158 J.C. Ambrose · R. Cyr
Fig. 2 Pathways of spindle assembly in eukaryotes. The self-organizational pathway in-
volves the nucleation of MTs around chromatin or the NE and their subsequent sorting
into a bipolar spindle via the action of motor proteins. The search-and-capture path-
way predominates when bipolarity of the spindle is established prior to NEB, thereby
predisposing MTs in a favorable orientation to establish kinetochore and interpolar MT
connections. Prophase spindle bipolarity is facilitated via polar organizers or the PPB,
which acts as an equatorial organizer
ence of these components is separated temporally during plant mitosis; both
F-actin and bridge MTs are operative in spindle positioning and orientation
during prophase, but after NEB, the bridge MTs are lost and F-actin takes over
the role of spindle positioning and subsequent phragmoplast guidance during
cytokinesis (Mineyuki and Palevitz 1990; Molchan et al. 2002). The idea that
astral MTs in animals are analogous to bridge MTs and actin cables in plants
as spindle-orienting structures is supported by the observations that each of
these structures are under tension, providing a pulling force on the associated
spindle in the direction of the sites of cortical anchorage (Hahne and Hoff-
man 1984; Aist et al. 1991; Goodbody et al. 1991). The key difference is that
astral MTs act solely at the spindle poles, bridge MTs act in the phragmoso-
mal plane, and F-actin acts at both of these sites (Traas et al. 1987; Lloyd and
Traas 1988).
Figure 2 summarizes the above pathways of spindle formation in eu-
karyotes, with modifications accounting for the diverse mechanisms utilized
within each fundamental pathway.
AcknowledgementsThanks to Deb Fisher for critical reading of this manuscript along
with support from the US Department of Energy and the National Science Foundation.