Functional and morphological assessment of age-related changes in the choroid and outer retina in pigeons

We sought to determine if choroidal and outer retinal deterioration occur w ith age in pigeons, as they do in other species, and investigated the relat ionship between age-related retinal and choroidal changes. In 64 pigeons ra nging in age over the pigeon lifespan (0.5-20 years), we measured some or a ll among the following parameters: choroidal blood flow (ChBF) by laser Dop pler flowmetry, choroidal thickness and choriocapillary vessel abundance by LM histology, choriocapillary endothelial cell transport specializations b y EM histology, acuity by behavioral methods, and degenerating photorecepto r abundance and total photoreceptor abundance by LM histology. Regression a nd Receiver Operator Curve (ROC) analyses were used to characterize the pat tern of age-related changes and determine the ages at or by which significa nt changes occurred, For the 45 birds for which we measured choroidal param eters, choriocapillary vessel abundance showed a curvilinear decline with a ge and half of this decline occurred by 3.5-4.6 years. The endothelial cell transport specializations called channels also declined curvilinearly with age. Choroidal thickness was slightly increased between the ages of 3-6 ye ars, and thereafter declined steadily so that choroidal thickness in the ol dest birds was half that in the youngest. ChBF showed an abrupt decline of about 20% at 4 years and a Further 20% decline thereafter. In the 53 birds for which we obtained visual acuity and/or photoreceptor data, we observed a curvilinear decline in acuity (with half the decline having occurred by 8 years) and a prominent stepwise decline of about 20% in photoreceptor abun dance at 4.7 years, followed by further decline thereafter. The period of m ajor photoreceptor loss coincided with ages during which about 10% of photo receptors appeared to show degenerative changes (4-8 years of age), Using p artial correlation analysis with the common effect of age held constant, Ch BF was found to have a positive correlation with acuity. Our results show t hat ChBF and choroidal vascularity decline significantly with age in pigeon s, as do acuity and photoreceptor abundance, Our statistical analyses sugge st that prominent choroidal vascular decline preceded the visual decline, a nd that there is a positive relationship between choroidal and visual funct ions. Thus, our findings an consistent with the view that age-related decli ne in choroidal function might contribute to age-related vision loss in pig eons.