Thus, one gradient of activity per axis is not sufficient to establish retinotopic LOUREIRIN-B connections, because a single cue gradient would cause all axons to migrate to and branch at one end of the map. Instead, counterbalanced forces are thought to be required, with each axon branching where these opposing forces balance out. Conflicting models have been postulated about the second mapping force. The most accepted model proposes that this second force is produced by a decreasing rostro-caudal gradient of EphA7 which repels nasal optic fibers and prevent them from branching in the rostral tectum/colliculus. However, as optic fibers invade the tectum/ colliculus throughout the highest part of this gradient, this model cannot explain how the axons invade the tectum/colliculus without being repelled by EphA7. Moreover, the role of tectal EphAs has not been evaluated in non mammalian vertebrates throughout in vivo experiments. Although differences in retinotectal/collicular mapping have been mainly described between fishes and amphibian versus birds and mammals, EphA3 and ephrin-A6 are only expressed in birds visual system and recent works suggest that some molecular mechanisms of retinotectal/ collicular mapping diverge between birds and mammals. Thus, the results obtained with collicular EphAs are not necessarily applicable to tectal EphA3. Given its decreasing rostro-caudal gradient in the chicken tectum, we hypothesized that EphA3 could account for the second activity necessary for retinotectal mapping along the rostro-caudal axis. Through functional in vitro and in vivo experiments, we demonstrated that the tectal gradient of EphA3 ectodomain is necessary to map nasal RGC axons on tectal surface by promoting nasal RGC axon growth toward the caudal Pimozide tectum and inhibiting branching rostrally to their appropriate termination zone. Furthermore, the promotion of axon growth by tectal EphA3 allows us to explain how the optic fibers invade the tectum. Therefore, opposite tectal gradients of EphA3 and ephrin-As counterbalance each other during retinotectal mapping. To investigate the function of tectal EphA3 we first analyzed its developmental pattern of expression by performing immunohistochemistry during the embryonic development 24 to 18 days of incubation – and the early postnatal period �Cnewly hatched, postnatal days 3 and 7 -. The study was achieved on sections coinciding with the rostro-caudal developmental gradient axis of the chicken optic tectum. This allowed us to distinguish between the most developed rostral pole, and the less developed caudal pole. We showed that EphA3 is expressed along the main cellular layers during the embryonic development. The stratum opticum formed by the optic fibers is also labeled mainly in the rostral tectum where the EphA3-positive temporal fibers arrive. The levels of expression of EphA3 in the superficial layers �C which include the SO and the retino-recipient layers- produce a decreasing rostro-caudal gradient which extends along the entire tectal axis. This gradient can be appreciated at E5�CE6 �Cwhen the EphA3-positive temporal RGC axons have not yet invaded the tectum- and presents the highest level of expression around E12 when the retinotectal mapping is taking place. The EphA3 expression decreases postnatally when the retinotectal mapping has finished. As axonal ephrin-As are the potential receptors for tectal EphA3, we analyzed the developmental pattern of expression of ephrin-A2 by performing immunocytochemistry on retinal sections coinciding with the naso-temporal axis from E4 to P7.