Mitotic Spindle Assembly and Function 143
and cell type (Bakhuizen et al. 1985; Venverloo and Libbenga 1987; Kat-
suta et al. 1990). Upon reaching the division plane, migration ceases and the
nucleus becomes anchored in place, its position being more resistant to cen-
trifugation or cytoskeletal-disrupting drugs (Venverloo and Libbenga 1987;
Katsuta et al. 1990; Panteris et al. 2006). In highly vacuolated cells such as
those found inNautilocalyxleaves, cytoplasmic strands initially radiate out
in all directions from the centralized nucleus, and then later become more
or less restricted to a dense disc-like structure, the phragmosome, which de-
fines the future division plane (Sinnott and Bloch 1940). In addition to the
phragmosome, several polar cytoplasmic strands may remain, which supple-
ment the phragmosome in nuclear positioning and division plane determin-
ation (Traas et al. 1987; Lloyd and Traas 1988). Both the polar strands and the
phragmosome contain MTs and actin, the cytoskeletal elements responsible
for nuclear migration and positioning. The phragmosome is thought to play
a role in division plane determination, although its role may not be essen-
tial since it is not obvious in some cell types, such as the densely cytoplasmic
meristematic cells of root tips. Similarly, the phragmosome itself may not play
a direct role in division plane determination; instead it may be a visible mani-
festation of cellular polarity in large vacuolated cells, this same polarity being
present also in smaller cytoplasmic cells.
In addition to the migration of the nucleus to the division site, changes
in the arrangement of cortical MTs are also indicative of ensuing cell div-
ision. Concomitant with or shortly after nuclear migration, the transversely
oriented MTs of the cortex begin to accumulate in a dense band encircling
the nucleus, while simultaneously becoming depleted in the distal cortical
regions of the cell. This preprophase band (PPB) of MTs is unique to the
vegetative cells of land plants, and forecasts the future plane of cell division
(Mineyuki 1999). As the cell nears prophase, MTs become more numerous
and densely packed within the PPB as it narrows into a tight band (Mineyuki
et al. 1991; Vos et al. 2004). For reasons that will be discussed throughout this
work, it is appropriate for this cortical structure to be considered an integral
component of the plant mitotic spindle.
2.3
Development of the Prophase Spindle
The first sign of the prophase spindle is the appearance of a cytoplasmic clear
zone (seen with light microscopy) largely devoid of organelles, around the
prophase nucleus (Bajer 1957). In narrow cells, this thin layer later becomes
asymmetrically distributed into two cone-like projections along the future
long axis of the spindle. These were termed “polar caps” by early light mi-
croscopists (Robyns 1929). It was later shown that the clear zone and polar
caps exhibit birefringence under polarizing light microscopy (Inoue and Ba-
jer 1961), and were subsequently shown to contain MTs (Bajer and Molè-Bajer