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Biomedical subjects

J M Tiffany

Publications and source records attributed to J M Tiffany.

At least 37 records · Page 2Linked to original sources
PubMed1993

The casual level of meibomian lipids in humans.

Using a modified skin surface lipid measuring instrument, the Meibometer, the amounts of meibomian lipid on the lid margins (the casual levels) of 421 subjects aged 1 to 94 years were measured. The lowest levels were found in children younger than 14 years (means +/- S.E.: males = 1.48 +/- 0.17, females = 1.53 +/- 0.17 micrograms lipid/mm2 lid margin surface) and rose with age, the highest levels being found in males aged 60-69 years (mean +/- S.E. = 3.26 +/- 0.18 micrograms lipid/mm2 lid margin surface). Male levels were higher than females in the 20-29 years age group but male and female levels became indistinguishable above the age of 50. These age- and sex-related changes differed markedly from those seen in skin surface lipid levels. The age-related changes in the casual levels of lid lipids remain unexplained although an endocrinological mechanism modified by morphological factors is considered.

Adolescent↗
PubMed1992

Ocular surface residence times of artificial tear solutions.

Solutions of hydroxypropylmethylcellulose (HPMC) and polyvinyl alcohol (PVA) are widely used as artificial tears. However, their usefulness is limited by the short duration of their effect. Dilute sodium hyaluronate (SH) solutions exhibit non-Newtonian rheology with high viscosities at low shear rates, which would be expected to enhance their ocular surface residence time. Using quantitative gamma scintigraphy, estimates of the ocular surface residence times of 0.3% HPMC, 1.4% PVA, and 0.2% SH were made in six patients with keratoconjunctivitis sicca. The sodium hyaluronate solution had a mean half-life on the ocular surface of 321 s, significantly longer than hydroxypropylmethylcellulose (44 s; p = 0.012) and polyvinyl alcohol (39 s; p = 0.013).

Biological Availability↗
PubMed1991

The viscosity of human tears.

Blinking involves high rates of shear within the tear film, requiring a low tear viscosity to avoid damage to epithelial surfaces. Conversely, in the open eye, a higher viscosity is desirable to resist drainage and film break-up. Samples of human tears were collected with moderate stimulation from 5 adult males, 3 with normal and 2 with marginally-dry eyes. The apparent viscosity at 22 degrees C was found using a Couette-type rheometer over the range of shear rate 2-160 sec-1. Marked shear-thinning was apparent in all samples, with little apparent difference between normal and dry-eye tears. Although a power-law equation could be fitted over part of the range, analyses according to either the Casson plastic model as used for rabbit tears (with a low yield-point indicative of some very loose initial gel-like structure) or the Steiger-Ory model (a true pseudoplastic model with no initial yield point) were inconclusive over the range of shear rate studied. The descriptive model of Cross gave zero-shear viscosity values of 4.4, 7.1 and 8.3 mPa.sec for normal tears, and 27.1 and 31.1 mPa.sec for dry-eye tears; the corresponding time constants were 0.13, 0.27 and 0.38 sec for normal tears, and 2.7 and 2.9 sec for dry-eye tears. These time-constants can be considered as an approximate relaxation time, indicating the time taken for the tear film to stabilise after a blink.

Adult↗
PubMed1991

Factors responsible for tear ferning.

The biochemical agents involved in fern formation on drying tears were investigated by studying the different patterns of ferning of tears from normal and pathological eyes, mucus glycoprotein, saliva, serum, and various salt solutions. We conclude that the ferning phenomenon observed when a drop of tears is allowed to dry at room temperature on a clean microscope slide is to a large extent determined by the electrolyte concentration, especially the ratio of monovalent sodium and potassium ions to divalent calcium and magnesium ions. The presence of a biopolymer is essential, but this need not specifically be mucus as previously thought. The test may therefore be useful clinically in indicating the need for further tear analysis.

Electrolytes↗
PubMed1990

Lipid synthesis in vitro by rabbit Meibomian gland tissue and its inhibition by tetracycline.

