The development of the tectum in Xenopus laevis: an autoradiographic study.

The development of the optic tectum in Xenopus laevis has been studied by the use of autoradiography with tritiated thymidine. The first part of the adult tectum to form is the rostroventral pole; cells in this position undergo their final DNA synthesis between stages 35 and 45 or shortly thereafter. Next, the cells comprising the ventrolateral border of the tectum form. These cells undergo their final DNA synthesis at or shortly after stage 45. Finally the cells comprising the dorsal surface of the adult tectum form, mainly between stages 50–55. This part of the tectum originates from the serial addition of strips of cells medially, which displace the pre-existing tissue laterally and rostrally. The formation of the tectum is virtually complete by stage 58. The tectum in Xenopus thus forms in topographical order from rostroventral to caudo-medial. The distribution of labelled cells, several stages after the time of injection of isotope, indicates that, at any one time, a segment of tectum is forming which runs normal to the tectal surface and includes all layers from the ventricular layer out to the surface. In Xenopus, therefore, the times of origin of tectal cells appear to be related not to cell type or tectal layer but to the topographical position of the cells across the surface of the tectum.