The albino chick as a model for studying ocular developmental anomalies, including refractive errors, associated with albinism

Abstract

Albinism is associated with a variety of ocular anomalies including refractive errors. The purpose of this study was to investigate the ocular development of an albino chick line. The ocular development of both albino and normally pigmented chicks was monitored using retinoscopy to measure refractive errors and high frequency A-scan ultrasonography to measure axial ocular dimensions. Functional tests included an optokinetic nystagmus paradigm to assess visual acuity, and flash ERGs to assess retinal function. The underlying genetic abnormality was characterized using a gene microarray, PCR and a tyrosinase assay. The ultrastructure of the retinal pigment epithelium (RPE) was examined using transmission electron microscopy. PCR confirmed that the genetic abnormality in this line is a deletion in exon 1 of the tyrosinase gene. Tyrosinase gene expression in isolated RPE cells was minimally detectable, and there was minimal enzyme activity in albino feather bulbs. The albino chicks had pink eyes and their eyes transilluminated, reflecting the lack of melanin in all ocular tissues. All three main components, anterior chamber, crystalline lens and vitreous chamber, showed axial expansion over time in both normal and albino animals, but the anterior chambers of albino chicks were consistently shallower than those of normal chicks, while in contrast, their vitreous chambers were longer. Albino chicks remained relatively myopic, with higher astigmatism than the normally pigmented chicks, even though both groups underwent developmental emmetropization. Albino chicks had reduced visual acuity yet the ERG a- and b-wave components had larger amplitudes and shorter than normal implicit times. Developmental emmetropization occurs in the albino chick but is impaired, likely because of functional abnormalities in the RPE and/or retina as well as optical factors. In very young chicks the underlying genetic mutation may also contribute to refractive error and eye shape abnormalities.

Intact globe inflation testing of changes in scleral mechanics in myopia and recovery

“…To quantify the effects of form deprivation treatments, refractive errors and axial ocular dimensions were measured using static retinoscopy and high-frequency A-Scan ultrasonography respectively, under gaseous anesthesia (1.5% isoflurane in oxygen) in both cases (Rymer et al, 2007). Refractive errors are expressed as spherical equivalent errors (SER), i.e.…”

Retinal image degradation by optical aberrations and light scatter in normal and albino chick eyes

“…The underlying genetic abnormality, which is expressed as an inability to synthesize melanin in ocular and extraocular tissues, is presumed to be responsible for the latter abnormalities (either directly or indirectly) and may be directly responsible for observed refractive errors. [17] …”

Correlation of visually evoked intrinsic optical signals and electroretinograms recorded from chicken retina with a combined functional optical coherence tomography and electroretinography system

“…Considering that the diameter of the fOCT imaging beam is 2.5 mm at the cornea and the diameter of the naturally dark dilated chicken pupil was between 3 and 3.5 mm, it is likely that the observed slow changes in the retinal layer reflectivity are due to vignetting of the imaging beam resulting from stimulus-induced pupil constriction and subsequent dilation. According to previously published research, [19][20][21] visually evoked pupil constriction in chickens begins ∼100 ms after the stimulus onset and occurs more rapidly than the subsequent dilation. The pupil dynamics timing and behavior described by Barbur et al 21 match well with the changes observed in the slow IOSs from the choroid (pale blue line in Fig.…”

“…Figure 3(a) shows that the a-wave (the negative peak of the ERG trace) appears ∼15 ms after the visual stimulus onset, which correlates well with previous ERG studies in living chicken retina and indicates normal photoreceptor function. 19,20 The corresponding IOSs measured in the inner retinal layers, from the NFL to the ONL, show very small (∼1% to 5%) positive changes and peak between 27 ms and 33 ms post-flash onset, which appear to correlate with the end of the steep rise of the b-wave (the positive peak in the ERG recording). Furthermore, these IOSs show a subsequent decrease in reflectivity, with minima between 67 ms and 73 ms after stimulus onset for the individual inner retinal layers.…”