A stochastic model for retinocollicular map formation

We examine results of gain-of-function experiments on retinocollicular maps in knock-in mice [Brown et al. (2000) Cell 102:77]. In wild-type mice the temporalnasal axis of retina is mapped to the rostral-caudal axis of superior colliculus (SC). The established map is single-valued, which implies that each point on retina maps to a single termination zone (TZ) in SC. In homozygous Isl2/EphA3 knock-in mice the map is double-valued, which implies that a single point on retina maps to two TZs in SC. This is a reflection of the fact that only about 50 percent of cells in retina express Isl2. In heterozygous Isl2/EphA3 knock-ins the map is intermediate between the homozygous and wild-type: it is single-valued in temporal and double-valued in the nasal parts of retina.