This report deals with the as yet undetermined issue of whether
cell-surface associated microtubules in
certain cochlear epithelial cells are centrosomally nucleated and subsequently
migrate to microtubule-capturing sites located at the surface regions in
question. Alternatively, the cells may possess additional
nucleating sites which are noncentrosomal and surface-associated. These
alternative possibilities have been
investigated for highly polarised epithelial cells called supporting cells
in the mouse and guinea pig organ of
Corti using antibodies to pericentrin and γ-tubulin. There is
substantial evidence that both proteins are
essential components of microtubule-nucleating sites in cells generally.
Each mature supporting cell possesses
a large microtubule array that is remotely located with respect to its
centrosome
(more than 10 μm away).
The antibodies bind to a cell's centrosome. No binding has been
detected at 2 other microtubule-organising
centres that are associated with the ends of the centrosomally-remote
microtubule array while it is being
constructed. Such arrays include thousands of microtubules in some of
the cell types that have been
examined. If all a cell's microtubules are nucleated by its centrosome
then the findings reported above imply
that microtubules escape from the centrosomal nucleating site and migrate
to a new location. Furthermore,
capture of the plus and minus ends of the errant microtubules
is
taking place because both ends of a
centrosomally-remote microtubule array are attached to sites that are
precisely positioned at certain cell
surface locations. Minus ends are locating targets with an exactitude
comparable to that which has been
demonstrated for plus ends in certain cell types. These cells apparently
operate a single control centre
strategy for microtubule nucleation that is complemented by precise positioning
of plus and minus end-capturing sites at the cell surface.