156 J.C. Ambrose · R. Cyr
1978). Similarly, regions of overlap between adjacent bridge MTs within the
cytoplasm between the PPB and nucleus could also provide sites for force
generation via a sliding-filament mechanism.
Considerable evidence supports the presence of tension between the PPB
and prophase nucleus/spindle: (1) cytoplasmic strands rapidly recoil upon
laser ablation (Hoffman 1984; Goodbody et al. 1991); (2) prophase nuclei
are often somewhat flattened within the PPB plane, suggesting pulling forces
at the nuclear equator (Burgess 1970; Panteris et al. 1991; Granger and Cyr
2001); (3) PPBs copurify with isolated prophase nuclei, indicating a phys-
ical link between the PPB and nucleus (Wick and Duniec 1984); (4) or-
ganelle motility decreases inside mature phragmosomes (Ota 1961; Mineyuki
et al. 1994) and prophase nuclei are less easily displaced by centrifugation
than interphase nuclei, indicating increased gelation within mature phragmo-
somes (Pickett-heaps 1969; Mineyuki and Furuya 1986; Mineyuki and Pale-
vitz 1990). Although these findings provide compelling evidence for tensile
forces between the PPB and prophase spindle, the mechanism by which these
interactions facilitate bipolarity along an axis perpendicular to the plane of
the PPB remains speculative. We favor the idea that this is mediated, at least
in part, by intervening bridge MTs.
4.5
The PPB and Centrosome Share Similar Functions
Because of their ubiquitous presence across diverse phyla and intimate associ-
ation with spindle poles in animal cells, the long-standing view has been that
centrosomes are essential for spindle formation and function. Although com-
pelling, a large body of evidence challenges this notion. First, higher plants
and certain animal cell types (e.g. oocytes) lack discrete MTOCs and yet
still form bipolar spindles. Second, numerous studies have shown that nor-
mal bipolar spindles can still form in the absence of centrosomes, even in
cells that normally contain these structures (Phalle and Sullivan 1998; Khod-
jakov et al. 2000; Khodjakov and Rieder 2001; Megraw et al. 2001; Wadsworth
and Khodjakov 2004). Third, by disruption of key pole determinants such as
NuMA, a loss of pole focusing can be induced even in cells in which centro-
somes are still present (Gaglio et al. 1997).
It was initially believed that the “inside-out” or self-organizational mech-
anisms of spindle assembly observed in acentrosomal systems were not oper-
ative in centrosome-containing cells, however this is not the case. More recent
experiments have shown these pathways are also operative in centrosome-
containing cells (Maiato et al. 2004; Wadsworth and Khodjakov 2004). Simi-
larly, when centrosomes are introduced into acentrosomal self-organizational
systems, they become the dominant sites of MT nucleation and contribute
to spindle formation (Heald et al. 1997). In light of these findings, the cur-
rent understanding of centrosome function has shifted away from that of