Rabbit Meibomian gland tissue was incubated with radioactive acetate, propionate, valine, leucine or isoleucine. The time-course of incorporation of radioactivity into total lipids from acetate and isoleucine was studied in Hanks' or Krebs medium. Incorporation of acetate or isoleucine into lipid classes was followed by TLC, and of acetate label into individual fatty acids and alcohols by GLC with radioactivity detection. Radioactivity was highest in the minor acid components C16:1 and C18:1. Levels in branched fatty acids fell with chain length, reflecting the time-course of successive chain elongations. Use of amino acids or propionate as precursors suggested that molecules containing a specific branched structure were preferentially incorporated; this indicates a binding preference at the level of the fatty acid synthase which might explain the very high proportion of anteiso-branched structures in the rabbit secretion. Incorporation of label into total lipids was significantly reduced by tetracycline in the medium, with 50% reduction at about 1.5 mg/ml for acetate and 0.7 mg/ml for isoleucine.

Amino Acids↗
PubMed1990

Measurement of wettability of the corneal epithelium. I. Particle attachment method.

Surface wettability of the adult rabbit cornea, as indicated by the epithelium/air surface tension, was estimated from the attachment of particles of several different types from suspension in dimethyl sulphoxide-saline bathing solutions. The mean surface tension of corneas where mucus had been removed by rinsing with N-acetyl cysteine was 67.5 +/- 0.6 mN/m, and for corneas retaining their native covering of mucus was 68.3 +/- 0.8 mN/m. Thus the wettability of the normal cornea does not depend on the presence of mucus to overcome its previously-supposed hydrophobicity. Removal of mucus by wiping gave a damaged surface for which no surface tension could be determined within the range of solution surface tensions studied.

Acetylcysteine↗
PubMed1990

Measurement of wettability of the corneal epithelium. II. Contact angle method.

Adult rabbit corneas were saline-rinsed, mucolytic-rinsed, or wiped to remove mucus. The corneas were allowed to dry in air; sessile droplets of saline were placed on the surface at various drying times, and their equilibrium contact angle theta measured. Several stages of drying can be distinguished: 1) an initial period when excess water is present, with either complete spreading of droplets or very low value of theta; 2) a 'plateau' region lasting about 20-25 min in which theta is low and changes slowly with time, and reflects the intrinsic wettability of the surface; 3) a region when more severe drying takes place and theta rises sharply. Computed tables are used with known values of theta and the surface tension of the saline drops, to give the surface tension of the corneal surface and the interfacial tension between cornea and saline. Acetylcysteine-washed corneas showed a plateau value of about 16 degrees and corneal surface tension of 69.3 mN/m. Rinsed corneas remained fully wettable (theta = 0 degrees) for longer than 90 min; hence the plateau value must also be near zero and corneal surface tension about 72 mN/m. Mechanically-wiped corneas dried more rapidly and theta rose continuously with no marked plateau value, indicating much lower wettability due to cellular damage (about 40 mN/m).

Acetylcysteine↗
PubMed1989

Tear film stability and tear surface tension.

Surface tension has been measured by the method of Ferguson and Kennedy on small (0.3-0.4 microliter) samples of tears from 65 normal and 35 dry eyes. Non-invasive break-up time (NIBUT) was also measured on the same patients as an indication of tear film stability. The mean (+/- SD) surface tension value was 43.6 +/- 2.7 mN/m for normals, and 49.6 +/- 2.2 mN/m for dry eyes. All NIBUT values for dry eyes were below 20 sec (8.9 +/- 5.1 sec, mean +/- SD, n = 35) while 53% of normal values were 30 sec or over. A negative correlation was found between surface tension and NIBUT for both dry eyes and normals. Comparison of surface tension results for tears and a variety of standard solutions indicated that mucus makes the greatest contribution, even at low concentrations, but the effect of proteins can be large, especially in inflammatory states where serum proteins leak from conjunctival vessels. The measurement of surface tension is simple and direct, and in conjunction with the NIBUT value can help in classification of tear quality in questionably dry eye cases.

