Biomedical subjects

C F Wildsoet

Publications and source records attributed to C F Wildsoet.

29 records · Page 2Linked to original sources

PubMed1997

Active emmetropization--evidence for its existence and ramifications for clinical practice.

There is increasing evidence from animal studies in support of the concept of an active emmetropization mechanism which has potentially important clinical ramifications for the management of refractive errors. Recent research into refractive development and emmetropization is reviewed, with emphasis given to work involving the chick, tree shrew and monkey, which represent the three most widely used animal models in this field. The findings of this research are reviewed in a clinical context. Compensatory eye growth responses to focusing errors imposed by lenses represent the most compelling evidence for active emmetropization. These observations are complemented by other evidence showing recovery from induced refractive errors such as form-deprivation myopia. Of the animals listed above, chicks show the most impressive emmetropization, being able to compensate fully (using choroidal and scleral mechanisms) to lens powers ranging from +15 D to -10 D. The range of lens powers eliciting appropriate compensatory responses is narrower in the tree shrew and monkey, and the response patterns generally are also more complex to interpret. These data relate to young animals and together indicate refractive plasticity during development. Extrapolation of these findings to humans predicts that natural emmetropization will be inhibited in neonates by early intervention with prescription lenses, and that refractive correction of myopia will lead to accelerated progression. This convincing evidence for active emmetropization warrants due consideration in developing clinical management strategies for refractive errors.

Animals↗

PubMed1996

Effects on the compensatory responses to positive and negative lenses of intermittent lens wear and ciliary nerve section in chicks.

This study examined the ocular compensation to lens-induced defocus in chick and the effect of interrupting lens wear on a daily basis. Eyes fitted with +10 D lenses at hatching compensated rapidly, with almost complete compensation after 4 days of lens wear; they had decreased vitreous chamber depth compared to normal eyes and were thus hyperopic when the lenses were removed. In contrast, adaptation to the -10 D lenses was much slower, was still incomplete after 9 days of lens wear, and in this case, eyes had increased vitreous chamber depth and were myopic without the lenses. Adaptation improved when lens wear was delayed until 7 days after hatching. The effect of interrupting lens wear by periods of normal vision varied with the sign of the lenses worn. Hyperopia was always seen in response to +10 D lenses, although the magnitude of the response decreased as the duration of lens wear was decreased. In contrast, even brief periods of normal vision, i.e., 3 hr, prevented the development of myopia in response to the -10 D lenses; this apparent sensitivity to normal vision is similar to that reported for form-deprivation myopia. Ciliary nerve section used here to eliminate accommodation did not alter these response patterns.

Accommodation, Ocular↗

PubMed1995

Measurement of monochromatic ocular aberrations of human eyes as a function of accommodation by the Howland aberroscope technique.

Further development of the objective version of the Howland and Howland [(1976) Science, 193, 580-582; (1977) Journal of the Optical Society of America, 67, 1508-1518] aberroscope technique for measuring ocular aberrations is described. Compensation for refractive corrections and calibration is discussed. The technique was used to investigate the effect of accommodation upon the monochromatic aberrations of the right eyes of 15 subjects. Coma and coma-like aberrations were the dominant aberrations for most people at different accommodation levels, thus confirming previous findings. Variations in aberrations were considerable between subjects. About half the subjects showed the classical trend towards negative spherical aberration with accommodation. Changes in spherical aberration with accommodation in this study were less than found in previous studies where all monochromatic aberration was considered to be spherical aberration.

Accommodation, Ocular↗

PubMed1995

Monochromatic aberrations and myopia.

The monochromatic aberrations present in the eyes of a group of 21 young myopic subjects and 16 young emmetropic subjects were measured along the visual axis at three levels of accommodation. The aberrations were measured using a modified aberroscope technique which makes use of a retinal camera to photograph the shadow image of the aberroscope grid on the retina, while accommodation levels of 0, 1.5 and 3.0 D were induced consensually. Fourth-order aberrations were significantly different between the emmetropic and myopic groups, with the myopes showing lower fourth-order terms. A high proportion of the aberroscope grids photographed in the myopic eyes were too highly aberrated to permit analysis.

Accommodation, Ocular↗

PubMed1993

Chromatic aberration and accommodation: their role in emmetropization in the chick.

