Postnatal ocular development in the beagle dog: A histomorphological and immunohistochemical study

The beagle dog is a commonly used nonrodent species in juvenile toxicity studies, supporting pediatric drug development, and is a relevant model in ophthalmic research. However, reference data on postnatal ocular development are limited, complicating the interpretation of histologic findings in immature tissues, especially when age-matched controls are unavailable. The present study provides a comprehensive histologic and immunohistochemical characterization of postnatal ocular development in 68 beagle dogs from birth to 6 months of age, at closely spaced timepoints. At birth, eyelids were fused, and all ocular structures were immature. Major developmental events occurred primarily within the first 2 months and included corneal epithelial stratification around eyelid opening (~postnatal day 14), regression of hyaloid and pupillary membrane vascular remnants, high proliferative indices across multiple structures, and retinogenesis. Notably, the retina still contained the fetal outer neuroblastic layer at birth. Histology and immunohistochemistry (rhodopsin, synaptophysin, glial fibrillary acidic protein, Ki-67, calbindin, cleaved caspase-3, and von Willebrand factor) showed that critical stages of retinogenesis occurred predominantly during the first 3 postnatal weeks, initiating within the visual streak and progressing in a central-to-peripheral gradient-paralleling human fetal retinal development, which begins in the foveal/parafoveal macula. Most ocular structures reached histomorphologic maturity between 2 and 6 months. These data provide essential references to distinguish normal developmental changes from test article-related effects in juvenile toxicity studies and enhance the translational relevance of the beagle dog for pediatric and ophthalmic research.