Interplay Between Spindle Architecture and Function - International Review of Cell and Molecular Biology - page 88

k-fiber MTs and may be more or less numerous. They appear to increase the
rigidity of the spindle to resist to pole-to-pole compression and may provide
a pool of MTs to help maintain k-fibers. Because kinetochores and chromo-
some arms can move via motor proteins along spindle MTs toward the cen-
ter of the spindle, spindle MTs provide a k-fiber-independent mechanism
that promotes chromosome congression to the metaphase plate ( Cai
et al., 2009; Guo et al., 2013 ). However, the role of spindle MTs has infre-
quently been studied explicitly. In one case, fission yeast spindle MTs exten-
ding from each pole were shown to be dispensable for meiotic spindle
function ( Akera et al., 2012 ); however, in other systems, spindle MTs have
a more pronounced role. Depletion of spindle MTs from Xenopus egg
extract spindles by addition of a tubulin-sequestering protein revealed a role
for spindle MTs in establishing proper MT dynamics within the k-fiber as
well as for maintaining spindle size ( Houghtaling et al., 2009 ). In general,
larger spindles appear to contain a higher proportion of spindle MTs com-
pared with k-fibers, indicating that they primarily play a structural role.
In addition to the k-fiber and spindle MTs that make up the body of the
spindle, a population of astral MTs projects outward toward the cell cortex
from the MTOC or centrosome at or near the spindle poles. Astral MTs
appear less tightly bundled than MTs of the spindle body and, though found
in most spindles, they appear to be absent from female meiotic spindles and
plant spindles that lack centrosomes. Astral MTs may span the distance from
the spindle pole to the cortex and thus provide the cues and forces to prop-
erly orient and position the metaphase spindle, which is required for asym-
metric cell divisions important for cell fate determination, tissue
organization, and development ( Noatynska et al., 2012 ). However, in very
large embryonic cells, single astral MTs are not long enough to reach the cell
periphery, and the mechanism by which astral MTs contribute to metaphase
spindle positioning is unclear ( Mitchison et al., 2012 ). A prominent feature
of astral MTs is their increase in length and density at the metaphase to ana-
phase transition in both somatic and early embryonic cells, which likely con-
tributes to their function in positioning the spindle and cleavage furrow
( Wuhr et al., 2010 ). If astral MTs were induced to lengthen inappropriately
in HeLa cells, however, spindles oscillated dramatically in early anaphase due
to blebbing of the cell cortex at the poles, indicating that proper astral MT
dynamics is essential to coordinate spindle position with cortical contractility
( Rankin and Wordeman, 2010 ).
The relative partitioning of MTs among the different populations varies
in different cell types and species, but the high frequency at which all three