Identification of the Mouse Uveoscleral Outflow Pathway Using Fluorescent Dextran

Abstract

purpose. This study was undertaken to assess directly whether there is uveoscleral outflow in the mouse eye by monitoring the movement of intracamerally injected fluorescent dextran.

methods. After anesthesia, NIH Swiss mice received intracameral injection of 1.5 μL of 0.2 pg/μL 70-kDa dextran conjugated to tetramethyl-rhodamine and to lysine. After survival times of 10, 20, 60, and 120 minutes, the experiments were terminated by transcardial perfusion with 2% paraformaldehyde. The eyes were enucleated and embedded in paraffin, and sections were prepared. These sections were then analyzed by fluorescence microscopy.

results. Fluorescent tracer in the eyes of animals that survived for 10 minutes was prominent in the iris root and ciliary processes and was of moderate intensity in the adjacent sclera. Moderate intensity fluorescence also was observed in the trabecular meshwork and adjacent cornea. At 20 minutes, intense fluorescence was observed in the ciliary processes and the ciliary muscle. This fluorescence in the ciliary muscle extended from the posterior edge of the ciliary muscle’s tail into the anterior choroid. At 60 minutes, the fluorescence in the choroid extended to the equator and adjacent sclera. The intensity of the fluorescence within the ciliary processes of these eyes was substantially reduced when compared with the 20-minute-survival eyes. At 120 minutes, label was observed only within trabecular meshwork and Schlemm’s canal.

conclusions. These results indicate that at least a portion of aqueous outflow in the mouse eye is through the uveoscleral outflow pathway.

The uveoscleral outflow pathway is a drainage route for aqueous humor from the anterior chamber that successively includes extracellular spaces within the iris root, the ciliary muscle, the anterior choroid and suprachoroidal space, and the adjacent sclera. ^{1 2 3 4 5 6 7} Aqueous humor then passes through the sclera into the extraorbital fat tissue and eventually into the lymphatic drainage. ^{8 9 10} There has been increasing interest in this pathway, perhaps related to the observations that topical application of certain prostaglandins (PGs) and PG analogues lowers intraocular pressure (IOP) by increasing uveoscleral outflow, and that some of these drugs are useful in the treatment of glaucoma. ^{11 12 13} The uveoscleral outflow pathway has been characterized in the eyes of monkeys, humans, rabbit, dog, and pig eyes. ^{1 5 6 14 15 16} However, it is presently unknown whether the mouse eye has a uveoscleral outflow pathway. 

It has been demonstrated recently that topical application of the PG analogue latanoprost, which lowers IOP and increases uveoscleral outflow in monkey and human eyes, also lowers IOP in mouse eyes. ^{17 18} These observations suggest that there may be a functional uveoscleral outflow pathway through which the IOP-lowering effect of latanoprost is mediated in the mouse eye. This study was undertaken to assess directly whether there is uveoscleral outflow in the mouse eye. 

Methods

Animals

All experiments were performed in compliance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. NIH Swiss mice, 9 to 11 weeks old and weighing 25 g, were obtained from Harlan Sprague-Dawley (San Diego, CA) and housed in clear plastic cages. The cages contained white pine shavings for bedding and were covered loosely with lids containing an air filter. The environment was kept at 21°C with a 12-hour light–dark cycle maintained by an automatic timer. 

Tracer Preparation

The tracer was 70-kDa dextran that was covalently conjugated to tetramethyl-rhodamine and to lysine residues (Molecular Probes, Eugene, OR). The presence of the lysine residues allows covalent cross-linking of the labeled dextran to tissue proteins by aldehyde fixatives. Hence, aldehyde fixation at the end of the survival phase stabilizes the position of the label in the aqueous outflow pathways during subsequent tissue embedding, sectioning, and clearing. To prepare a stock solution, the dextran was dissolved in phosphate-buffered saline (PBS) at a concentration of 2 mg/mL. Subsequently, the stock solution was clarified by centrifugation at 14,000g for 20 minutes and sterilized by passage through a 0.22-μM syringe filter (Millipore, Bedford, MA). An aliquot of this stock solution was diluted to 0.2 pg/μL with sterile PBS just before injection. This concentration was determined to be optimal in a series of pilot experiments. 

