Time-lapse in vivo imaging of the morphological development of Xenopus optic tectal interneurons.

Excitatory and inhibitory interneurons play a key role in the establishment of neuronal responses and circuit properties in the brain. Despite their importance in brain function, the structural development of interneurons has not been well studied. We used in vivo time-lapse imaging in intact anesthetized Xenopus tadpoles to determine the morphological events underlying the development of interneuron dendritic arbor structure. Single optic tectal neurons were labeled with DiI and imaged at daily intervals over 4 days in intact albino Xenopus tadpoles. The same neurons were also imaged at shorter intervals to determine the dynamic rearrangements in arbor branches that accompany large-scale arbor growth. Tectal interneurons, like projection neurons, develop from neuroepithelial cells located near the ventricular layer. They elaborate complex dendritic arbors over a period of 2 days. Short-interval time-lapse images reveal that tectal interneuron arbors have rapid rates of branch additions and retractions. We identified four patterns of interneuron arbor development, based on the cell morphology and types of structural rearrangements that occur over the development of the neuronal arbor. A surprising feature of interneuronal development is the large extent of structural rearrangements: many interneurons extend transient processes so that the neuronal structure is dramatically different from one day to the next. Because the majority of synaptic contacts are formed on dendrites, the structural changes we observed in interneuronal dendritic arbors suggest that optic tectal circuits are extremely plastic during early stages of development.