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Biomedical subjects

D M Maurice

Publications and source records attributed to D M Maurice.

15 recordsLinked to original sources
PubMed1992

Use of fluorometry in assessing the efficacy of a cation-sensitive gel as an ophthalmic vehicle: comparison with scintigraphy.

Gelrite, a heteropolysaccharide that forms a gel in the presence of cations, was tested in humans for its efficacy as an ophthalmic vehicle by a nonivasive fluorometric technique. Fluorescein was used as the tracer, and its concentration in the anterior chamber was used as the principal measure of bioavailability. The gel afforded a twofold increase in penetration of fluorescein compared with an isotonic buffer solution; this increase is slightly more than can be obtained with simple viscous vehicles. The increase in penetration caused by Gelrite was confirmed by measurements of the contact time of fluorescein in the tear film with the cornea. Earlier experiments with scintigraphy suggested a considerably greater contact time of fluorescein with the cornea when Gelrite was used. However, this increased contact time may be because the technique also measures radioactive tracer that had dried out on the lid margins. Accordingly, significant quantities of fluorescein could be eluted from the lids after the penetration experiments were completed.

Adult↗
PubMed1992

A microfluorometer for measuring diffusion of fluorophores across the cornea.

A microscope has been modified to a confocal spatially scanning fluorometer so that the concentration profile of a fluorophore across an isolated cornea can be measured. A slit of light is focused through one half of the objective to excite fluorescence. A confocal slit is placed at the image-plane of the microscope to collect the fluorescent light from the tissue which passes through the other half of the objective. The fluorescent light falls on to the cathode of a photomultiplier whose output is amplified by a lock-in amplifier. Scanning across the cornea is achieved by a stepper motor coupled to the fine focus of the microscope. The performance of the instrument has been assessed by studying the transport of fluorescein, carboxyfluorescein, and rhodamine B through intact rabbit corneas. A depth resolution of about 10 microns has been achieved with a 40x objective. This resolution is sufficient to determine concentration gradients in the epithelium as well as the stroma and to partially resolve the endothelium. The potential errors in the technique, resulting from limited resolution, light scattering and absorption, quenching of fluorescence, and binding of the fluorophores, are discussed.

Animals↗
PubMed1991

The loss of fluorescein, fluorescein glucuronide and fluorescein isothiocyanate dextran from the vitreous by the anterior and retinal pathways.

The pathways by which fluorescein (F), fluorescein glucuronide (FG) and fluorescein dextran (FD) leave the vitreous body of the rabbit were examined by measuring the concentration distribution of the injected fluorophores in sections of the frozen eyes. The contours of F, as already known, show that it leaves the vitreous predominantly across the retinal surface. Mathematical analysis of the concentration gradient leads to an average outward permeability coefficient of 1.4 x 10(-3) cm min-1 for the retinal layers. The contours of FG and FD show that they leave predominantly by diffusion into the posterior chamber, encountering only a minor barrier at the anterior hyaloid membrane. The anterior contours indicate that there can be no substantial posteriorly directed fluid flow through the vitreous; if it occurs its velocity across the retinal surface must be less than 2 x 10(-5) cm min-1. The contours of FD near the posterior pole of the retina suggest that such a flow may be taking place. Some time after the systemic administration of F, an analysis of the rate of loss of fluorescence from the vitreous body shows that this corresponds to the movement of FG out through the anterior chamber. Its value bears little relationship to the condition of the blood-vitreal barrier.

Animals↗
PubMed1991

The kinetics of tear fluid under the lower lid.

A very small drop of fluorescein solution was placed at the bottom of the lower fornix in human volunteers and its appearance in the tear film was measured. This was quite variable, but its appearance time averaged about 4 min and the peak of fluorescence 8 min. The time was shortened by blinking. The appearance time was too fast to be accounted for by simple diffusion in an unmixed tear fluid. Under normal circumstances, the release of the dye from the fornix is faster than its loss from the conjunctival sac, so that it does not seem to be a controlling factor in this loss.

