THE ROLE OF MICROTUBULES AND MICROTUBULE-ORGANIZING CENTERS 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 tr ansmission electron microscopy. Throughout ontogeny the migrating mito chondria were codistributed with cytoplasmic microtubules which were p reserved after the application of conventional preparation techniques for transmission electron microscopy. Both the basal body of the conne cting cilium and the second centriole located in the vicinity of the b asal body were demonstrated to act as microtubule-organising centres ( MTOCs) from which axonemal and cytoplasmic microtubules originated. Th e 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 radia l size-gradients within developing and mature cone inner segments. Thu s the consistent finding of microtubules and MTOCs in the structurally polarised cones represents an extreme example of the capacity of cell s to regulate the transport and distribution of organelles.