infrastructure of Blood-Retinal Barrier Permeability
in Rat Phototoxic Retinopathy
Gary E. Korre, Roy W. Bellhorn, and Margaret 5. Burns
It has been shown previously that the blood-retinal barrier (BRB) of rats with phototoxic retinopathy
is permeable to sodium fluorescein and to fluoresceinated dextrans as large as 32A ESR (EinsteinStokes radius). The leakage presumably occurs from retinal capillaries that have invaded the retinal
pigment epithelium (RPE) and become fenestrated. In this report, the ultrastructural tracers horseradish peroxidase and catalase were used to further localize the leakage site, and to evaluate the size
limit of molecules penetrating the phototoxic BRB. Horseradish peroxidase (HRP: 30A ESR) freely
penetrates the BRB of phototoxic rats, since it is present in the retinal extracellular space 10 min
after intravenous injection. HRP penetrates the fenestrae of capillaries which invade the RPE from
the retina. It then diffuses along the pericapillary space of the intraepithelial capillaries, which is
confluent with that of their parent retinal capillaries, and into the retinal extracellular space. HRP
thus circumvents the tight junctions between RPE cells and between capillary endothelial cells, which
appear intact in thin sections. Catalase (52A ESR) does not freely penetrate the BRB of phototoxic
rats. As long as 40 min after intravenous injection, catalase is still confined to the lumen of fenestrated
capillaries in the RPE, retinal capillaries, and the choriocapillaris. Although present in intraendothelial vesicles, no evidence of deposition in the pericapillary space is observed. It is concluded fenestrated
capillaries in the RPE are a major site where blood-borne tracers penetrate the BRB in phototoxic
retinopathy. Invest Ophthalmol Vis Sci 24:962-971, 1983
Light-induced (phototoxic) retinopathy in rats
models the blood-retinal barrier (BRB) in disease.1'2
Fluoresceinated dextrans as large as 32A ESR (Einstein-Stokes radius) penetrate the BRB of phototoxic
rats, presumably at sites where retinal capillaries invade the retinal pigment epithelium (RPE) and become fenestrated.1"4 These observations raised questions that a study using ultrastructural tracers could
clarify: (1) What route do tracers take into the retina
after leaking across the fenestrated capillaries in the
RPE? (2) What contribution do opened tight junctions or vesicular transport across endothelial cells
make to the leakage? (3) What is the size limit of
molecules penetrating the phototoxic BRB?
To answer these questions, we have administered
intravenously ultrastructural tracers of two different
sizes—horseradish peroxidase (HRP) and catalase—
to rats with advanced phototoxic retinopathy. We
then determined the tracer's behavior in relation to
fenestrated capillaries in the RPE and to known components of the normal BRB—tight junctions between
RPE cells and capillary endothelial cells, and intraendothelial vesicles. Our observations indicate that leakage across fenestrated capillaries in the RPE and subsequent diffusion into the retina via their pericapillary
space is the major mechanism of barrier breakdown
in phototoxic rats. Tight junctions between RPE cells
and capillary endothelial cells remain intact, and
transendothelial vesicular transport of tracers was not
observed. The observations extend those made using
fluorescent markers.3'4
Materials and Methods
Tracer studies were undertaken in 26 albino rats:
eight control and five phototoxic rats each for studies
on horseradish peroxidase and catalase. Phototoxic
retinopathy was produced by .exposure to fluorescent
light, as previously described.2 Observations were
made 9-12 months after light challenge, when the
retinopathy is advanced (Figs. 1 A, B). Rats were anesthetized intraperitoneally with sodium pentobarbital
(40 mg/kg body weight) and ketamine hydrochloride
(15 mg/kg body weight) during experiments, and
killed afterwards by an overdose of sodium pentobarbital.
From the Department of Ophthalmology, Albert Einstein College of Medicine/Montefiore Medical Center, Bronx, New York.
Presented at the Annual Meeting Association for Research in
Vision and Ophthalmology (ARVO) Sarasota, Florida, May 1982.
Supported by National Eye Institute grant #EY 02038 to RWB
and an unrestricted grant from Research to Prevent Blindness, Inc.
Submitted for publication: August 3, 1982.
Reprint requests: Gary E. Korte, PhD, Department of Ophthalmology, Montefiore Medical Center, 111 East 210th Street,
Bronx, NY 10467.
