Disk formation in retinal cones of Tupaia belangeri (Scandentia) |
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Authors: | Wolfgang Knabe H-J Kuhn |
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Institution: | Anatomisches Institut der Georg-August-Universit?t, Kreuzbergring 36, D-37075 G?ttingen, Germany Fax: 0551–397043, DE
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Abstract: | Existing hypotheses on the mode of disk formation in the photoreceptor cells of mammals appear to be incompatible: (1) plasma
membranes of adjacent evaginations form a disk which, subsequently, is internalized by a disk rim; (2) pinocytotic vesicles
are pinched off from the plasma membrane and fuse into a larger vesicle, which flattens and forms a disk. We have studied
the development of the cone outer segment and the disk formation in Tupaia
belangeri by transmission electron microscopy. During the first two postnatal weeks, the distal part of the single cilium, which is
inserted apically on the inner segment, becomes balloon-shaped. Apical to the axoneme, it contains tubular and vesicular material,
which, most probably, has been detached from the axonemal microtubules. These tubules and vesicles do not contribute to disks.
The balloon-shaped expansion, later retained as the ciliary backbone, establishes the contact with the pigment epithelium.
Formation of disks, from the 12-day-old Tupaia onwards, occurs between adjacent evaginations at the outer segment base. The initial disk rims are “hooked” to the ciliary
axonemal microtubules. The axonemal microtubules are involved in the initiation and in the alignment of the disks. Disk rim
formation and, thus, internalization of disks proceeds from the base to the apex of the outer segment, that is, from the younger
to the older disks. In the adult Tupaia, an uneven progression of disk rim formation on both sides of the axoneme is found among consecutive disks. The seemingly
incompatible hypotheses on the mode of disk formation reflect a heterochrony of the internalization of membranes and of the
disk formation among different mammals and, possibly, between cones and rods.
Received: 24 July 1997 / Accepted: 10 September 1997 |
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Keywords: | Retina Photoreceptor cells Outer segment Disk formation Tree shrew Tupaiabelangeri (Scandentia) |
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