Adult↗
PubMed1991

Loss of fluorescein across the conjunctiva.

The rate of disappearance from the tear film of fluorescein and rhodamine dextran instilled together into the human eye was compared. In many cases fluorescein disappeared more rapidly, which was attributed to its penetration across the conjunctival surface. A corresponding mean fluorescein permeability across this surface of 2.5 x 10(-5) cm min-1 was calculated. This route of loss of fluorescein from the tears leads to an average overestimate of tear turnover of 25%.

Adult↗
PubMed1991

Synchrotron x-ray diffraction studies of the cornea, with implications for stromal hydration.

The intermolecular and interfibrillar spacings of collagen in bovine corneal stroma have been measured as a function of tissue hydration. Data were recorded from low- and high-angle x-ray diffraction patterns obtained using a high intensity synchrotron source. The most frequently occurring interfibrillar spacing varied from 34 nm in dry corneas to 76 nm at H = 5 (the hydration, H, is defined as the ratio of the weight of water to the dry weight). The most frequently occurring intermolecular Bragg spacing increased from 1.15 nm (dry) to approximately 1.60 nm at normal hydration (H approximately 3.2) and continued to increase only slowly above normal hydration. Most of the increase in the intermolecular spacing occurred between H = O and H = 1. Over this hydration range the interfibrillar and intermolecular spacings moved in tandem, which suggests that the initial water goes equally within and between the fibrils. Above H = 1 water goes preferentially between the fibrils. The results suggest that, even at normal hydration, water does not fill the interfibrillar space uniformly, and a proportion is located in another space or compartment. In dried-then-rehydrated corneas, a larger proportion of the water goes into this other compartment. In both cases, it is possible to postulate a second set or population of fibrils that are more widely and irregularly separated and therefore do not contribute significantly to the diffraction pattern.

Animals↗
PubMed1990

Cohesive strength of corneal lamellae.

Strips of rabbit stroma were prepared, and the force required to tear them apart along their length was determined. This amounted to an average of 10 g mm-1 width of tissue and it is independent of the depth of the plane in which the splitting of the cornea takes place. It could not be determined, however, whether the cohesive strength of the tissue is due to occasional collagen fibrils binding it together, interweaving of the lamellae or enmeshing of the collagen fibrils by ground substances. Other corneas were split by blunt dissection in vivo and allowed to recover for various periods of time, when the reformed strength of the split was measured. It was found to be negligible for about 5 days and rose quickly to 0.25-0.5 of the value of the untouched cornea. No increase in the force of adhesion of the split stroma was observed if an extract of corneal epithelium or a suspension of platelets was injected into the wound.

Adhesiveness↗
PubMed1990

The pH in the precorneal tear film and under a contact lens measured with a fluorescent probe.

The reaction of the precorneal tear film of the human eye was determined non-invasively by instilling pyranine, a pH-sensitive fluorescent dye. The mean value was 7.83 (S.D. +/- 0.10) and it takes up this value immediately on opening the eye after the lids had been kept closed. The HCO3 system seems to be responsible for only a portion of the buffering power of the tear film. When a drop buffered to pH 6.4 with 0.075 M PO4 was instilled, the tears returned to their normal value in about 7 min, consistent with the washout times of solutes in the conjunctival sac. A pH of 7.3 was established in the tear fluid behind contact lenses either gas permeable or impermeable, probably as a result of their restricting the loss of CO2 from the eye.

Adult↗
PubMed1990

Staining of the conjunctiva and conjunctival tear film.