The roles of chromatic aberration and accommodation as cues to emmetropization in the chick were investigated. Myopia was induced monocularly by lid suture for a period of 1-2 weeks from hatching, after which eyes were reopened and the recovery process followed. Monochromatic light (ML) rearing conditions and ciliary nerve section surgery were used to eliminate chromatic aberration and accommodative activity respectively. Control animals were reared in white light (WL). When accommodation was left intact, chickens reared under monochromatic light were able to recover normally. However, ciliary nerve section produced hyperopia, deepening of the anterior chamber and a tendency towards axial lens thinning, irrespective of the light conditions used. Hyperopic refractive errors peaked at 4 weeks (mean refractive errors: +5.7 D, +4.21 D for ML, WL groups respectively, 4 weeks), with the ML group still exhibiting significant hyperopia at 7 weeks. Ciliary nerve section did not prevent the myopic response to lid suture (mean refractive errors: -22.65 D; -25 D for ML, WL groups respectively, 1 week) nor the elimination of myopia when eyes were reopened. These data indicate that neither accommodation nor chromatic aberration are fundamental to the gross operation of the emmetropization process although they may be essential for the fine tuning of refraction.

Accommodation, Ocular↗

PubMed1993

The spatial organization of tyrosine hydroxylase-immunoreactive amacrine cells in the chicken retina and the consequences of myopia.

We examined the spatial organization of the putative dopaminergic amacrine cells in the chicken retina and how this organization was affected by myopic eye enlargement. Myopia was produced by monocular lid suture for 4-7 months from hatching. Dopaminergic amacrine cells (TH-IR) were labelled by tyrosine hydroxylase immunohistochemistry. The somata of the TH-IR cells were usually located at the inner border of the inner nuclear layer; they gave rise to a dense plexus in stratum 1 (S1) of the inner plexiform layer, to a sparse plexus in stratum 3 (S3), and to short spiny dendrites at the border of strata 4 and 5 (S4/S5). The long thin processes in S1 and S3 could seldom be traced to their cell of origin, whereas the S4/S5 dendrites formed discrete fields that tiled the retina with little overlap. Lid suture resulted in retinal expansion of between 25-70%, but the total number of TH-IR amacrine cells was unaltered. Per retina, there were about 4700 TH-IR amacrine cells which showed a 3:1 density gradient from central to peripheral retina. The size of the S4/S5 dendritic fields increased proportionately in the expanded retinae, thus maintaining their coverage across the retina. The increase was achieved through scaled growth of the S4/S5 dendrites, involving both terminal and non-terminal dendrites. These findings suggest that the expansion of retinal neurons during myopia occurred through normal, albeit excessive, growth mechanisms.

Animals↗

PubMed1992

Retinal topography in the koala (Phascolarctos cinereus).

Nissl-stained retinal wholemounts were used to investigate the topographical organization of the ganglion cell layer of the koala (Phascolarctos cinereus); the visual resolution limit of this animal was subsequently estimated from retinal ganglion cell density data. Two types of cells could be differentiated on the basis of their size and staining characteristics: a subpopulation of presumed ganglion cells, consisting of medium to large cells with Nissl substance in the cytoplasm and pale uniformly staining nuclei, and a further subpopulation of small, densely staining cells. The latter group were presumed to be neuroglia and displaced amacrine cells. Iso-density contour maps were prepared from total cell counts and also counts of presumed ganglion cells; in all cases, the density of cells was greatest in the inferior retina where there was an area of peak density occurring as a poorly developed, horizontal streak that extended across the inferior retina. The inferior position of the streak in the koala contrasts with reports of the superior position of streaks in other marsupials. Peak cell densities of 2370 cells/mm2 and 1480 cells/mm2 were recorded for the total cell population and the presumed ganglion cell subpopulation, respectively. The latter value is equivalent to a visual resolution of 2.4 cycles/degree, based on sampling theory and a square packing paradigm, placing the koala close in visual performance to two other marsupials, the Australian Northern native cat and the American Virginia opossum.

Animals↗

PubMed1990

Saccadic oscillations facilitate ocular perfusion from the avian pecten.

