. Moreover, consistent with the sequence of neuronal differentiation observed in vitro, the growth of the axon precedes that of the dendrites, although both emerge from a common primary process in this type of tectal neuron.