CILIOGENESIS IN PHOTORECEPTOR CELLS OF THE TREE SHREW RETINA

Transmission electron microscopy of the retinal cones from several pre natal, young postnatal and adult tree shrews (Tupaia belangeri) reveal s that the centrioles, from which the ciliary precursors of the outer segments grow out, are not transported into a pre-existing inner segme nt, but are positioned under the apical embryonic day 20 and thus prec edes initial formation of inner segment formation. Ciliogenesis starts before or on embryonic day 20 and thus precedes initial formation of the inner segment by 20 days, which is half the gestation period. Thus , the maturation of the outer segment covers a considerably longer per iod than has been previously described. Published observations from ot her mammals can be interpreted as conforming with the situation in Tup aia. In other vertebrates, compared to mammals, marked heterochronies do occur. In Tupaia, the centrioles and the cilium are located close t o the central longitudinal axis of the photoreceptor precursor cell fr om the 20-day-old embryo to the 5-day-old juvenile. In this position t he microtubule apparatus originating from the centrioles should be mos t effective in transporting the mitochondria into the inner segment. I n the 12-day-old tree shrew, when transport of the mitochondria into t he inner segment has been completed, centrioles and cilium have shifte d into an eccentric position and the light-collecting megamitochondria have approached the disks of the outer segment. This eccentric positi on is maintained in all later developmental stages. In certain of the retinal areas of the adult Tupaia, the connecting cilia of neighbourin g cones are always positioned on the same side of the inner segments.