Cell News 3/2013
18
RESEARCH NEWS
On Spindle Length and Shape.
Simone Reber
Max-Planck-Institute of Molecular Cell Biology and Genetics, Pfotenhauer Strasse 108, 01307 Dresden,
The internal organization of a cell is critical for its viability and
specifically tailored to its function. Therefore, number, size, and
geometry of intracellular organelles are important and must be
tightly regulated according to cell cycle state and cell type. A
classic example for the tight control of organelle number is the
centrosome cycle. Centrosomes are the major microtubule-orga-
nizing centre of animal cells and their number is linked to the
cell cycle state. In interphase, centrosome number is limited to
one centrosome per cell. Before mitosis, centrosomes duplicate
precisely once to become the poles of the bipolar mitotic spind-
le
1
. Aberrant centrosome numbers can result in the generation
of abnormal mitotic spindles and thus be a source of chromoso-
me instability, a hallmark of cancer
2
. Examples of cells adjusting
their internal organization are the active size regulation of the
mitochondrial meshwork in muscle cells or the over-proliferation
of the ER in secretory cells. Muscle cells can respond to chan-
ges in energy demands by modifying the rates of mitochondrial
biogenesis to induce compensatory changes in mitochondrial
capacity
3
. B cells express immunoglobulins (Ig) on their surface
but do not secrete antibodies. Upon binding of a specific antigen
to the B cell receptor, B cells proliferate and differentiate into
plasma cells, each of which secretes thousands of antibodies per
second. This massive Ig production correlates with an expansion
of the ER
4
. As basic physiological processes such as molecular
transport rates across membranes are intrinsically size-depen-
dent, their efficiency will vary with changes in organelle surface
area or volume
5
. Thus to understand cell organization, it will be
critical to understand how cells sense and control the number,
size and shape of their organelles.
While approaching near to complete proteomic parts lists of cel-
lular structures and organelles, mechanisms that control their
defined shape and size remain poorly understood. One reason
why this question has been so hard to answer is that the size of
an organelle is generally not simply set by a “ruler” (Figure 1a)
but is an emergent property of molecular collectives (Figure 1b).
“Emergence” describes the way complex properties and patterns
of a system arise by numerous elements, which interact by rela-
tively simple rules. Examples include the generation of an infi-
nite variety of six-sided snowflakes from frozen water in snow
6
(Figure 1c). Similarly, “flocking”, the coordinated motion of ani-
mals observed in bird flocks, fish schools, or insects swarms, is
Figure 1. Emergent Properties on Different Length Scales:
a)
Xenopus
spindle length is not set by a “ruler” (a) but is a collective behaviour problem (b). (c) Snow crystal. Scale bar ~1 mm Image courtesy Kenneth
G. Libbrecht (
). (d) Starling flock. Scale bar ~10 m. Image courtesy Robert Wolstenholme (
.
robwolstenholme.co.uk/). (e) Metaphase spindle assembled in Xenopus egg extracts. Scale bar 10 µm.
