DISK FORMATION IN RETINAL CONES OF TUPAIA-BELANGERI (SCANDENTIA)

Existing hypotheses on the mode of disk formation in the photoreceptor cells of mammals appear to be incompatible: (1) plasma membranes of a djacent evaginations form a disk which, subsequently, is internalized by a disk rim; (2) pinocytotic vesicles are pinched off from the plasm a membrane and fuse into a larger vesicle, which flattens and forms a disk. We have studied the development of the cone outer segment and th e disk formation in Tupaia belangeri by transmission electron microsco py. During the first two postnatal weeks, the distal part of the singl e cilium, which is inserted apically on the inner segment, becomes bal loon-shaped. Apical to the axoneme, it contains tubular and vesicular material, which, most probably, has been detached from the axonemal mi crotubules. These tubules and vesicles do not contribute to disks. The balloon-shaped expansion, later retained as the ciliary backbone, est ablishes the contact with the pigment epithelium. Formation of disks, from the 12-day-old Tupaia onwards, occurs between adjacent evaginatio ns at the outer segment base. The initial disk rims are ''hooked'' to the ciliary axonemal microtubules. The axonemal microtubules are invol ved in the initiation and in the alignment of the disks. Disk rim form ation and, thus, internalization of disks proceeds from the base to th e apex of the outer segment, that is, from the younger to the older di sks. In the adult Tupaia, an uneven progression of disk rim formation on both sides of the axoneme is found among consecutive disks. The see mingly incompatible hypotheses on the mode of disk formation reflect a heterochrony of the internalization of membranes and of the disk form ation among different mammals and, possibly, between cones and rods.