Biology Reference
In-Depth Information
cells' cleavage plane varies, and has been linked to the alternatively proliferative and
differentiative division, the latter producing daughter cells that become postmitotic
neurons (Chenn and McConnell
1995
). The apparently aperiodic tumble of these
cells' mitotic spindles that precedes the selection of the cleavage plane orientation is
well documented (Haydar et al.
2003
). Neuroblasts are among the first candidates for
further application of the systems-biomechanical models developed for the simpler
and more regular oscillatory movement.
The spectrum of potential applications to movements of the cell body within the
cell boundary that are not known to be periodic is further illustrated by the problem
of distribution of nuclei in fungi. Upon germination of conidia in
Aspergillus nidu-
lans
, for example, the nuclei divide and distribute along the germ tube in a
microtubule-dependent and dynein-dependent fashion (Oakley and Morris
1980
;
Xiang et al.
1994
).
Returning to the review of comparatively regular oscillatory movements, oscilla-
tions of the nucleus during meiosis in the fission yeast
Schizosaccharomyces pombe
have a period of approximately 10 min (Chikashige et al.
1994
). The movement
depends on dynein (Yamamoto et al.
1999
). In its absence brought about by genetic
interference with dynein, alignment of homologous chromosomes and recombina-
tion are suppressed (Yamamoto et al.
1999
). The cited works speculate that the
movement facilitates chromosome disentanglement.
A further class of objects for the periodic models is represented by fertilized eggs
of various invertebrates. Their first division is accompanied by oscillations of the
mitotic spindle, which appear to be a mechanism of search for the morphogeneti-
cally correct spindle orientation. So, for example, Dan and Inoué (
2008
) describe
the preparation for the first division in the Atlantic surf clam
Spisula solidissima
. As
one semiaster assumes the central position in the egg, the other is pushed out to the
periphery, and its rays are bent flat (cf. the symmetry instability model in Fig.
13
).
Oscillations of this aster ensue, during which the spindle is pivoted at the position
of the central aster. The oscillations appear to be a mechanism for searching for the
morphogenetically correct orientation of the spindle, in which it stabilizes before
division.
The flattening of the “rocking” aster in this well-described case is analogous to
the flattening of the microtubules against the synapse in the cytotoxic T lympho-
cytes (Kuhn and Poenie
2002
). In the lymphocytes, accumulation of the microtu-
bule motor dynein on the cell cortex in the area of the synapse has been described
(Combs et al.
2006
). The rocking motion of the spindle during the second round of
divisions in
Caenorhabditis elegans
appears to belong to the same class: The periph-
eral semiaster oscillates about the morphogenetically correct position, its flattening
against the flattened cell-cell contact area is observed, and the dynein adapter dyn-
actin is found on the cortex in the contact area (Skop and White
1998
). These obser-
vations place special demands on the theoretical explanation of this apparently
widely represented class of oscillatory motion of the cell body. It must consider
large microtubule bends and dynein motor action on microtubules contacting the
cortex tangentially.