0146-0404/83/0700/962/$ 1.30 © Association for Research in Vision and Ophthalmology
962
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933110/ on 06/18/2017
ULTRASTRUCTURE OF BLOOD-RETINAL DARKER / Korre er al.
No. 7
963
B
Fig. 1. Light microscopic histology of the retina of phototoxic rats, 10 minutes after intravenous injection of HRP. A, Retinal capillaries
(arrow) enter the RPE. The capillary profiles appear circular due to perfusion fixation. Their black outline is due to HRP reaction product.
Note apposition of RPE and inner nuclear layer (INL) of the retina, due to photoreceptor loss. Arrowheads, choriocapillaris apposed to
Bruch's membrane (3 titn plastic section stained with toluidine blue) (XI25). B, Higher magnification micrograph of retinal capillaries
(arrows) entering the RPE. The capillaries appear collapsed due to immersion fixation and are black due to HRP reaction product.
Arrowhead, leakage of HRP into the inner nuclear layer of the retina. Bruch's membrane overlies the RPE and is stained black due to HRP
reaction product (3 ^im plastic section, unstained) (X800).
Horseradish peroxidase (Sigma, Type II; 150-200
U/mg protein; 0.3-0.4 mg/g body weight in 0.5-0.75
ml saline) or catalase (Sigma, Type C-100; 30,00040,000 U/mg protein; 1-2 ml of an aqueous suspension) was injected into a cannulated femoral vein.
Ten or 40 min later the eyes were enucleated. The
cornea, lens, and vitreous were removed and the posterior eye cup immersed for 3-4 hrs in 2% paraformaldehyde and 2% glutaraldehyde in 0.1 M phosphate
buffer, pH 7.2. Some animals were perfused through
the heart with fixative prior to immersing the eyes in
it. The eyecups were sliced into narrow wedges ra-
diating about the optic disc. The wedges were rinsed
overnight at 4 C in phosphate buffer and then incubated at room temperature in a solution of diaminobenzidine and hydrogen peroxide for localizing
horseradish peroxidase5 and catalase.6'7 Control
wedges were incubated in the absence of diaminobenzidine or hydrogen peroxide. The wedges were
osmicated in 2% osmium tetroxide in 0.1 M phosphate buffer, dehydrated in graded concentrations of
methanol and embedded in epoxy resin.
Two-micron thick sections were cut midway between the optic disc and ora serrata. This provided
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933110/ on 06/18/2017
964
INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / July 1983
Vol. 24
Fig. 2. Thin-sectioned capillary in the pigment epithelium of a phototoxic rat which received an intravenous injection of HRP 10 minutes
prior to killing. A, Black reaction product fills the lumena (L) of the intraepithelial capillary and the choriocapillaris, their pencapillary
spaces, and Bruch's membrane (BM). Focal continuities (white arrowheads) between the intraepithelial capillary's pericapillary space and
Bruch's membrane make it possible for HRP leaking across the choriocapillaris to directly enter the pericapillary space. RPE cells abut the
capillary, on which they form basal folds. Area at blunt arrow is enlarged in Figure 2B. V, apical villi of RPE cells (X75OO). B, Reaction
product appears continuous across endothelial fenestra (arrowheads) of the intraepithelial capillary seen in Figure 2A. Intraendothelial
vesicles (arrows) are also filled with reaction product. L, capillary lumen (X2O.3OO).
a survey section containing the wedge's cut surfaces,
where reaction product is most dense. These regions
could then be trimmed for thin sectioning at known
distances from the razor-cut surface. Thin sections
were stained with lead citrate8 and uranyl acetate and
examined in a Zeiss EM-9S-2 electron microscope.
Tracer sizes are those accepted by other investigators910: horseradish peroxidase = 30A ESR and catalase = 52A ESR. Tissue from one normal and one
phototoxic rat was stained en-bloc with tannic acid
(1% in the aldehyde fixative) or uranyl acetate (0.5%
in saline, after osmication) to help visualize intercellular junctions.