Both the distribution of tear fluid over the conjunctiva and any injury to the conjunctival epithelium can be made visible by instilling a fluorescent solution into the eye and observing with an appropriate combination of excitation and barrier filters. Sulphorhodamine B, which has an orange fluorescence that can be separated from the green natural fluorescence of the ocular tissues, gives a greater contrast than fluorescein. The tear film is seen to cover the surface of the conjunctiva and to be concentrated in its folds. Small circular areas of thin tear film appear transiently in the neighbourhood of the limbus after a blink. Occasional cells stain on the normal conjunctiva, particularly in the interpalpebral area. The density of the staining increases in dry eye conditions. Conjunctival trauma is sensitively revealed by the method, and its healing can be followed. Hard contact lenses are seen to traumatize continually the inferior limbal conjunctiva in symptomless wearers.

Conjunctiva↗
PubMed1985

The absence of corneal toxicity with low-level topical anesthesia.

By means of an osmotic pump we infused 0.3% proparacaine solution continuously into the central stroma of a rabbit cornea. An area of anesthesia about 6 mm in diameter was maintained for several days. The epithelium remained unaffected and healed normally where it was scraped away in the anesthetic area. Similarly, the repeated instillation into the rabbit's eye of 0.05% proparacaine eyedrops at ten-minute intervals throughout the day resulted in continual anesthesia without signs of epithelial dystrophy. It seems that the toxic level of proparacaine is above that required for anesthesia.

Anesthesia, Local↗
PubMed1985

Kinetics of macromolecules injected into the subretinal space.

Small, experimental, non-rhegmatogenous retinal detachments (blebs) in rabbit eyes resorbed 50% more slowly when filled with autologous serum than with Hanks' solution. To study the fate of large molecules in the subretinal space, carboxyfluorescein and several sizes of FITC-dextrans were injected into blebs and their movement followed by fluorophotometry. Carboxyfluorescein diffused quickly into the vitreous and was gone from the space after 8 hr. FITC-dextran 10-S (smaller than albumin) also diffused readily into the vitreous and took about 30 hr to be eliminated from the subretinal space. The diffusion of FITC-dextran 70-S and 150-S (both larger than albumin) was markedly slower, and roughly 80% of the 150-S was still present in the subretinal space after 3 days. Since the subretinal fluid in all of these blebs resorbed within 10 hr, the physiologic mechanisms for fluid resorption and elimination of large substances appear to be independent. Damaging the RPE barrier with sodium iodate allowed even the larger FITC-dextrans to exit from the subretinal space.

Absorption↗
PubMed1978

Transplantation of tissue-cultured corneal endothelium.

Cultured endothelial cells have been shown to regain their physiological function when replaced in the rabbit eye. Corneas were wiped free of native endothelium and seeded with cultured cells. After an incubation period, full-thickness buttons were cut from these corneas and transplanted into recipient animals. Clear grafts were obtained only when the donor cells were derived from cultures less than a month old. Light and scanning electron microscopy showed the endothelial cells of these grafts to be present as a slightly irregular monolayer on the posterior surface of the cornea. In corneas made edematous by benzalkonium chloride, the clear graft remained surrounded by thick and cloudy host tissue. In those grafts with 3H-thymidine--labeled cells, radioactivity was limited to the host tissue.

Animals↗
PubMed1977

A simple conjunctival biopsy.

Biopsy of conjunctival goblet cells may be carried out by pressing a piece of Millipore filter on the conjunctival surface. This can then be stained with PAS and hematoxylin and observed under the microscope after the filter has been cleared with oil. Either spots of mucous corresponding to the openings of the goblet cells are seen, or the filter adheres to the surface and pulls off one or more layers of epithelial cells and goblet cells. The procedure can be carried out on the unanesthetized eye and does not cause discomfort.

Animals↗
PubMed1976

Automatic recording of corneal thickness in vitro.

An addition to the specular microscope is described which allows it to record the thickness of the excised cornea automatically as a function of time. The focus of the instrument is scanned mechanically through the tissue, and the position of the reflecting surfaces is detected by a photo-electric system and marked on a chart recorder. The system is able to follow thickness changes over periods of many hours and with an accuracy greater than obtainable by manual operation. This system has been helpful in the evaluation of a new medium which considerably extends the useful lifetime of the corneal endothelial fluid pump.

Animals↗