Tracer Injection

Before injection of the tracer, the mice were anesthetized by intraperitoneal injection of a mixture of ketamine (100 mg/kg, Ketaset; Fort Dodge Animal Health, Fort Dodge, IA) and xylazine (9 mg/kg, TranquiVed, Vedco, Inc., St. Joseph, MO). To minimize stress, the animals were gently immobilized in a plastic film restrainer (Braintree Scientific Inc., Braintree, MA), and the injection was delivered with a 30-gauge needle. Anesthesia was usually obtained within 4 minutes and absence of tail-pinch reflex was determined before proceeding. A drop of sterile PBS was instilled in each eye to avoid corneal desiccation. This protocol reliably produces a sufficiently deep plane of anesthesia for ocular procedures in mice. ¹⁸  

The tracer injections were made using a microprocessor-controlled motorized microsyringe fitted with a glass micropipet (UltraMicro-Pump II; World Precision Instruments [WPI], Inc., Sarasota, FL). The glass microneedle was made from borosilicate glass tubing (outer diameter, 1.0 mm; inner diameter 0.58 mm; KwikFil; WPI). The tip of the glass microneedle was drawn by pipette puller (P-87; Sutter Instruments, Novato, CA), and a 30 μm-wide beveled tip was produced by a microgrinder (Micropipette Beveler; WPI). The microneedle, microneedle holder, and syringe were filled with mineral oil to eliminate air-compression–related inaccuracy during the injection. The injection volume was 1.5 μL. Thus, based on the total amount of tracer injected (0.3 pg) and the volume of the anterior chamber (estimated to be approximately 2 μL) the concentration of tracer in the aqueous after the injection was approximately 0.8 to 1.5 pg/μL. 

Guided by a micromanipulator, the microneedle tip was advanced through the cornea of the left eye and visualized in the anterior chamber by stereomicroscope. Care was taken to avoid needle tip contact with either the iris or lens. After insertion, any dimpling of the cornea at the site of microneedle penetration was removed by a slight withdrawal of the microneedle position. To minimize injection-associated IOP elevation, the injection was delivered over 8 minutes. At 2 to 5 minutes after the end of the injection, the microneedle was gradually withdrawn. Absence of leakage of the bright red tracer through the injection site was confirmed after withdrawal of the microneedle by stereomicroscopic inspection. Survival time after the initiation of tracer injection was 10, 20, 60, or 120 minutes; there were three animals in each experimental group. 

Histologic Analysis

At the desired survival times, each mouse was killed by CO₂ inhalation. The vascular bed was rinsed by transcardial perfusion with PBS followed by 2% paraformaldehyde freshly prepared in 0.12 M phosphate buffer [pH 7.4]. After enucleation, both eyes were fixed an additional 3 hours in fixative, dehydrated through graded ethanol series, transferred to xylenes, and embedded in paraffin. Midsagittal 6-μm-thick sections were cut and then mounted on gelatin-coated glass slides. After drying, the slides were cleared in xylenes, rehydrated through a graded ethanol series, and rinsed in PBS. The slides were then exposed to freshly prepared 1% sodium borohydride dissolved in PBS for 10 minutes to reduce autofluorescence. ¹⁹ Finally, the slides were rinsed in PBS, and a coverslip was mounted (Fluoromount G; Southern Biotechnologies Associates, Birmingham, AL). 

The sections were examined by fluorescence microscope with rhodamine excitation and emission filters, and images were captured with a cooled charged-coupled device digital camera (Spot Camera; Diagnostic Instruments, Sterling Heights, MI). Control sections from the contralateral eyes of each mouse that were not injected with tracer were examined at the same time. 

For comparison, paraffin sections of untreated mouse eyes were stained with toluidine blue or hematoxylin and eosin and evaluated by bright field microscopy. 

Results

The ciliary body in the mouse contains well-developed ciliary processes and trabecular meshwork (Fig. 1) . As shown in the inset, the ciliary muscle is posterior to the trabecular meshwork and between the caudal ciliary processes and sclera. This has been reported by Smith et al. ²⁰ The posterior tail of the ciliary muscle extends toward the choroid between the pigmented epithelium of the retina and the sclera. 

At each time point analyzed, similar distributions of tracer were observed in each of the three animals examined. Examination of eyes fixed 10 minutes after the initiation of tracer injection revealed intense fluorescence in the iris root and in the stroma of the anterior ciliary process (Fig. 2) . In the latter case, this signal extended to the tip of the ciliary process stroma and surrounded the blood vessel that lies at the tip of the process. Trabecular meshwork also was intensely fluorescent. Moderate fluorescence was observed in the adjacent sclera and also in the corneal stroma. There was minimal tracer in the ciliary muscle, and tracer was not observed in the retina or posterior to the ciliary muscle. This fluorescence appeared to be specific to the tracer in the aqueous humor, because fluorescence was not observed within the anterior segment of the contralateral eye (Fig. 2B) . In addition, fluorescence was not observed in sections of the injected eye when using the microscope filter set for fluorescein. 