Results
Ten minutes after intravenous injection, horseradish peroxidase reaction product is observed in the
pericapillary space of capillaries in the RPE and crossing between the retina and RPE (Figs. 1-3). Numerous examples of tracer-filled pericapillary spaces continuous with tracer-filled retinal extracellular spaces
were observed where capillaries cross the retina-RPE
boundary and in the subjacent retina (remnant outer
plexiform or inner nuclear layers: Figs. 3, 4A). Tracer
was not observed in the pericapillary space of capillaries in the inner plexiform layer even after 40 min
circulation time. However, reaction product was constantly present in their lumen and some intraendothelial vesicles (Fig. 4B).
Horseradish peroxidase seems to enter the pericapillary space of the intraepithelial capillaries by
penetrating their fenestra (Fig. 2A). Bellhom et al2
have described these fenestra and their occurrence in
many profiles of intraepithelial capillaries. We ob-
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933110/ on 06/18/2017
UURA5TRUCTURE OF DLOOD-RETINAL BARRIER / Korre er QI.
No. 7
965
A
Fig. 3. A, Capillary (L, lumen) bridging the RPE and retina (R), from a phototoxic rat that received an intravenous injection of HRP
10 min prior to killing. Reaction product occurs in the pericapillary space spanning the RPE-retina boundary (arrows), and is continuous
with reaction product in the retinal extracellular space, seen in the area indicated by a blunt arrow in Figures 3B and C. Curved arrow,
RPE process extending along capillary (X525O). B, C, Area denoted by the blunt arrow in Fig 3A, from the same section (B) and a nearby
area in an adjacent section (C). Blunt arrows denote where reaction product in the pericapillary space is continuous with reaction product
in the retinal extracellular space (X46,000).
served HRP reaction product in the pericapillary
space of both fenestrated and nonfenestrated capillary
profiles in the RPE. This could arise in two ways:
(1) Passage across fenestra outside the plane of section; (2) By diffusion across Bruch's membrane from
the choriocapillaris at sites where Bruch's membrane
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933110/ on 06/18/2017
966
INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / July 1983
Vol. 24
B
Fig. 4. Portions of retinal capillaries in the remnant outer plexiform layer (A) and the inner plexiform layer (B) of a phototoxic rat that
received an intravenous injection of HRP 10 min prior to killing. Reaction product is present in the lumen (L) and some intraendothelial
vesicles (arrowheads) of both capillaries, but in the pericapillary space (arrows) only of the outermost retinal capillary, ie, that nearest the
leaky intraepithelial capillaries. Note tracer in retinal extracellular space in A, (A: XI 8,400; B: X41,200).
and the pericapillary space of the intraepithelial capillaries are continuous focally (Figs. 2A, 5). This provides a route by which HRP may diffuse along the
pericapillary space of intraepithelial capillaries and
into the retina (Fig. 3).
Numerous HRP-laden vesicles occur in the endothelial cells of capillaries in the RPE and adjacent
retina. They cannot be interpreted as evidence for
vesicular transport from the capillary lumen, as their
contents could be HRP that was taken up after en-
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933110/ on 06/18/2017
No. 7
ULTRA5TRUCTURE OF DLOOD-RETINAL CARRIER / Korre er ol.
t
967
I
BM
RPE
Fig. 5. Capillary in the RPE of a phototoxic rat which received an intravenous injection of catalase 40 min prior to killing. Black catalase
reaction product is confined to the lumen (L), the enlarged pericapillary space remaining unstained. Bruch's membrane (BM) is also
unstained, since catalase does not penetrate the choriocapillaris.10 Arrows: continuity between Bruch's membrane and the pericapillary
space. Arrowheads: endothelial fenestra (as identified at higher magnification). V, apical villi of RPE cell surrounding the capillary (X15,600).
tering the pericapillary space via fenestra of the capillaries in the RPE or the choriocapillaris (Fig. 2A).
This ambiguity is addressed in experiments using
catalase. Catalase is retained in the lumen of capillaries in the RPE, the retina, and the choriocapillaris
even after 40 min circulation time (Figs. 5, 6). This
shows that the fenestra of the intraepithelial capillaries do not pass catalase and suggests that vesicular
transport across endothelial cells does not contribute
to the deposition of tracer in the retinal extracellular
space, even though some intraendothelial vesicles are
filled.
