THE DYNAMICS OF DENDRITIC STRUCTURE IN DEVELOPING HIPPOCAMPAL SLICES

Time-lapse fluorescence confocal microscopy was used to directly visua lize the formation and dynamics of postsynaptic target structures (i.e ., dendritic branches and spines) on pyramidal neurons within developi ng hippocampal tissue slices. Within a 2 week period of time, pyramida l neurons in cultured slices derived from early postnatal rat (postnat al days 2-7) developed complex dendritic arbors bearing numerous posts ynaptic spines. At early stages (1-2 d in vitro), many fine filopodial protrusions on dendrite shafts rapidly extended (maximum rate similar to 2.5 mu m/min) and retracted (median filopodial lifetime, 10 min), but some filopodia transformed into growth cones and nascent dendrite branches. As dendritic arbors matured, the population of fleeting late ral filopodia was replaced by spine-like structures having a low rate of turnover. This developmental progression involved a transitional st age in which dendrites were dominated by persistent (up to 22 hr) but dynamic spiny protrusions (i.e., protospines) that showed substantial changes in length and shape on a timescale of minutes. These observati ons reveal a highly dynamic state of postsynaptic target structures th at may actively contribute to the formation and plasticity of synaptic connections during CNS development.