Organisation of the thalamofugal visual projections in chick embryos, and a sex difference in light-stimulated development

The organisation of the thalamofugal visual projections to the forebrain has been determined in chick embryos by injecting retrograde tracers (true blue and either fluorogold or diamidino yellow) into the left or right hyperstriatum. The embryos were injected on day 19 of incubation and allowed to survive for a further 4 days. Unlike chicks posthatching, the embryos were found to have no asymmetry of the thalamofugal visual projections, irrespective of whether they had received 24 h of light exposure prior to injecting the tracer dyes or had been kept in darkness. The light exposure did, however, lead to a significant increase in the ratio of the number of cell bodies in the thalamus labelled contralaterally to the injection site to the number labelled ipsilaterally (CI ratio) in male embryos. The elevation of the CI ratio appeared to be due to an increase in the number of contralateral projections from each side of the thalamus to the hyperstriatum on the other side of the forebrain. Thus, growth of these visual projections is promoted by light experience during the later stages of embryonic development. Most likely, light stimulation promotes increased arborisation of end-terminals in the hyperstriatum. Development of the thalamofugal visual projections in female embryos was not influenced by exposure to light, a finding consistent with earlier studies demonstrating that circulating oestrogen either inhibits or over-rides the sensitivity of these developing neurones to light stimulation.