Tight junctions between RPE cells and among capillary endothelial cells appear intact in thin sections
(Figs. 7A-C). Even where RPE cells abut on intraepithelial capillaries, they retain apparently well-developed junctional complexes with their neighboring
RPE cells. Numerous examples of HRP present only
on the choroidal side of the junctional complex were
observed. HRP was observed on the retinal side of
RPE and endothelial tight junctions only near sites
where retinal capillaries entered the RPE, ie, where
HRP could approach both sides of the tight junction
due to leakage out of intraepithelial capillaries. The
observations using catalase also suggest intercellular
junctions remain intact; catalase was constantly arrested on the luminal side of capillary interendothelial
junctions in the retina and in the RPE.
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933110/ on 06/18/2017
968
INVESTIGATIVE OPHTHALMOLOGY & VI5UAL SCIENCE / July 1983
Vol. 24
Fig. 6. Capillary in the remnant outer plexiform layer of the retina of a phototoxic rat which received an intravenous injection of catalase
40 min prior to killing. Reaction product is confined to the capillary lumen (L) and some intraendothelial vesicles (arrowheads). No evidence
of deposition is seen in the pericapillary space (arrows), whose density equals that of control slices of tissue (X18,400),
Discussion
The major leakage site in the BRB of rats with
phototoxic retinopathy occurs at capillaries that invade the RPE from the retina (Fig. 8). The pericapillary space of these intraepithelial capillaries receive
bloodborne tracers via: (a) the fenestra of their endothelial cells, or (b) the fenestra of the choriocapillaris, followed by diffusion across Bruch's membrane
and into their pericapillary space. Once tracer is in
the pericapillary space of intraepithelial capillaries,
it can diffuse into the retina along the pericapillary
space of capillary segments bridging the RPE-retina
boundary. Thus, the permeability of endothelial fenestra and any hindrance to diffusion provided by the
basement membrane in the pericapillary space (as by
coulombic charge or molecular sieving910) would be
major determinants of the permeability of the BRB
in phototoxic rats. The relative contribution of the
choriocapillaris and the intraepithelial capillaries to
leakage into the retina would depend on their number
of fenestra and how extensive is the continuity between Bruch's membrane and the pericapillary space
of intraepithelial capillaries. The possibility exists that
HRP passage across intraepithelial capillary fenestra
is only apparent, due to HRP from the choriocapillaris "refluxing" up against them.
Tight junctions between RPE cells and capillary
endothelial cells remain intact in rats with phototoxic
retinopathy. They appear similar to those described
in normal rats.1' However, tight junctions that appear
intact in thin sections may also leak tracers.1213 Thus,
a tracer smaller than HRP, such as microperoxidase
(10A ESR14) may leak across apparently intact tight
junctions, even though HRP does not. Such a study
would further verify the integrity of these junctions
and determine if the mechanism of BRB breakdown
described above is the only one operating in phototoxic retinopathy. This is an important question as
tight junctions between RPE cells or endothelial cells
may open in some other conditions, eg, dog diabetic
retinopathy, after lens extractions in monkeys, or
during rat retinal dystrophy.15"17
Our observations using catalase indicate that vesicular transport across endothelial cells does not contribute to the barrier breakdown in rats with phototoxic retinopathy. As long as 40 min after intravenous
injection catalase remained in the lumen and some
intraendothelial vesicles of capillaries in the RPE,
retina, and choroid. No evidence of catalase deposi-
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933110/ on 06/18/2017
No. 7
ULTRASTWJCTURE OF BLOOD-RETINAL BARRIER / Korre er ol.
969
V
Fig, 7. Examples of junctional complexes connecting RPE cells of phototoxic rats. The cells in Figures 7A and C abut on intraepithelial
capillaries; the cells in Figure 7B are from a stretch of RPE devoid of intraepithelial capillaries. A, B, In a rat which received HRP 10 min
prior to killing, reaction product is restricted to the choroidal side of the junctional complex (JC), the extracellular space on the retinal side
of the junctional complex being free of tracer (arrowheads). V, apical villi of RPE (X52,4OO). C, In tissue stained en bloc with tannic acid,
the several types of intercellular junctions comprising the junctional complex are resolved. Membrane fusions corresponding to tight
junctions (arrowheads) are interspersed along an extensive zonula adherentes. Arrow, gap junction. V, apical villi of RPE cells. N, nucleus
of RPE cell (X47,500).
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933110/ on 06/18/2017
970
INVESTIGATIVE OPHTHALMOLOGY b VISUAL SCIENCE / July 1983
Bruch's
membrane
RETINA
tion in the pericapillary space was observed, such as
vesicles releasing their contents at the abluminal
plasma membrane.
