Regulation of the rostrocaudal axis of the optic tectum: histological study after rostrocaudal rotation in quail-chick chimeras.

Transplantation of the quail tectum primordia after rotation of its rostrocaudal axis through 180 degrees into a chick embryo was carried out at around the 10-somite stage. Our previous study showed that the rostrocaudal specificity was not determined at the stage when the transplantation was carried out in view of the retinotectal map formation. Here, we report the cytoarchitectonic development of the rotated tectum by comparing it with the quail tectum development. In the quail and chick embryos, the rostral part of the tectum had a thicker wall and was in more advanced cytoarchitectonic differentiation than the caudal part. Such cytoarchitectonic gradient along the rostrocaudal axis of the tectum became discernible on day 5.5 of incubation (E5.5) in quail embryos. In the rotated tectum, the rostral part, though it was originally caudal, had a thicker wall and more advanced cytoarchitecture than the caudal part at E5.5 and E6.5. Our results suggest that the rostrocaudal gradient of the cytoarchitecture is regulated to that of the host by E5.5.