144 J.C. Ambrose · R. Cyr
1969; Sakai 1969). Because of their close association with the nuclear envelope
(NE), the MTs of the clear zone and polar caps will subsequently be referred to
herein as perinuclear MTs. Perinuclear MTs are initially distributed randomly
along the surface of the NE, radiating out into the cytoplasm in all directions
(De Mey et al. 1982; Mineyuki et al. 1991). Close examination of these perinu-
clear MTs inHaemanthusendosperm reveals that most of them are organized
into groups of 2–3 MTs, called MT-converging centers (MTCCs), which collect
together at one end on the NE, while the other ends radiate into the cyto-
plasm (Bakhuizen et al. 1985; Smirnova and Bajer 1994). It was assumed that
the converging ends of the MTs within MTCCs were minus ends, based on
the known capacity of the nucleus to spawn MT growth (Stoppin et al. 1994).
Visualization of growing MT plus ends with GFP::EB1 later confirmed that
MTs grow predominately away from the nucleus at this stage ( 80 %away, 20 %
toward), consistent with the minus ends being anchored at the nuclear surface
(Dhonukshe et al. 2005).
Concurrent with PPB formation and narrowing, MTs become more nu-
merous on the surface of the nucleus, radiating out into the cytoplasm and
often reaching the cortical regions of the cell. In later prophase, these ra-
diating MTs become more dynamic and shorter in all regions except in the
plane of the PPB, where they continue to interact with the PPB site (Mineyuki
et al. 1991; Dhonukshe et al. 2005). As the PPB narrows, MTs radiating from
the nucleus and connecting to the cortex become progressively restricted
to the same region of the cortex that is occupied by the PPB, appearing as
spokes in a wheel from the pole view (Wick and Duniec 1983, 1984; Mineyuki
and Palevitz 1990; Nogami et al. 1996). At the same time, existing perinu-
clear MTs become sorted asymmetrically into two poles (forming the polar
caps), the axis of which defines the prophase spindle and corresponds to
the future spindle axis (Schmit et al. 1983). In cells with a PPB, the axis
of the prophase spindle is typically perpendicular to the plane of the PPB.
MTs within the poles run parallel to the future spindle axis, often extending
along the surface of the nucleus where they may interdigitate in the equa-
torial region; or they may extend out laterally into the cytoplasm to contact
the PPB region (Pickett-Heaps and Northcote 1966; Burgess 1970; Bakhuizen
et al. 1985). MTs of the prophase spindle typically converge at pointed poles,
which may appear annular in structure (Wick and Duniec 1984; Marc and
Gunning 1988; Liu et al. 1993). In some species, such as soybean suspension
cultures, prophase spindle poles are comparatively broad (Wang et al. 1991).
MTs may also become associated with protrusions or invaginations near the
polar regions of the NE (Bajer and Molè-Bajer 1969; Hanzely and Schjeide
1973), or may also occasionally penetrate into the nucleus (Pickett-Heaps
and Northcote 1966).
Although no discrete microtubule-organizing centers (MTOCs) have been
observed in the polar caps of higher plants, extensive ER is present through-
out the polar caps, and has been observed to frequently coalign and/or inter-