THE evolution of the eye is constrained by two conflicting requirements--good vascular perfusion of the retina, and an optical path through the retina that is unobstructed by blood vessels. Birds are interesting in that they have higher metabolic rates and thicker retinas than mammals, but have no retinal blood vessels. Nutrients and oxygen must thus reach the neurons of the inner retina either from the choroid through 300 micron of metabolically very active retina, or from the pecten, a pleated vascular structure protruding from the head of the optic nerve into the vitreous chamber, and more than a centimetre away from some retinal neurons. Despite the diffusional distance involved, several lines of evidence indicate that the pecten is the primary source of nutrients for the inner retina: the presence of an oxygen gradient from pecten to retina, the large surface area produced by macroscopic folds and by microscopic infoldings of the luminal and external surfaces of the capillary endothelium, extrusion of circulating fluorescein, high content of carbonic anhydrase and alkaline phosphatase, and retinal impairments after pecten ablation. Another peculiarity of birds, their saccadic oscillations, occur with a large cyclotor-sional component during every saccadic eye movement. In different species, saccades, which occur at intervals of 0.5-40 s, have up to 13 oscillations with frequencies of 15-30 Hz and ampliá-tudes of about 10 degrees. Therefore, as much as 12% of some birds' total viewing time may be subject to the image instability caused by the oscillations. Using fluorescein angiography, we show here that during every saccade, the pecten acts as an agitator which propels perfusate towards the central retina much more effectively than is observed during intersaccadic intervals.

Animals↗

PubMed1988

Kainic acid-induced eye enlargement in chickens: differential effects on anterior and posterior segments.

Intravitreal injections of kainic acid were used to examine the significance of normal retinal activity for eye growth in chickens, this acid being chosen because of its known, selective neurotoxic effects on cells in the chicken retina. A 6 nmole dose of kainic significantly reduces amacrine cell numbers when used in very young chickens, while higher doses of kainic acid also affect bipolar and horizontal cell numbers. The effects of intravitreal injection of kainic acid on eye growth were assessed 4 weeks after treatment. A 200 nmole dose of kainic acid, used with day-old and 14-day-old chickens, had opposing effects on the anterior and posterior segments of the eye; while growth of the anterior segment was inhibited, the posterior segment was enlarged, predominantly in the equatorial direction. A 20 nmole dose of this acid similarly affected growth in 14-day-old chickens, but in day-old chickens, the anterior segment was also enlarged and the overall eye enlargement had an axial bias. Myopia was the most common refractive error associated with both patterns of development. A 2 nmole dose of kainic acid was without effect on eye growth. Parallels are drawn between these eye enlargement phenomena and those described in chickens whose visual environments have been manipulated. Our results indicate that normal retinal activity is fundamental to normal eye growth in chickens, and furthermore, that growth of the anterior and posterior segments of the chicken eye are independently regulated.

Aging↗

PubMed1985

The effect of illumination and foveal fusion lock on clinical fixation disparity measurements with the Sheedy Disparometer.

The Sheedy Disparometer is a clinical instrument designed to measure fixation disparity and hence determine forced vergence--fixation disparity curves. The target design includes a central 1.5 degrees fusion-free area. The effect of introducing a central fixation target to the instrument is investigated along with the influence of ambient illumination level on fixation disparity. All curve parameters, i.e. curve type, slope, X- and Y-intercepts were found to significantly alter with the introduction of a foveal lock. Illumination changes did not cause significant differences in any parameter.

Adult↗

PubMed1984

The contribution of diffusion to the entry of catecholamines into guinea-pig trachealis smooth muscle cells.

In smooth muscle or cartilage preparations of guinea-pig trachea incubated in 0.1 mumol/l 3H-isoprenaline, extraneuronal uptake inhibitors (corticosterone, normetanephrine, phenoxybenzamine) caused only partial inhibition (64-75%) of the formation of 3-O-methylisoprenaline. Thus, isoprenaline appeared to be exposed to catechol-O-methyltransferase by diffusional entry as well as by extraneuronal uptake in both the smooth muscle and cartilage regions of the trachea. Fluorescence microphotometric measurements of catecholamine uptake in trachealis smooth muscle cells, when tissues were incubated in 200 or 1,200 mumol/l isoprenaline, adrenaline or noradrenaline in the absence and presence of extraneuronal uptake inhibitors (corticosterone, normetanephrine, phenoxybenzamine), showed that the contribution of diffusional entry to the uptake of the amines into the cells fitted with the order of their lipophilicities, viz. isoprenaline much greater than adrenaline greater than noradrenaline. A kinetic analysis of the uptake of isoprenaline into the trachealis smooth muscle cells was carried out in the absence and presence of 100 mumol/l corticosterone. The kinetic analysis (a) showed that the corticosterone-resistant component of total uptake was not saturable, supporting the view that it represented diffusional entry of isoprenaline into the cells, and (b) provided Km and Vmax values (112 mumol/l and 101 F/min, respectively) for the saturable extraneuronal uptake of isoprenaline into the cells. The study provided evidence for marked diffusional entry of the lipophilic amine isoprenaline into guinea-pig trachealis smooth muscle cells. The diffusional entry of adrenaline was much less and that of noradrenaline negligible.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