Dendritic dynamics in vivo change during neuronal maturation

In vivo imaging of optic tectal neurons in the intact Xenopus tadpole permi ts direct observation of the structural dynamics that occur during dendriti c arbor formation. Based on images of single Dil-labeled neurons collected at daily intervals over a period of 6 d, we divided tectal cell development into three phases according to the total length of the dendritic arbor. Du ring phase 1, the cell differentiates from a neuroepithelial cell type and extends an axon out of the tectum. The total dendritic branch length (TDBL) is <100 mu m. During phase 2, when TDBL is 100-400 mu m, the dendritic arb or grows rapidly. During phase 3, when TDBL is >400 mu m, the dendritic arb or grows slowly and appears stable. Neurons at different positions along th e rostrocaudal developmental axis of the tectum were imaged at 2 hr interva ls over 6 hr and at 24 hr intervals over several days. Images collected at 2 hr intervals were analyzed to determine rates of branch additions and ret ractions. Morphologically complex, phase 3 neurons show half the rate of br anch additions and retractions as phase 2 neurons. Therefore, rapidly growi ng neurons have dynamic dendritic arbors, and slower-growing neurons are st ructurally stable. The change in growth rate and dendritic arbor dynamics f rom phase 2 to phase 3 correlates with the developmental increase in synapt ic strength in neurons located along the rostrocaudal tectal axis. The data are consistent with the idea that strong synaptic inputs stabilize dendrit ic arbor structures and that weaker synaptic inputs are permissive for a gr eater degree of dynamic rearrangements and a faster growth rate in the dend ritic arbor.