Biomedical subjects

L D Beazley

Publications and source records attributed to L D Beazley.

94 records · Page 6Linked to original sources

PubMedDate not supplied

Topographies of retinal cone photoreceptors in two Australian marsupials.

Microspectrophotometry indicates the presence of at least three cone visual pigments in two Australian marsupials, the fat-tailed dunnart (Sminthopsis crassicaudata) and honey possum (Tarsipes rostratus). Here we have examined the distribution of cone types using antisera, JH455 and JH492, that recognize short-wavelength-sensitive (SWS) and medium-to-long-wavelength-sensitive (M/LWS) cone opsins, respectively. SWS cones were concentrated in dorso-temporal retina in the dunnart with a shallow decreasing gradient extending to the periphery (2300-1500/mm2). In the honey possum, SWS cones showed a uniform distribution (2700/mm2), except for a slight increase in a narrow peripheral band (3100/mm2). In both species, M/LWS cones dominated and their distributions were similar to those of retinal ganglion cells: a horizontal streak in the dunnart (31,000-21,000/mm2) and a shallow mid-ventral to peripheral gradient in the honey possum (37,000-26,000/mm2). A low number of cones remained unlabeled when the antisera were combined revealing further minority cone population(s). We discuss cone distributions in relation to visual capabilities and requirements of the species.

Animals↗

PubMedDate not supplied

The ipsilateral retinal projection in the fat-tailed dunnart, Sminthopsis crassicaudata.

The population of retinal ganglion cells which project ipsilaterally in the brain was examined in the fat-tailed dunnart, Sminthopsis crassicaudata, following injection of horseradish peroxidase into one optic tract. Retinae were examined as wholemounts and optic nerves as serial sections. In addition, visual fields were measured ophthalmoscopically. Ipsilaterally projecting ganglion cells were located temporal to a line which ran vertically through the middle of the area centralis and extended medially to define a ventrolateral crescent. Temporal to the naso-temporal division, a mean of 77% of ganglion cells projected ipsilaterally; these cells represented 20% of the total ganglion cell population. The magnitude and retinal location of the ipsilateral projection correlated with the extensive binocular field which measured 180 deg in the vertical (from 20 deg below the horizontal axis to 70 deg beyond the zenith) and 140 deg in horizontal meridian. Ipsilaterally projecting axons were restricted to the lateral third of the optic nerve along its length, sharing territory with contralaterally projecting axons.

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PubMedDate not supplied

Experimental eye enlargement in mature animals changes the retinal pigment epithelium.

Form deprivation has been shown to result in myopia in a number of species such that the eye enlarges if one eye is permanently closed at the time of eye opening. In the quokka wallaby, the eye grows slowly throughout life. After form deprivation, the eye enlarges by 1-1.5 years of age to the size of that in a 4-6-year-old animal and the number of multinucleated retinal pigment epithelial (RPE) cells in the enlarged retina remains much lower than would be expected in eyes of comparable size. Here we have repeated the experiment but examined animals at 4 years of age. The sutured eye grew significantly larger than did its partner. Numbers of RPE cells were comparable between sutured and partner eyes but were lower than in normal animals of similar age. Reductions in RPE cell density were greater in nasal than in dorsal or ventral retina and were not seen in temporal retina. The distribution of multinucleated cells was quite different in the sutured and open eyes. As in normal eyes, partner eyes had most multinucleated cells in ventral retina, while in the sutured eyes such cells were located mainly in the far periphery. In conclusion, the RPE is significantly changed by the eye enlargement process. However, it is not known whether this change results from an active part played by the RPE in the retinal expansion process or whether the changes are simply a result of a passive increase in area of the RPE.

Aging↗

PubMedDate not supplied

Electrophysiological evidence for transient topographic organization of retinotectal projections during optic nerve regeneration in the lizard, Ctenophorus ornatus.

In the lizard, Ctenophorus ornatus, anatomical studies have revealed that optic axons regenerate to visual centers within 2 months of nerve crush but that, from the outset, the regenerated projections lack topographic order (Beazley et al., 1997; Dunlop et al., 1997b). Here we assess the functional topography of the regenerated retinotectal projections by electrophysiological recording of extracellular multiunit responses to visual stimulation and by observing the lizards' ability to capture live prey. At the completion of the electrophysiology, DiI was applied locally to the retina and the topography of the tectal projection later assessed. Electrophysiology revealed that, at 2-4.2 months, responses were weak and habituated readily; no retinotopic order was detected. Between 4.5-6 months, responses were more reliable and the majority of lizards displayed a crude retinotopic order, especially in the ventro-temporal to dorso-nasal retinal axis. Although responses were variable between 6-9 months, they tended to be more reliable again thereafter. However, from 6-18 months, the projection consistently lacked topography with many retinal regions projecting to each tectal locus. Lizards, including those with electrophysiological evidence of crude retinotopy, were consistently unable to capture live prey using the experimental eye. Labelling with DiI confirmed the absence of anatomical retinotopy throughout. Taken together, the electrophysiological and anatomical data indicate that retinotopically appropriate axon terminals (or parts thereof) are transiently active whilst inappropriately located ones are silent. Presumably in lizard map-making cues fade with time and/or the mechanisms are lacking to stabilize and refine the ephemeral map. Moreover, the transient retinotopy is insufficient for useful visual function.

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