146 J.C. Ambrose · R. Cyr
MTs attach to kinetochores throughout prometaphase and metaphase, such
that the number of MTs in an individual K-fiber gradually increases until the
start of anaphase, at which point a drastic decrease in numbers is observed
(Jensen and Bajer 1973). This phenomenon may vary between species how-
ever, since birefringence increases throughout prometaphase but then ceases
abruptly at metaphase (when chromosome oscillations stop) inTilia ameri-
canaendosperm (Fuseler 1975).
During metaphase, a majority of spindle MTs reside within thick bundles
of k-fibers or interpolar bundles, and the degree of this bundling is exacer-
bated by the effects of taxol (Molè-Bajer and Bajer 1982, 1983). At this stage,
interpolar MT bundles have been shown to branch frequently and intermin-
gle with k-fibers and other interpolar bundles (Jensen and Bajer 1973). The
structure of an individual k-fiber resembles that of a fir tree, the base of
which is at the kinetochore, and the bough is at the poles (Bajer and Molè-
Bajer 1986; Palevitz 1988). Numerous MTs splay out (like branches) along the
length of the fiber, becoming tapered toward the pole. Indeed these micro-
tubule fir trees may represent a fractal manifestation of the fundamental plant
MT organizational unit, the MTCC (Smirnova 1998). The k-fibers are thus
higher order structures, composed of many short and overlapping MTs. In-
terestingly, gamma tubulin associates along the length of interphase MTs and
spindle MTs, and nucleates MT branches from preexisting cortical MTs at
an angle of 40 degrees (Murata et al. 2005). From examining micrographs of
MTCCs, it appears that most MTs within the MTCC branch at roughly a 40 de-
gree angle. It therefore seems possible that the nucleation of MTs occurs all
along the length of k-fibers, rather than predominately at the pole regions (as
is often depicted in textbooks). Similarly, chromatin may provide a source of
MT nucleation, since plant spindles recovering from cold- or drug-induced
MT depolymerization frequently appear as a collection of semi-autonomous
“minispindles”, each one comprised of an individual kinetochore of a given
chromosome with its own opposing fir-trees (each fir tree emanating from
a sister kinetochore), creating a diamond shape with a dark center (Bajer
1987; Cleary and Hardham 1988; Falconer et al. 1988).
Numerous ER is found throughout prometaphase and metaphase spindles,
frequently as long tubular structures closely apposed to MTs (Pickett-Heaps
and Northcote 1966), similar to that observed in prophase polar caps. Al-
though possibly involved in MT stabilization or nucleation, the function of
this ER is not known. Given the presence of crosslinks between this ER and
spindle MTs, ER may comprise a component of the theoretical “spindle ma-
trix”, an electron-dense substance proposed to provide a structural scaffold
for spindle MTs and motors (Pickett-Heaps and Northcote 1966; Pickett-
Heaps et al. 1984).
One intriguing question concerning spindle function is how MTs, which
are highly dynamic polymers that exhibit frequent transitions between
growth and shortening, give rise to the metaphase spindle, which maintains