HRP-laden intraendothelial vesicles in capillaries
in the inner nuclear layer may represent tracer picked
up at (a) the capillary lumen, (b) the pericapillary
space after leaking across the RPE or (c) HRP being
transported across endothelial cells by vesicles. Our
catalase observations cast doubt on, though do not
disprove, the latter possibility; HRP could be transported by vesicles even though catalase is not.
The absence of HRP in the pericapillary spaces of
the layers vitread to the inner nuclear layer as long
as 40 min after intravenous injection suggests that
these capillaries are not leaking due to either vesicular
transport or opened tight junctions. Essner et al18
came to similar conclusions in their study of dystrophic rats. As intraepithelial capillaries were not
observed in the animals they used, the leakage of retinal capillaries closer to the RPE was presumed due
to vesicular transport and/or opened interendothelial
tight junctions, and not leakage across the RPE itself.
HRP probably was not observed in the pericapillary
space of capillaries in the inner plexiform and more
vitread layers of phototoxic rats for several reasons:
(a) these capillaries remain impermeable to HRP. (b)
Longer than 40 min (our longest circulation time) is
needed for HRP to diffuse across the retina from its
Vol. 24
Fig. 8. Diagram illustrating how blood-borne tracers
can leak across the BRB
of phototoxic rats. Intravenously injected tracer
reaches the choriocapillaris
(small arrows in lumen of
top capillary) and intraepithelial capillaries in the RPE
(small arrows in lumen of
bottom capillary), which
arise from retinal capillaries. Tracer in the intraepithelial capillaries penetrates
their fenestra and enters the
pericapillary space (black).
This leads to the space between RPE cells (sealed off
by tight junctions) or directly into the retina (large
arrows). Tracer can also
reach the pericapillary space
of intraepithelial capillaries
by passing across the fenestrated choriocapillaris (top
capillary) and crossing
Bruch's membrane, which
is focally continuous with
the pericapillary space of the
intraepithelial capillaries.
leakage site in the RPE. The HRP may diffuse more
slowly along the pericapillary space as it dilutes out
in the vasculature (the source of this HRP). (c) The
HRP may build up in the pericapillary space, as by
electrostatic binding to basement membrane components, and clog it. Thus, the tracer may form its
own impediment to diffusion deeper into the retina,
(d) The HRP may be present in too low a concentration for us to detect.
The mechanisms outlined in Figure 8 may circumvent the BRB in other conditions in which retinal
capillaries invade the RPE. In rats with urethan retinopathy, in which retinal capillaries also invade the
RPE,19 we have found that HRP leaks into the retina
by the same route as described in our phototoxic rats
(unpublished observation: GK). In rats with hereditary retinal dystrophy, intravenously administered
retinol-binding protein leaks into the retina where
retinal capillaries invade the RPE (Fig. 5 from Reference 20). Conditions in which choroidal capillaries
penetrate the RPE and enter the retina would provide
a similar route by which blood borne molecules could
leak into the retina: leakage out of the fenestrated
choriocapillaris and diffusion along the pericapillary
space of choroidal capillaries entering the retina.21"23
Since both fenestrated and nonfenestrated capillaries
arising from the choroid have been observed in the
RPE and subretinal space in human senile macular
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933110/ on 06/18/2017
No. 7
ULTRASTRUCTURE OF BLOOD-RETINAL BARRIER / Korre er ol.
degeneration,2425 our observations in rats with phototoxic or urethan retinopathy may help us understand the mechanisms of abnormal permeability during the exudative phase of senile macular degeneration.
Key words: retina, blood-retinal barrier, permeability, ultrastructure, pathology, rat, phototoxic retinopathy, vasculopathy
Acknowledgments
The authors wish to acknowledge the excellent technical
assistance of Judith Channer and Noel Roa, and the secretarial assistance of Patricia Lynch.
References
1. Rabkin MD, Bellhorn MB, and Bellhorn RW: Selected molecular weight dextrans for in vivo permeability studies of rat
retinal vascular disease. Exp Eye Res 24:607, 1977.
2. Bellhorn RW, Burns MS, and Benjamin JV: Retinal vessel
abnormalities of phototoxic retinopathy in rats. Invest
Ophthalmol Vis Sci 19:584, 1980.
