Development of retinohypothalamic projections in the chick embryo.

The retinal projection to the hypothalamus was investigated in embryonic as well as hatched chick using wheat germ agglutinin-conjugated horseradish peroxidase as an anterograde tracer. The presence of an aggregation of anterogradely labeled terminals in the hypothalamus was used to identify the suprachiasmatic nucleus (SCN). The first clear labeling of retinal fibers in the SCN was found on embryonic day 16, whereas labeling was found in the other primary visual projections, in the diencephalon and mesencephalon, several days earlier. Thus, the retinohypothalamic projection is the last primary visual pathway to develop in the chick embryo.