Fig. 2.

Sprouting of lateral filopodia, growth cones, and collateral branches from pyramidal cell dendrites during early stages of differentiation (1 d in vitro). A, Low-magnification image of DiI-labeled CA1 pyramidal neuron somata (P) and dendrites (arrows) in a live tissue slice taken from a P2 animal. This is an extended-focus image made by combining a stack of five optical sections spanning 15 μm in theZ-dimension. At early developmental stages, dendrites are often tipped by complex growth cones (arrowhead).SR, Stratum radiatum; SP, stratum pyramidale.B, Through-focus sequence of boxed regionin A showing a segment of an apical dendrite shaft (arrow) having fine, lateral filopodial protrusions (arrowhead). For time-lapse imaging, multiple optical sections were collected to ensure that small dendritic protrusions were captured in their entirety. C, Time-lapse sequence of the same field as in B showing that numerous lateral filopodia (arrowheads) extend from, and retract back to, the dendrite shaft. Most filopodia are resorbed within a few minutes after first appearing. These images are composites of two or three optical sections from the middle of a five-image stack (shown in B).D, Persisting lateral filopodia sometimes evolve into growth cones. Time-lapse sequence shows a lateral filopodium (arrowheads) sprouting from an apical dendrite shaft (0–30 min), persisting for more than an hour (30–90 min), then rapidly developing into a complex, growth cone-like structure (arrow; 90–100 min). E,De novo sprouting of a growth cone leading to formation of a collateral dendrite branch. A faint, filopodial structure is seen first (40 min), then a growth cone develops (60–80 min) and spins out a new branch that persisted and grew to a length of at least 35 μm (120 min). For C–E, elapsed time is shown in minutes.