3. Bellhorn RW: Blood-retinal barrier abnormalities in rat phototoxic retinopathy. ARVO Abstracts. Invest Ophthalmol Vis
Sci 22(Suppl):63, 1982.
4. Bellhorn R and Korte G: Permeability of the abnormal bloodretinal barriers in rat phototoxic retinopathy. A clinicopathologic correlation study using fluorescent markers. Invest
Ophthalmol Vis Sci 24:972, 1983.
5. Graham RC Jr, Karnovsky MJ: The early stages of absorption
of injected horseradish peroxidase in the proximal tubules of
mouse kidney; ultrastructural cytochemistry by a new technique. J Histochem Cytochem 14:291, 1966.
6. Venkatachalam MA and Fahimi HD: The use of beef liver
catalase as a protein tracer for electron microscopy. J Cell Biol
42:480, 1969.
7. Novikoff AB and Goldfischer S: Visualization of peroxisomes
(microbodies) and mitochondria with diaminobenzidine. J
Histochem Cytochem 17:675, 1969.
8. Reynolds ES: The use of lead citrate at high pH as an electronopaque stain in electron microscopy. J Cell Biol 17:208, 1963.
9. Caulfield JP and Farquhar MG: The permeability of glomerular capillaries to graded dextrans. Identification of the basement membrane as the primary filtration barrier. J Cell Biol
63:883, 1974.
971
10. Pino RM and Essner E: Permeability of rat choriocapillaris to
hemeproteins. Restriction of tracers by a fenestrated endothelium. J Histochem Cytochem 29:281, 1981.
11. Hudspeth AJ and Yee AG: The intercellular junctional complexes of retinal pigment epithelia. Invest Ophthalmol 12:354,
1973.
12. Brightman MW, Hori M, Rapoport SI, Reese TS, and Westergaard E: Osmotic opening of tight junctions in cerebral endothelium. J Comp Neurol 152:317, 1973.
13. Martinez-Palomo A and Erlij D: Structure of tight junctions
in epithelia with different permeability. Proc Natl Acad Sci
USA 72:4487, 1975.
14. Smith RS and Rudt LA: Ocular vascular and epithelial barriers
to microperoxidase. Invest Ophthalmol 14:556, 1975.
15. Wallow IHL and Engerman RL: Permeability and patency of
retinal blood vessels in experimental diabetes. Invest Ophthal
Vis Sci 16:447, 1977.
16. Tso MOM: Pathology of the blood-retinal barrier. In The
Blood-Retinal Barriers, Cunha-Vaz J, editor. New York,
Plenum Publishing, 1980, pp. 235-250.
17. Caldwell RB, McLaughlin BJ, and Boykins LG: Intramembrane changes in retinal pigment epithelial cell junctions of
the dystrophic rat retina. Invest Ophthalmol Vis Sci 23:305,
1982.
18. Essner E, Pino RM, and Griewski RA: Permeability of retinal
capillaries in rats with inherited retinal degeneration. Invest
Ophthalmol Vis Sci 18:859, 1979.
19. Bellhorn RW, Bellhorn M, Friedman AH, and Henkind P:
Urethan-induced retinopathy in pigmented rats. Invest
Ophthalmol 12:65, 1973.
20. Bok D and Heller J: Autoradiographic localization of serum
retinol-binding protein receptors on the pigment epithelium
of dystrophic rat retinas. Invest Ophthalmol Vis Sci 19:1405,
1980.
21. Baum JL and Wise GN: Experimental subretinal neovascularization. Am J Ophthalmol 61:528, 1966.
22. Deutman AF: Significance of alteration of the outer bloodretinal barrier. In The Blood-Retinal Barriers, Cunha-Vaz J,
editor. New York, Plenum Publishing, 1980, pp. 365-374.
23. Archer DB, and Gardiner TA: Experimental subretinal neovascularization. Trans Ophthalmol Soc UK 100:363, 1980.
24. Grindle CFJ and Marshall J: Aging changes in Bruch's membrane and their functional implications. Trans Ophthalmol
Soc UK 98:172, 1978.
25. Sarks SH, Van Driel D, Maxwell L, and Killingsworth M:
Softening of drusen and subretinal neovascularization. Trans
Ophthalmol Soc UK 100:414, 1980.
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933110/ on 06/18/2017