Brain Research, 142 (1978) 343-352 343 © Elsevier/North-Holland Biomedical Press Postnatal development of retinal projections to the lateral geniculate body in Syrian hamsters KWOK-FAI SO, GERALD E. SCHNEIDER and DOUGLAS O. FROST* Department of Psychology, Massachusetts Institute of Technology, Cambridge,Mass. 02139 (U.S.A.) (Accepted September 14th, 1977) The hamster visual system is a convenient model for studying the factors governing the formation of orderly connections in the developing mammalian brain. The immaturity of the hamster nervous system at parturition makes it possible to study the development of some connections in postnatal animals. In the adult hamster, points in the visual field are represented along lines of projection within the dorsal and ventral nuclei of the lateral geniculate body (LGBd, LGBv) and the superior colliculus (SC). In the areas receiving input from the binocular part of the visual field, the terminal arbors of axons originating from the two eyes are found at opposite ends of the lines of projectionS, ~. As part of an effort to understand how this pattern of connections is established, we have studied the development of contra- and ipsilateral retinal pro- jections to LGBd 11. It has recently been demonstrated in the monkey visual systemS,9,10 and rat olfactory cortex s that two populations of axon terminals which are segregated from one another in a common target structure in adult animals, may have an overlapping distribution at early stages of development. We now report evidence that, in the LGBd of the hamster, the axons from the ipsilateral eye arrive after the axons from the contra- lateral eye have become distributed throughout the entire nucleus. These later-arriving axons arborize medially in the nucleus, overlapping initially with the contralateral input; then, within a few days' time the axons of the contralateral projection greatly decrease in density in the area of the ipsilateral innervation. Our experiments were carried out on 37 neonatal Syrian hamsters (Mesocricetus auratus Waterhouse). The time of impregnation was determined to within about half an hour by observation of mating behavior. The gestation period of the hamster is 16 days. We counted the first 24 h of postnatal life as day zero. At various postnatal ages (Table I), the pups were taken away from the nest and prepared for surgery. Before day 4, the only anesthesia was the mild hypothermia produced by removal of the pups from the nest. For animals injected on or after day 4, general anesthesia was induced by * Present address: Instltut d'Anatomie, Universit6 de Lausanne, Facult6 de M6decme, Rue du Bugnon 9, Lausanne 11 CHUV, Switzerland.