Eyes fixed 20 minutes after the initiation of tracer injection showed intense fluorescence in the ciliary process stroma, the ciliary muscle, and a short segment of the anterior choroid (Fig. 3) . Identification of the anterior choroid is supported by the presence of unstained round profiles surrounded by the fluorescent signal that are likely to be blood vessels (horizontal arrows). Fluorescence was not observed in the retina or within the posterior pole. 

An intense fluorescent signal in the eyes fixed 60 minutes after the initiation of tracer injection was observed in the choroid extending posteriorly to the equator and within the anterior and equatorial sclera (Fig. 4) . Moderate fluorescence was observed in the trabecular meshwork and Schlemm’s canal. Minimal fluorescence was observed in the ciliary processes and ciliary muscle. In some sections of the injected eyes, aqueous humor was retained and showed bright fluorescence adjacent to the surfaces of the iris anterior border layer and the corneal endothelium. There was reduced fluorescence in the midanterior chamber and fluorescence was not observed in the retina or in the posterior pole. 

Eyes fixed 120 minutes after the initiation of tracer injection of tracer injection contained moderate fluorescence in the trabecular meshwork and Schlemm’s canal (Fig. 5) . Minimal tracer was occasionally observed in the anterior choroid. Tracer was not observed elsewhere within the anterior segment or posterior pole. 

Discussion

After injection of fluorescent 70-kDa dextran into the anterior chamber of NIH Swiss mice, fluorescence was observed sequentially in tissue compartments that typically comprise the trabecular meshwork and uveoscleral outflow pathways. First, the tracer was visible in the trabecular meshwork and ciliary process stroma. Next, it was observed in the ciliary muscle and anterior choroid. Later, it was observed in equatorial choroid and within the stroma of adjacent equatorial sclera. Finally, there was a reduction of the fluorescent signal present in most of these structures, with retention of fluorescence observed only in the trabecular meshwork and Schlemm’s canal. Thus, in these latter two structures, there may have been binding or endocytosis of the fluorescent dextran. In contrast, the gradual elimination of fluorescence within the elements of the uveoscleral outflow pathway, indicates that there was minimal binding of the labeled dextran to these tissues. Hence, the progression of fluorescent signal through uveoscleral outflow pathway in the mouse probably reflects bulk transport of the labeled dextran as part of normal aqueous humor drainage from the anterior chamber. 

The sequence of tissues in which the fluorescent tracer appeared in the present study is the same as in monkey eyes in which uveoscleral outflow has been well characterized. In an early study, intracamerally injected anionic ferritin distributed through the extracellular spaces of the monkey ciliary muscle and choroid within 20 minutes. ⁶ The tissue distributions of intracamerally injected vinyl particles also confirmed bulk aqueous humor movement from the monkey anterior chamber through the ciliary muscle to choroid. ⁹ Additional studies identified the bulk movement of aqueous humor in the monkey uveoscleral outflow pathway from intraocular tissues to the sclera, and then to extraorbital tissues. ^{10 21 22} Similar labeling of the ciliary muscle, choroid, and sclera were obtained in a study of labeled albumin movements after intracameral injection into human eyes. ²³ The appropriateness of using fluorescent dextran as a marker for bulk flow in the mouse eye is supported by previous studies showing that fluorescent dextrans are stable in vivo and in vitro ²⁴ and that they are suitable for characterizing bulk flow of aqueous humor through the uveoscleral outflow pathway in monkey. ^{5 25} Thus, the movements of fluorescent dextran in the mouse eye observed in the present study suggests a uveoscleral outflow pathway exists in the mouse that shares many features of the uveoscleral outflow pathway of the monkey. 

Although there are striking similarities in the anatomic features of the uveoscleral outflow pathways of the mouse and monkey, there also may be differences between them. For example, the thin sclera of the mouse eye may be more permeable to transscleral movement of molecules from the suprachoroidal space to the extraorbital tissue space. This is suggested by the absence of observed fluorescence in the mouse posterior pole choroid compared with the appearance of tracer in the monkey posterior pole choroid. ³ It is possible that the amount of tracer injected was too low to deliver sufficiently observable label to the posterior pole. Another possibility is that the size of the dextran tracer may be important. Previously, Toris et al. ⁵ examined the ocular tissue distributions of 4-kDa, 40-kDa, and 150-kDa dextrans after intracameral injection in normal and inflamed monkey eyes. They observed good recovery of 40-kDa dextran in anterior uvea, anterior sclera, and posterior sclera of normal eyes. However, the most efficient recovery in the posterior sclera of inflamed eyes was with 150-kDa dextran. ⁵ Continuity of the posterior pole and equatorial choroidal spaces in mouse is suggested by the similar labeling of these compartments after subconjunctival injections of 70-kDa dextran. ²⁶ Thus, additional experiments with higher dose injections of 70-kDa dextran, as well as with higher-molecular-weight dextran tracers, may further clarify the properties of macromolecular transport to the posterior pole of the mouse eye by uveoscleral outflow. 

