Mechanisms Underlying the Initiation and Dynamics of Neuronal Filopodia: From Neurite Formation to Synaptogenesis - International Review of Cell and Molecular Biology - page 96

Figure 3.1 Neuronal filopodia. Panels A-E show primary chicken embryo sensory neu-
rons in vitro and panel F shows an example of a sensory axon in the living embryonic
spinal cord. (A) Growth cones are described as consisting of a peripheral (P) and central
(C) domain. The P-domain is characterized by lamellipodia (lam) and filopodia (f ). The
growth cone “neck” is denoted by the arrow and represents the site of axon consoli-
dation reflective of suppression of protrusive activity and formation of the axon shaft
proper. The sample was stained with phalloidin to specifically reveal actin filaments and
not soluble actin. (B) Two stills form a phase contrast timelapse sequence. Note that
during the 100 s period, the growth cone largely changes its shape and the distribution
of filopodia and lamellipodia. (C) Timelapse sequence from an axon expressing eYFP- β -
actin. Note that the filopodium denoted by the * initially extend outward (48 s) and the
buckles near its base (66 s, arrowhead). (D) Two stills from a phase contrast timelapse
sequence showing the emergence of lamellipodium from the shaft of a filopodium
(arrowheads). (E) Platinum replica electron micrograph of the actin cytoskeleton of an
axonal filopodium (collaborative work with Dr T. Svitkina, University of Pennsylvania).
Note that the actin filaments at the base of the filopodium become bundled together
(arrow) and form the shaft of the filopodium. (F) Example of an axon extending in the
living embryonic spinal cord. RFP-cortactin was expressed in sensory neurons through
in ovo electroporation and imaged live following acute dissection of the living spinal
cord ( Spillane et al., 2011 ). Note the presence of filopodia (f ) along the axon shaft and
at the growth cone.