Adolescent↗
PubMed1987

Identification by combined gas chromatography-mass spectrometry of constituent long-chain fatty acids and alcohols from the meibomian glands of the rat and a comparison with human meibomian lipids.

Constituent long-chain fatty acids and alcohols from the meibomian secretions of the rat were examined as trimethylsilyl (TMS) and methyl ester-TMS derivatives by capillary gas chromatography and by combined gas chromatography-mass spectrometry. The positions of double bonds and methyl branch points were determined by the mass spectra of picolinyl esters and nicotinates for long-chain fatty acids and alcohols, respectively. Fatty acids had chain lengths from C12 to C34 and were of the straight-chain iso, anteiso and monounsaturated types. The unsaturated acids had double bonds in the omega-7 and omega-9 positions. The alcohols had corresponding structures. In common with the constituent acids and alcohols of other meibomian secretions, the chain lengths of the constituents showed a biphasic distribution with maxima around C16-C18 and C25-C27. The profile was qualitatively similar to that obtained from human meibomian secretion but with some differences in the relative proportions of certain acids and alcohols.

Animals↗
PubMed1986

Wettability of the corneal surface: a reappraisal.

Previous attempts by others to measure the wettability of the corneal epithelium concluded that the surface was strongly hydrophobic, but used methods which seem likely to damage the epithelial cell surface. These experimental techniques were repeated and the effects on the surface analysed by scanning electron microscopy. It is concluded that such methods cannot establish either the hydrophobicity or hydrophilicity of the cornea.

Animals↗
PubMed1986

Refractive index of meibomian and other lipids.

The refractive indices of human and rabbit meibomian secretions, and of several natural lipid mixtures and eye ointments, have been measured as a function of temperature and wavelength. A value of 1.482 was found for meibomian oil at typical eye surface temperature, but varies in humans between ca. 1.46 and 1.53 over the visible spectrum.

Animals↗
PubMed1985

The role of meibomian secretion in the tears.

Meibomian lipid promotes the formation and stabilisation of the precorneal tear film. The deposited layer of this material at the lid margins prevents entry into the eye of skin surface fatty acids which would disrupt the film. The external oily layer also acts to reduce the rate of evaporative loss of fluid from the tear film. The relationship between composition of the secretion and its physical properties is discussed.

Animals↗
PubMed1984

Identification of meibomian gland lipids by gas chromatography-mass spectrometry: application to the meibomian lipids of the mouse.

Methods are described for the structural determination of the constituent alcohols, steroids and fatty acids from meibomian gland esters. The compounds, obtained from hydrolysed extracts, were separated and quantified as trimethylsilyl and methyl ester-trimethylsilyl derivatives by fused-silica capillary column gas chromatography. Structural information relating to the position of chain branching and the position of double bonds in the aliphatic chains of the acids and alcohols was obtained from the mass spectra of a number of other derivatives. Thus pyrrolidide-trimethylsilyl and picolinyl ester-trimethylsilyl derivatives provided information on both branching and unsaturation. The double-bond position in unsaturated fatty acids was also examined by use of trimethylsilyl derivatives of the derived glycols. The positions of chain branching of the alcohols were determined by the spectra of their acetate and nicotinate derivatives and by their gas-liquid chromatographic retention times. In meibomian gland lipids from the mouse, cholesterol was the major constituent; alcohols from the n-, iso-, anteiso- and unsaturated series were present with iso-C26 and anteiso-C27 being the most abundant. Mono-unsaturated fatty acids belonged mainly to the omega 9 series and saturated acids belonged to the iso-, anteiso- and n-series. Several 1,2-diols were also identified. The most abundant of these had iso-C16 and iso-C20 chains. GC-MS studies on the intact wax esters showed them to be composed of the branched-chain alcohols and both branched-chain and unsaturated acids.

Acetates↗