In addition to the observed fluorescence in the uveoscleral outflow pathway, fluorescence also was observed in the trabecular meshwork and Schlemm’s canal. This observation is consistent with an important role for trabecular meshwork outflow in the normal drainage of aqueous from the mouse anterior chamber. It is likely that despite delivering the injected dextran over the course of several minutes, there was elevation of IOP associated with the injection. Because conventional outflow is largely pressure sensitive, it may have increased conventional outflow of the tracer beyond normal in the present studies. ^{7 13} However, it is likely to have had little influence on the movement of tracer in the present study, because uveoscleral outflow facility appears to minimally affected by increased IOP in monkey eyes. ^{7 13} Physiological studies, as have been conducted in monkeys and other species, ^{1 14 27 28 29 30} are needed in the mouse eye to investigate these questions and to determine the proportion of total outflow that passes through the trabecular meshwork route versus the uveoscleral outflow pathway. Previous studies showing that fluorescent dextran can be used to measure aqueous humor dynamics in the eyes of cats and rabbits suggest that it also may be useful to measure outflow dynamics in mouse eyes. ^{31 32}  

In conclusion, the present study provides direct evidence supporting the presence of a mouse uveoscleral outflow pathway that has many similarities with the monkey uveoscleral outflow pathway. In particular, analogous structures within the uveoscleral outflow pathway appear to be involved in both the mouse and primates. These results, coupled with the prior observations of IOP reduction in the mouse eye after topical instillation of the PG analogue latanoprost, ^{17 18} raise the possibility that the mouse eye may be a useful model system in which to investigate general mechanisms of uveoscleral outflow regulation. Many mutant and transgenic mice strains are available, including some in which abnormally elevated IOP has been identified. ^{33 34 35 36} Thus, the mouse eye may provide new opportunities for studying the cellular and molecular mechanisms that influence uveoscleral outflow. 

 Supported in part by Grant EY05990 from the National Eye Institute (RNW), and the Joseph Drown Foundation (JDL).

 Submitted for publication December 28, 2001; revised March 11, 2002; accepted March 19, 2002.

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Figure 1.

 

The anterior segment tissues of the NIH Swiss mouse contain prominent ciliary body (CB) and retina (R) adjacent to the lens (L). The sclera (S) is thick near the cornea but typically is thinner than the retina throughout most of the globe. At high magnification, the ciliary muscle (arrow) can be observed. Magnification: ×100; (inset) ×420.

Figure 2.

 

Fluorescence microscopic images showing the distribution of fluorescent 70-kDa dextran in the ciliary processes (CP) and iris root 10 minutes after the initiation of tracer injection (A). In the uninjected contralateral eye (B), no fluorescence was observed. Note the presence of label in the stroma of the ciliary processes that surrounds the blood vessels at the tips of the processes. Magnification: (A) ×65; (A, inset) ×200; (B) ×55.

Figure 3.

 

Intraocular distribution of fluorescent 70-kDa dextran at 20 minutes after initiation of tracer injection. Note the presence of fluorescence in the ciliary processes (CP), ciliary muscle (CM), and choroid between the sclera (S) and retinal pigment epithelium (vertical arrow). Shown more clearly at higher magnification, the unlabeled round profiles (horizontal arrows) in the label path are likely to be choroidal blood vessels (inset A). To clarify interpretation of inset A, a similar image of the anterior sclera, choroid, and retina stained with hematoxylin and eosin is shown (inset B). Magnification: ×65, (insets A, B) ×160.

Figure 4.

 

At 60 minutes after initiation of tracer injection, 70-kDa dextran was present primarily in the choroid (C) and sclera (S). This is more easily seen in the high-magnification inset where the sclera containing substantial tracer is indicated by the closed arrows and sclera containing minimal tracer is indicated by an open arrow. Retinal pigment epithelium is indicated by a vertical arrow. Tracer was not observed in the retina (R). Note that the intensity of tracer in the ciliary processes (CP) was markedly reduced from that seen at 20 minutes (Fig. 3) , and that aqueous humor retained in the anterior chamber of this section contained bright fluorescence adjacent to the corneal endothelium and anterior surface of the iris. Magnification: ×71; (inset) ×175.

Figure 5.

 

At 120 minutes after initiation of tracer injection, 70-kDa dextran was present primarily in the trabecular meshwork and in Schlemm’s canal. Note the absence of fluorescence in the ciliary processes (CP), choroid, and sclera. R, retina. Magnification, ×60.

The authors thank Makoto Aihara, MD, PhD, for assistance in the development of the microinjection methods use in the study.