The role of microtubules and microtubule-organising centres during the migration of mitochondria.

The translocation of mitochondria towards the primitive inner segment of the cones in the tree shrew Tupaia belangeri was investigated by transmission electron microscopy. Throughout ontogeny the migrating mitochondria were codistributed with cytoplasmic microtubules which were preserved after the application of conventional preparation techniques for transmission electron microscopy. Both the basal body of the connecting cilium and the second centriole located in the vicinity of the basal body were demonstrated to act as microtubule-organising centres (MTOCs) from which axonemal and cytoplasmic microtubules originated. The megamitochondria in the inner segment of the retinal cones of Tupaia are unique among mammals with respect to their extraordinary size and to their ordered distribution characterised by longitudinal and radial size-gradients within developing and mature cone inner segments. Thus the consistent finding of microtubules and MTOCs in the structurally polarised cones represents an extreme example of the capacity of cells to regulate the transport and distribution of organelles.