MORPHOGENESIS OF MEGAMITOCHONDRIA IN THE RETINAL CONE INNER SEGMENTS OF TUPAIA-BELANGERI (SCANDENTIA)

The morphogenesis of the megamitochondria in the retinal cones of pren atal, young postnatal and adult tree shrews (Tupaia belangeri) was stu died by transmission electron microscopy and three-dimensional reconst ruction techniques. The initial assembly of the supranuclear cone mito chondria and their subsequent migration towards the developing inner s egment conform to the morphogenetic pattern known from other mammals. Within the first postnatal week, however, a marked increase in both th e number of the cristae and the matrix density occurs in the inner seg ment mitochondria of Tupaia. These mitochondria then grow, initially e xhibiting a basal-to-apical size-gradient. In the 17-day-old Tupaia, t his gradient is superseded by a radial size-gradient that, in addition to the single apical megamitochondrion, is characteristically found i n the adult Tupaia. The number of megamitochondria remains almost cons tant from day 12 of postnatal ontogenesis to the adult stage. Each meg amitochondrion consists of an apically located body from which several long processes project towards the base of the inner segment. In the older stages, the number of small mitochondria that most probably have budded off from the megamitochondrial processes clearly increases. We consider that megamitochondria in the cone inner segments of Tupaia a rise by the growth of a single mitochondrion and not by the fusion of smaller mitochondria.