IS ABNORMAL RETINAL DEVELOPMENT IN ALBINISM ONLY A MAMMALIAN PROBLEM - NORMALITY OF A HYPOPIGMENTED AVIAN RETINA

The central retina in hypopigmented mammals is underdeveloped. In the outer retina this deficit is confined to rods. Also, many ganglion cel ls in temporal regions project inappropriately to the contralateral he misphere. This study addresses the question of whether pigment-related abnormalities occur in the central retina of a non-mammal, the bird. Birds have a highly developed central retina, but unlike most mammals they do not have a significant uncrossed retinal projection. Consequen tly, examination of the retinae of hypopigmented birds will reveal whe ther there is a relationship between the two abnormalities. Also if on e of the primary effects of albinism is centred on rods, then albino b irds may not show a deficit, because their retinae are cone dominated. Retinae from normally pigmented and two forms of hypopigmented budger igars (Melopsittacus undulatus) were studied. Measurements of layer th ickness, cell density and cell size were made at a range of locations in the ganglion cell layer and in the inner and the outer nuclear laye rs. Estimates of cone numbers were also made. Each strain of bird had an area of increased retinal layer thickness in dorso-temporal regions , but not a fovea. Although there were variations in the measurements undertaken between the strains, none were pigment related or consisten t with the abnormality found in the central retina in albino mammals. Consequently, the underdevelopment of the central retina seen in hypop igmented mammals does not occur in this bird. There are two possible e xplanations for this result. First, normal mammalian retinal developme nt may depend partly on time-dependent interactions in the maturation of the retinal pigment epithelium and the neural retina. Although ther e is a common time table for the development of the mammalian visual s ystem when expressed in terms of the caecal period, which is between c onception and eye opening, the pace of retinal development in birds is accelerated, which may alter interactions between these regions. Seco nd, as the bird retina is cone dominated, any deficits in albino strai ns may be relatively minor.