Utilizing adult albino rats and lanthanum nitrate as a tracer, electron microscopic studies were done to provide additional information on the blood-optic nerve barrier. Following injection into the cervical artery, lanthanum was observed to fill the intercellular space of the optic nerve parenchyma. Diffusion from the prelaminar optic nerve into the juxta-optic nerve retina was, however, prevented by the Kuhnt tissue at the level of the rod and cone layer. In this region, tight junctions were found between the glial cells of the Kuhnt intermediary tissue and were continuous from the tight junctions of the retinal pigment epithelial cells to the junctions (zonulae adhaerentes) of the outer limiting membrane. From the level of the outer limiting membrane to the inner limiting membrane there was no diffusion of lanthanum into the adjacent retina despite the absence of tight junctions, and the lanthanum which had diffused from the choroid was never observed in the area of the inner limiting membrane of the optic nerve head. It is in this region that the functional barrier may exist.
Chloroquine retinopathy was produced experimentally in the eye of the albino corydoras (one of the tropical fish) by daily administration of chloroquine (0.1 mg per os). The enucleated eyes were examined from the 14th day to 3 months after the beginning of drug administration under light and electron microscopy. The first change of retina was the appearance of membraneous cytoplasmic body (MCB) in the cytoplasm of ganglion, amacrine, bipolar and horizontal cells. MCB might be degenerated lysosome. They showed lamellar figures or crystalline lattice-like structures. Secondarily, these MCB appeared in the inner segments of photoreceptor cells. The outer segments of rod cells disappeared, and then those of cone cells. Although photoreceptor cells were diminished in number in advanced degeneration, the cells of inner nuclear layer and ganglion cells were maintained in number. The presence of MCB dose not mean death of cells. The retinal pigment epithelial cells contained MCB in its cytoplasm only in severe degenerative cases, and did not show other remarkable changes. MCB also appeared in the cytoplasm of perictyes of retinal vessels. Chloroquine is considered to damage directly photoreceptor cells most severely.
The photoreceptor cell endings of goldfish retinas were examined using ultrathin and freeze-fracture techniques. The synaptic vesicles could be seen lying closely packed in rows on each side of the synaptic ribbon, and the vesicles nearest to the end of the ribbon could be seen to be in direct contact with the presynaptic membrane. The openings of the synaptic vesicles to the synaptic cleft could also be observed at the presynaptic membrane nearest to the end of the ribbon, that is, 100 nm distant from the center of the apex. In the freeze-fractured replicas of the presynaptic membrane, the band-shaped membrane particle aggregation was seen at the apex and a row of the crater-like or circular protuberances were observed at each side of the apex. These structures were considered to be changes of the presynaptic membrane organizations according to the synaptic vesicle exocytosis. It may be possible that the synaptic vesicle exocytosis in the photoreceptor cells occurs at the sites of the presynaptic membrane nearest to the apical end of the synaptic ribbon, and that the exocytotic sites form a line on each side of the apex.
The celestial goldfish were systematically reared from fertilization for life, revealing developmental processes of retinal degeneration. The eyes began to protrude laterally at the age of 90 days, rotated antero-dorsally at 120 days, and thus the ‘celestial eye’ was completed. The retina developed to normal mature structure by the age of 50 days. No degenerative findings were revealed before the start of eye extrusion. The first morphological change, irregular distribution of melanin granules in the pigment epithelial layer and disorientation of the outer segments of photoreceptors, was detected at the age of 90 days. At 120 days, the pigment epithelial layer lost melanin granules at places and was occupied by phagocytes. Photoreceptor cells were destroyed, and phagocytes, containing melanin granules, appeared in the inner retina and/or choroid. The retinal degeneration started simultaneously with eye protrusion. The regular telescopic-eye goldfish have large, extended eyes as those of celestial goldfish, but they develop no retinal degeneration. Celestial goldfish seems to be an interesting new type of hereditary retinal degeneration in vertebrates.
Electron microscopic studies were done on the structural alterations of retinal pigment epithelial cells occurring under osmotic stress utilizing rhesus monkeys and lanthanum nitrate as a tracer. Fluorescein fundus angiography revealed leaking points of fluorescein from the choroid to the retina, and various degenerative alterations of the retinal pigment epithelial cells were observed in these leaking areas. Some cells included many vacuoles in their cytoplasm, but diffusion of lanthanum from the choroid to the outer segment of visual cell was obstructed by the tight junctions between the retinal pigment epithelial cells. Other cells were severely damaged with ruptured membranes. Abundant tracer material diffused through the cytoplasm into the subretinal space. Other cells were broken down in an isolated manner and here marked extravasation of lanthanum into the subretinal space was observed.
Using a freeze-fracture technique, the fine structure of the Schlemm's canal and trabecular meshwork in the monkey was studied with special reference to intercellular junctions. Gap junctions were observed between the endothelial cells of Schlemm's canal and between the trabecular meshwork cells. Maculae occludentes were rarely observed between the endothelial cells of the inner wall of Schlemm's canal. The trabecular meshwork cells appeared to act in an electrotonic and metabolic syncytium.
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