An Electron Microscopic Study on the Morphogenesis of the Outer Segment of the Retinal Rod

Abstract

The development of the outer segment of the rod of the retina of the albino rat has been studied with electron microscope on the eyes from the 13th day of gestation until the 14th day after birth.
At the early sta g e (from the 13th day of gestation to the 17th day of gestation) of the development of the optic cup, one of the diplosomes in the cells at the outer most portion of the neuroblastic layer extended the cilium which contained ciliary tubules. The bulge of cytoplasma of the primitive inner segment could not be found in this stage. The protruding processes were directed toward outwards and kept close contact with the pigment epithelial cells.
At the following sta ge (from the 18th day of gestation to the 5th day after birth), this primitive outer segment elongated its length and enlarged like cudgel. The plasma membrane showed an expansion in the tip of the cilium. The gradual enlargement of the cytoplasma at the tip of the cilium continued on opposite sides of the ciliary tubule.
On the 6th or 7th day after birth, several large vesicles could be found s uddenly in the distal portion of the cilium. These large vesicles seemed to be associated with protein synthesis in the inner segment and the processes of the pigment epithelial cells. Namely, these may be a precursor material for the membrane formation and may induce the plasma membrane invagination.
On the 8th day after birth, the plasma membrane invagination originating from the tip and all sides of the primitive outer segment appeared and were observed for one or two days. Thus, the lamellar membranous structure appeared suddenly on the 8th day after birth. Most of the invaginated disks were finally pinched off from the plasma membrane, however, only a few were continuous with it.
Thereafter, these primitive rod sacs were arranged from inwards to outwards in a position perpendicular to the long axis of the outer segment seen in mature animal.